Mecanisms underlying sensory interactions and - Pastel

Mecanisms underlying sensory interactions and complex
perceptions
David Labbe
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David Labbe. Mecanisms underlying sensory interactions and complex perceptions. Life Sciences
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THÈSE
pour obtenir le grade de
Docteur
de
l’Institut des Sciences et Industries du Vivant et de l’Environnement
(Agro Paris Tech)
Spécialité : Sciences de la vie et santé
présentée et soutenue publiquement
par
LABBE David
le 12 Décembre 2008
MECANISMES SOUS-JACENTS AUX INTERACTIONS
PERCEPTUELLES ET PERCEPTIONS COMPLEXES
Mechanisms underlying sensory interactions and complex perceptions
Directeur de thèse : Nathalie MARTIN
Travail réalisé : Centre de Recherche Nestlé, Département interactions alimentconsommateur Vers-chez-les-Blanc, 1000 Lausanne, Suisse
Devant le jury :
M. Henry- Eric Spinnler, Professeur, AgroParisTech, Paris, France...............................................Président
Mme Catherine Rouby, Professeur Associé, HDR, Université Claude Bernard Lyon 1,
Lyon, France....................................................................................................................................Rapporteur
M. Rick Schifferstein, Professeur Associé, University of Technology
Delf, Pays-Bas …............................................................................................................................Rapporteur
Mme Catherine Dacremont, Professeur, Centre Européen des Sciences du Goût,
UMR CNRS(5170)-Université de Bourgogne-INRA, Dijon, France ..............................................Examinateur
Mme Nathalie MARTIN, Professeur Associé, HDR, Centre de Recherche Nestlé,
Lausanne, Suisse........................................................................................................Directeur de thèse
Résumé
Les interactions perceptuelles entre différentes modalités sensorielles affectent la perception des
aliments. Si ce phénomène a été largement étudié entre des stimuli olfactifs et gustatifs en
solution aqueuse, peu d'études ont été menées avec des produits existants dans le commerce.
Similairement, peu de travaux sont dédiés à l'étude des interactions multi-sensorielles impliquant
l'olfaction, le goût et la perception tactile en bouche et à l'étude des perceptions dites complexes,
c'est-à-dire qui impliquent plusieurs modalités sensorielles. Le premier objectif de ma thèse était
l'exploration des mécanismes sous-jacents aux interactions perceptuelles existant entre l'olfaction
et la gustation durant la consommation de produits réels, et en solution avec des stimuli olfactifs à
une concentration infraliminaire. Cette approche a ensuite été étendue aux perceptions olfactive,
gustative et tactile (en bouche) et à la perception complexe "rafraîchissante". Il a été mis en
évidence que la familiarité d'un produit et la stratégie d'attention durant l'exposition affectent de
façon critique les interactions perceptuelles. Pour la première fois il a été démontré qu'une
concentration infraliminaire d'odorant associé au goût sucré (fraise) augmente la perception
sucrée d'une solution de sucrose. La multiplicité des interactions sensorielles présente dans un
milieu alimentaire complexe a également été mis en évidence à un niveau bimodal (ex: entre la
perception amer et le froid) et tri-modal (ex: entre la perception olfactive, sucrée et froide).
Finalement il été démontré que la perception rafraîchissante est construite sur la base d'une
combinaison de déterminants sensoriels, d'habitudes alimentaires ainsi que de facteurs
hédoniques et psychophysiologiques tels que l'énergie mental. Pour conclure, les connaissances
acquises par ce travail soulèvent d'autres interrogations notamment à propos des mécanismes
neuronaux sous-tendant la mémorisation des associations perceptuelles et des conditions
requises en termes de durée et de fréquence d'exposition pour la mise en place de ces
interactions.
Mots-clés: Olfaction, Gustation, Tactile, Interactions perceptuelles, Processus cognitifs,
Perception rafraîchissante
Summary
Perceptual interactions between different sensory modalities affect overall perception of food.
This phenomenon has been extensively investigated between olfactory and tastant stimuli in
aqueous solutions. But few studies assessed olfactory and taste interactions in existing products
(commercially available). Similarly multi-sensory perceptual interactions involving olfaction, taste
and in mouth tactile perceptions and complex perceptions (i.e. a perception which is driven by
more than one sensory dimension) have been poorly investigated. The first objective of my thesis
was to investigate mechanisms underlying perceptual interactions between bimodal olfactory and
taste perceptions in existing products and then in solutions with olfactory stimuli at a subthreshold
concentration. This approach was extended to olfactory, taste and in mouth tactile perceptions
and finally to "refreshing" complex perception. I showed that product familiarity and attentional
strategy applied during exposure are critical factors modulating perceptual interactions. I
demonstrated for the first time that subthreshold odorant concentrations related to sweet taste (i.e.
strawberry) increase perceived sweetness of a sucrose solution. The multiplicity of perceptual
interactions in complex food systems was demonstrated since I identified bimodal (e.g. between
bitterness and coldness) and tri-modal (e.g. between mint aroma, sweetness and coldness)
perceptual interactions. Finally I showed that refreshing perception is driven by positive and
negative sensory drivers, food habits, together with hedonic and psychophysiological factors such
as mental energy. To conclude, knowledge acquired during this work raised new questions; in
particular related to neural mechanisms underlying memorization of perceptual associations and
required conditions in terms of exposure duration and frequency for construction of such
interactions.
Keywords: Olfaction, Taste, Tactile, Perceptual interactions, Cognitive processes, Refreshing
perception
Acknowledgement
This research work was funded by Nestec S.A. and carried out at the Nestlé Research
Center (NRC) at Lausanne in Switzerland under the supervision of Nathalie Martin head
of the NRC Sensory Science Group and formally Associate Professor at AgroParisTech.
I gratefully acknowledge Nathalie Martin, my NRC PhD supervisor for giving me the
opportunity to conduct a PhD, for her trust, her high motivation and encouragement, her
efficient managing skill, our fruitful discussions, the time she spent to review and improve
my manuscript papers and finally her friendship.
I warmly thank the members of my PhD committee Thierry Thomas Danguin, Thomas
Hummel and Jeff Brunstrom for their interest in my work, our profitable meetings and their
valuable advice and constructive criticism.
I would like to thank Julie Hudry for our discussion about neuro-imagery, memory and
cognitive processes, Nicolas Godinot for sharing his abundant views and hypotheses
about my work but also about a non exhaustive list of various topics such as homeopathy
or politics.
Carla Vaccher, Coralie Keller, Thierry Waeber, Florie Gilbert and Santo Ali for their
support in the planning and conducting sensory experiments and clinical studies, Gaëlle
Schlup-Ollivier for her technical knowledge and competencies in ice cream formulation
and Andreas Rytz, Nicolas Antille, Sabrina Rami and Nicolas Pineau for their involvement
regarding design and statistical analysis and Elizabeth Prior and Peter Leathwood for
reviewing the English of thesis papers and PhD manuscript.
I am very grateful to Marcel Juillerat, Head of the NRC Food and Consumer Interaction
Department and Peter Van Bladeren, Head of the NRC for giving me the opportunity to
accomplish this PhD research with ideal conditions in terms of resources such as
experimental material and fund for participating to international conferences.
I wish to express all my gratitude to Eric Spinnler for kindly accepting to take over the
function of AgroParisTech PhD contact.
I would like to warmly thank François Sauvageot who transmitted to me his passion for
sensory evaluation, nine years ago.
A special thanks goes to Sonia Provent for designing the book cover.
Finally I dedicate this thesis to my family.
Content
SYNTHESE EN FRANÇAIS..................................................................................................... 1
INTRODUCTION...................................................................................................................... 8
1
PART 1: LITERATURE REVIEW ..................................................................................... 10
1.1
1.1.1
1.1.2
EXAMPLES OF OBSERVED SENSORY INTERACTIONS....................................................... 10
Sensory interactions between olfaction and taste.................................................. 10
Sensory interactions involving tactile perception ................................................... 11
1.2
1.2.1
1.2.2
1.2.3
MECHANISMS UNDERLYING SENSORY INTERACTIONS .................................................... 12
Sensory interaction induced by physicochemical interactions ............................... 12
Role of perceptual mechanisms on sensory interactions....................................... 14
Neural correlates of perceptual interactions .......................................................... 19
1.3
1.3.1
1.3.2
1.3.3
COMPLEX PERCEPTION ............................................................................................... 21
Creaminess........................................................................................................... 21
Freshness ............................................................................................................. 21
Refreshing ............................................................................................................ 22
SCIENTIFIC APPROACH ...................................................................................................... 23
LIST OF ORIGINAL COMMUNICATIONS ............................................................................. 30
2
2.1.1
2.2.3
PART 2 PERCEPTUAL MULTI-SENSORY INTERACTIONS .......................................... 32
2.1
OLFACTORY AND TASTE PERCEPTUAL INTERACTIONS .................................................... 32
Role of product familiarity and exposure strategy with stimuli
at suprathreshold level ........................................................................................................... 32
2.1.2
Impact of subthreshold olfactory stimuli on taste perception.................................. 52
2.2
OLFACTION, TASTE AND TACTILE PERCEPTUAL INTERACTIONS ....................................... 78
2.2.1
Validation of the sensory diversity induced by formulated products....................... 78
2.2.2
Impact of olfaction on taste, trigeminal and texture perceptions ............................ 83
Setting up of the range of liquid viscous products for the investigation of refreshing complex
perception 94
3
3.1
PART 3: COMPLEX PERCEPTION: REFRESHING........................................................ 96
SENSORY BASIS OF REFRESHING PERCEPTION: ROLE OF PSYCHOPHYSIOLOGICAL FACTORS AND
FOOD EXPERIENCE ................................................................................................................... 96
3.2
3.2.1
3.2.2
SENSORY FOUNDATIONS OF REFRESHING PERCEPTION ............................................... 116
Sensory determinants of refreshing..................................................................... 116
Temporal Dominance of Sensation and Sensory Profiling: A Comparative Study 127
3.3
ROLE OF PHYSIOLOGICAL FACTORS IN REFRESHING PERCEPTION:
MENTAL ENERGY AND SALIVA .................................................................................................. 134
3.3.1
The impact of refreshing perception on mental energy: changes in mood, cognitive
performance and brain oscillations ....................................................................................... 134
Modulation of saliva flow, saliva lubricating properties and related lingering perceptions by
refreshing frozen snacks ...................................................................................................... 153
3.3.2
4
4.1
4.2
PART 4: GENERAL DISCUSSION AND PERSPECTIVES............................................ 167
FAMILIARITY AND EXPOSURE STRATEGY MODULATE PERCEPTUAL INTERACTIONS .......... 167
MULTIPLICITY OF PERCEPTUAL INTERACTIONS BETWEEN OLFACTORY, TRIGEMINAL AND TACTILE
PERCEPTIONS ........................................................................................................................ 167
4.3
FOOD EXPERIENCE CONTRIBUTES TO CONSTRUCTION OF REFRESHING PERCEPTION ..... 168
4.4
ALTERNATIVES TO ATTRIBUTE RATING (SENSORY PROFILING) FOR FURTHER EXPLORING
PERCEPTUAL INTERACTIONS AND UNDERSTANDING CONSUMER PREFERENCE ............................ 169
4.5
ROLE OF MEMORY AND NEURAL INTEGRATION PROCESSES IN THE CONSTRUCTION OF PERCEPTUAL
ASSOCIATION ......................................................................................................................... 171
4.6
ANTHROPOLOGICAL APPROACH OF PERCEPTUAL INTERACTIONS .................................. 172
REFERENCES..................................................................................................................... 173
1
Synthèse en français
Problématique
Afin de formuler des aliments avec des propriétés sensorielles satisfaisant les
consommateurs, il est essentiel de mieux comprendre les facteurs influençant la
perception des aliments. Parmi ces facteurs, les interactions perceptuelles sont
reportées dans la littérature comme ayant un fort impact sur la perception. Cependant
si de nombreuses d'études sont consacrées aux interactions bimodales entre olfaction
et gustation en solution aqueuse: 1) aucune étude n'a été menée dans des produits
existants dans le commerce avec pour but l'étude de l'impact de la familiarité du
produit sur les interactions perceptuelles; et 2) peu de travaux sont dédiés à l'étude
des interactions perceptuelles impliquant l'olfaction, la gustation ainsi que la perception
tactile en bouche alors que paradoxalement l'ensemble de ces trois modalités
sensorielles est systématiquement impliqué pendant la consommation des aliments. Il
s'avère également que les perceptions complexes, c'est-à-dire qui ne sont pas
expliquées par une seule dimension (par exemple onctueux), et généralement
appréciés par les consommateurs, sont de façon surprenante peu étudiées
scientifiquement.
Le but de cette thèse est d'apporter de nouvelles connaissances concernant les
mécanismes sous-jacents aux interactions perceptuelles entre olfaction et gustation
puis entre olfaction, gustation et perception tactile en bouche pour finalement explorer
l'origine de la perception rafraîchissante. Les démarches et résultats sont expliqués
dans un premier temps et ensuite discuté dans une seconde partie.
Démarche et résultats
Suite aux différentes questions soulevées dans le cadre de la revue de littérature
(Partie 1), le sujet d'étude est organisé en deux parties comme présenté ci-dessous.
Partie 2: Interactions perceptuelles multi-sensorielles
Dans un premier temps nous nous sommes consacrés à l'étude des interactions
perceptuelles entre l'olfaction et la gustation dans des produits existant dans le
commerce et plus spécifiquement à l'étude de l'impact de la familiarité vis-à-vis du
produit sur les interactions perceptuelles. Trois boissons amères et non sucrées
différentes en termes de familiarité ont été utilisées: une boisson café familière, une
boisson cacao reconstituée avec de l'eau (moins familière qu'une boisson café car
reconstituée avec de l'eau), et du lait contenant de la caféine. Dans chaque produit a
été ajouté un arôme vanille, sans composante gustative, mais perceptuellement
associé au goût sucré. Pour chaque produit, les versions sans arôme et avec arôme
ont été décrites par profil sensoriel. Nous avons démontré que la nature des
interactions sensorielles entre olfaction et goût est modulée par la familiarité du produit.
Comme supposé, l'ajout d'un odorant vanille dans la boisson café diminue la
perception de l'amertume intrinsèque du café par l'effet suppressif de la sucrosité
induite par l'arôme. Cependant quand l'arôme est ajouté dans le lait amer non familier,
l'amertume de ce dernier est augmentée.
--M
Meecchhaanniissm
mss uunnddeerrllyyiinngg ppeerrcceeppttuuaall iinntteerraaccttiioonnss aanndd ccoom
mpplleexx ppeerrcceeppttiioonnss--
Synthèse en français
2
Ensuite, toujours dans le cadre des interactions entre olfaction et gustation agissant
durant la consommation de produits réels, l'impact de la stratégie attentionnelle mise
en place lors de la description de l'odeur de huit cafés a été évalué en comparant les
résultats: 1) d'un groupe de panélistes expérimentés qui a utilisé la méthode de profil
sensoriel, cette méthode induisant une stratégie attentionnelle analytique puisque
chaque dimension sensorielle est appréhendée et décrite individuellement; et 2) d'un
groupe de consommateurs de café qui a utilisé la méthode du tri avec libre génération
de vocabulaire. Cette méthode de tri a été choisie car elle implique une stratégie
d'attention synthétique, la perception de l'odeur café étant appréhendée de façon
globale afin de pouvoir mener à bien cette épreuve. Cette approche est proche de la
stratégie d'attention généralement adoptée spontanément pendant la consommation
dans la vie courante. Les résultats ont mis en évidence que la nature de la stratégie
attentionnelle mise en place lors de la description aromatique de huit cafés affecte
fortement la caractérisation et la disposition relative des produits sur la carte
sensorielle. Les consommateurs ont généré des termes appartenant au lexique de la
gustation tels qu'amer ou sucré alors que les cafés étaient uniquement flairés.
Nous nous sommes intéressés par la suite à l'impact de stimuli olfactifs présentés à un
niveau supraliminaire et infraliminaire sur l'intensité de la perception sucrée avec 1) un
odorant communément associé au goût sucré (fraise) et; 2) un odorant non familier
mais précédemment co-exposé avec du sucrose pendant cinq séances réparties sur
une semaine. Nous avons démontré que l'ajout de l'arôme fraise à un niveau supra et
infraliminaire dans une solution de saccharose à 15 g/L augmente la sucrosité.
Cependant l'odeur nouvellement associée au goût sucré par co-exposition augmente
la perception sucrée de la solution de saccharose uniquement lorsque l'odorant est
présenté à un niveau supraliminaire mais n'a pas d'effet lorsque présenté à un niveau
infraliminaire.
Finalement nous avons étendu notre champ d'investigation aux perceptions olfactive,
gustative et tactile perçues dans un fluide visqueux sucré. La perception tactile a été
abordées sous deux aspects: 1) la perception trigéminée (froid en bouche); et 2) la
perception proprioceptive (épaisseur en bouche). En suivant un plan factoriel à deux
niveaux, une gamme de huit produits aromatisés à la pêche et une gamme de huit
produits aromatisés à la menthe ont été formulées. Les deux gammes contenaient en
plus de l'odorant, de l'acide citrique et un agent cooling. Les deux espaces produits ont
été validés en termes de variété sensorielle par une épreuve de tri avec libre choix de
vocabulaire. Nous avons mis en évidence une large variété sensorielle entre les
produits estimée suffisante pour l'étude par profil sensoriel des interactions
sensorielles présentes dans ce modèle complexe. Les résultats de l'épreuve de tri ont
également mis en évidence que le terme rafraîchissant est fréquemment cité pour
décrire les groupes mis en place. Il s'avère donc que la richesse sensorielle des
produits semble induire une perception complexe rafraîchissante. Les résultats du
profil sensoriel ont mis en évidence qu'une grande diversité d'interactions
perceptuelles affectent la perception des produits avec des interactions de type: 1)
bimodale, par exemple entre la perception trigéminée froide et l'amertume et; 2) trimodale, par exemple entre la perception olfactive menthe, le goût sucré et la
perception trigéminée froide.
--M
Meecchhaanniissm
mss uunnddeerrllyyiinngg ppeerrcceeppttuuaall iinntteerraaccttiioonnss aanndd ccoom
mpplleexx ppeerrcceeppttiioonnss--
Synthèse en français
3
Partie 3 Perception complexe: rafraîchissant
Suite aux résultats précédemment décrits, la perception rafraîchissante a été étudiée
plus en profondeur dans une dernière partie. Dans un premier temps une revue de
littérature a montré que plusieurs déterminants sensoriels sont associés à la
perception rafraîchissante et a mis en évidence l'importance de certains déterminants
psychophysiologiques dans la construction de cette perception telle que la sensation
de désaltération, de bouche hydratée (par opposition à une sensation généralement
décrite comme "bouche sèche" ou "pâteuse") et d'augmentation de l'énergie mentale
(terme regroupant les notions d'activité corticale, de performance cognitive et d'éveil
perçu). De plus l'expérience alimentaire joue un rôle clé dans la mise en place de la
perception rafraîchissante.
Cependant afin de connaître la contribution respective des différentes modalités
sensorielles dans la perception rafraîchissante, une gamme de produits visqueux
proche de celle utilisée précédemment, mais contenant en plus deux niveaux
d'épaississant (xanthan) afin d'élargir l'espace sensoriel, a été décrite sensoriellement
par un panel expérimenté en utilisant une liste de descripteurs et en termes d'intensité
rafraîchissante par un groupe de 160 consommateurs. La technique statistique de
cartographie des préférences interne a montré que: 1) plus les produits sont perçus
sucrés, moins ils sont perçus rafraichissants par la majorité des 160 consommateurs
et; 2) les 160 consommateurs ont été segmentés en trois groupes de taille similaire
pour lesquels les déterminants sensoriels de la perception rafraîchissante varient. En
effet, une forte intensité froide, une forte intensité acide et une faible intensité épaisse
ont été mis en évidence comme étant pour chacun des trois groupes le principal
déterminant sensoriel de la perception rafraîchissante.
La perception temporelle, c'est-à-dire au cours du temps, induite par ces mêmes
produits après consommation a été évaluée par la technique de Dominance
Temporelle des Sensations (DTS). La méthode DTS permet de suivre l'évolution au
cours du temps d'un plus grand nombre d'attributs comparé aux méthodes classiques
de Temps Intensité limitées à un ou deux termes. Les résultats de 48 panélistes
entraînés à cette méthode ont montré que la perception laissée en bouche diffère
entre produits jusqu' à trois minutes après la consommation des produits. De plus, en
combinant ces résultats avec ceux obtenus lors du précédent test avec les 160
consommateurs, il s'avère que ces perceptions rémanentes pourraient être des
facteurs importants sous-jacents à la perception rafraîchissante.
Finalement nous avons comparé l'impact de la consommation de deux produits
congelées (contenant principalement de l'eau et du saccharose) et de même valeur
énergétique, mais variant en intensité rafraîchissante, sur deux facteurs
psychophysiologiques associés à la perception rafraîchissante: la sensation
d'hydratation en bouche et l'énergie mentale. Le produit le plus rafraîchissant contenait
un agent cooling et de l'acide citrique. La sensation d'hydratation en bouche a été
évaluée en mesurant le flux salivaire et les propriétés lubrifiantes de la salive au
moyen d'un tribomètre, un appareil reproduisant artificiellement les mouvements de
friction entre la langue et le palais. L'énergie mentale a été mesurée lors d'une
seconde étude en termes d'activité corticale par électroencéphalogramme, de
performance cognitive durant une tache attentionnelle et de la sensation d'éveil par la
notation d'une liste d'attributs relatifs à l'état d'humeur ressenti. Ces différents
paramètres ont été mesurés avant et après consommation des produits. Suite à la
consommation du produit le plus rafraîchissant, le flux salivaire est plus important et la
--M
Meecchhaanniissm
mss uunnddeerrllyyiinngg ppeerrcceeppttuuaall iinntteerraaccttiioonnss aanndd ccoom
mpplleexx ppeerrcceeppttiioonnss--
Synthèse en français
4
salive produite est plus lubrifiante, c'est à dire qu'elle induit une plus faible force de
friction entre la langue et le palais. De même les mesures relatives à l'énergie mentale
ont montré que le produit le plus rafraîchissant augmente l'activité corticale,
principalement dans les fréquences alpha et beta, ainsi que les performances durant
la tâche de vigilance. Cependant les deux produits n'ont pas significativement modifié
l'éveil perçu par les sujets.
Discussion générale et perspectives
Les résultats de la partie consacrée à l'étude des interactions multi sensorielles ont
montré que les interactions perceptuelles diffèrent entre des solutions modèles et des
produits réels, puisque la familiarité du produit module ces interactions; et entre des
approches attentionnelles analytique et synthétique.
L'augmentation de la perception sucrée et la diminution de l'amertume des boissons
café et cacaotée causées par l'ajout d'odorant vanille étaient attendues. En effet
l'association perceptuelle existant entre l'odeur vanille et le goût sucré est à l'origine de
l'augmentation de la sucrosité et par conséquent de la diminution de l'amertume par
un phénomène d'interaction suppressive entre les goûts sucré (induit par l'odeur) et
amer. Cependant l'augmentation de l'amertume du produit non familier (lait cafeiné)
par l'ajout de ce même odorant vanille reste inexpliquée. Ce produit a probablement
été perçu de façon déplaisante par le biais de phénomènes de néophobie alimentaire
et de rejet du goût amer largement décrits dans la littérature. L'ajout de vanille a
certainement renforcé l'aspect non familier et donc déplaisant du produit. Le caractère
déplaisant du produit a pu être reporté sur l'échelle dédiée à l'attribut amer, cette
dimension sensorielle ayant une forte connotation hédonique négative (contrairement
à la dimension sucrée généralement associée au plaisir). Pour résumer l'augmentation
de l'amertume pourrait être la conséquence d'interactions entre dimensions sensorielle
et hédonique.
Ces résultats ouvrent des perspectives d'études futures de neuro-imagerie en utilisant
la technique d'imagerie par résonance magnétique fonctionnelle (IRMf) pendant la
consommation de ces mêmes produits. Le but est de pouvoir expliquer d'un point de
vue neuronal les différences de perception gustative observées après la
consommation de produits familiers et non familiers aromatisés à la vanille, c'est-à-dire
l'augmentation de la sucrosité et la diminution de l'amertume induites par le produit
familier et l'augmentation de l'amertume induite par le produit non-familier. Deux
hypothèses sont proposées:
- 1) une augmentation de l'activité de l'aire gustative primaire associée au goût sucré
et au goût amer lors de la consommation de produit familier et non familier,
respectivement qui signifierait que les modulations d'intensité gustatives perçues sont
uniquement la conséquence d'une différence de traitement des stimuli sensoriels à un
niveau périphérique (récepteurs sensoriels) et/ou à un niveau cortical primaire;
- 2) des différences induites par les deux produits au niveau de l'activation des aires
intégratives (par exemple le cortex orbitofrontal) où convergent et sont intégrées les
informations gustatives et olfactives ce qui validerait que des mécanismes cognitifs
tels que le plaisir localisés dans ces même aires cérébrales modulent les processus
d'intégration et consécutivement la perception gustative.
--M
Meecchhaanniissm
mss uunnddeerrllyyiinngg ppeerrcceeppttuuaall iinntteerraaccttiioonnss aanndd ccoom
mpplleexx ppeerrcceeppttiioonnss--
Synthèse en français
5
La seconde hypothèse semble la plus plausible et validerait l'explication proposée
pour expliquer l'augmentation d'amertume causée par le produit non familier mis en
évidence par les résultats psychophysiques, c'est à dire une interaction entre les
dimensions sensorielle (amertume) et hédonique (déplaisant)
Les différentes stratégies d'attention mise en place par un panel sensoriel et par des
consommateurs (c'est à dire dans la vie courante) modifie les interactions
perceptuelles aussi bien lors de la description de produits familiers (l'arôme de café)
que lors de la co-exposition à un nouvel odorant et à du saccharose. En effet
considérer un produit de façon analytique: 1) pendant l'entraînement, c'est à dire en
apprenant à disséquer ses caractéristiques individuellement; et 2) pendant le profil
sensoriel où chaque attribut est évalué indépendamment, réduit l'effet des interactions
sensorielles qui au contraire est augmenté lors d'une approche synthétique (ou
holistique). Cependant la première étude consacrée au rôle de la familiarité a tout de
même mis en évidence des interactions perceptuelles alors que l'approche utilisée
était analytique (profil sensoriel). La persistance d'interaction peut être expliquée par
l'entraînement qui n'a pas été mené spécifiquement sur les produits évalués par la
suite mais sur des références externes. Par conséquence, les panélistes n'ont pas
appris à décomposer les dimensions sensorielles des produits évalués ce pourrait
expliquer que des interactions sensorielles ont pu quand même être observées avec
une méthode analytique.
Afin de pouvoir évaluer l'efficacité de cette méthode analytique à mettre en évidence
des interactions perceptuelles, il serait intéressant lors de futures études de combiner
la notation standard d'attributs à l'aide d'échelle (profil sensoriel) avec d'autres
méthodes qui ne nécessitent pas l'utilisation d'échelle comme par exemple l'épreuve
de catégorisation. En effet le principe de cette méthode est de catégoriser un groupe
de produits sur une dimension perceptuelle par exemple l'intensité du goût sucré, les
catégories proposés par l'expérimentateur pouvant être à deux niveaux "faible
intensité sucrée" et "forte intensité sucré", ou à trois niveaux incluant en plus des deux
catégories précédentes la catégorie "moyennement sucré", ou plus. Ainsi en
catégorisant un groupe de produits formulés avec plusieurs concentrations de sucre et
d'odorant (par exemple vanille), il est possible de déterminer si l'odorant influence ou
non la catégorisation des produits sur la perception sucrée et par conséquent si des
interactions perceptuelles existent.
Nos travaux dédiés à l'impact d'un stimulus olfactif présenté à un niveau infraliminaire
sur la perception sucrée, nous ont permis d'observer: 1) qu'un odorant communément
présent dans des aliments sucrés augmente la sucrosité d'une solution de saccharose
quand l'odorant est présenté au dessus du seuil de perception (supraliminaire) mais
aussi quand l'odorant est présenté en dessous du seuil de détection (infraliminaire); et
2) qu'un odorant non-familier mais co-exposé de façon répétée pendant une semaine
avec du saccharose augmente également par la suite l'intensité de la perception
sucrée d'une solution de saccharose quand il est présenté à un niveau supraliminaire
mais n'a pas d'effet quand il est présenté à un niveau infraliminaire. La littérature
rapporte que les associations construites au cours de la vie entre différentes
dimensions sensorielles nécessitent un apprentissage implicite et ensuite une
consolidation impliquant la mémoire à long terme (de plusieurs jours à plusieurs mois)
et à très long-terme (de plusieurs mois à la vie). A un niveau cérébral, des neurones à
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l'origine uni modaux et ne pouvant donc répondre qu'à une seule dimension
sensorielle (par exemple olfactive) pourrait avoir évolués chez l'homme en neurones
bimodaux suite aux stimulations répétées durant la vie à des même paires de stimuli
olfactif et gustatif (par exemple arome fraise et goût sucré). Ces neurones bimodaux
pourraient donc répondre à des stimuli olfactifs et gustatifs. Cette hypothèse semble
être assez largement partagée comme étant le corrélat neuronal sous-tendant les
interactions perceptuelles d'autant que des neurones bimodaux et multimodaux ont été
identifiés chez le singe dans le cortex orbitofrontal. Dans notre étude, l'augmentation
de la sucrosité est induite par un stimulus olfactif congruent avec le goût sucré
présenté à un niveau supraliminaire mais aussi à un niveau infraliminaire. L'activation
des récepteurs olfactifs, même à une concentration qui n'induit pas de perception
consciente, pourrait tout de même conduire à une activation de neurones bimodaux
olfactif-gustatif, en admettant que de tels neurones existent chez l'homme. Cependant
dans le cas d'un nouvel odorant brièvement co-exposé avec du saccharose (durant
une semaine), le processus d'apprentissage et de consolidation au niveau de la
mémoire à court terme semble être suffisant pour induire des interactions
perceptuelles quand présenté à un niveau supraliminaire mais insuffisant quand
présenté à un niveau infraliminaire.
D'autres études psychophysiques sont nécessaires pour répondre à de nombreuses
questions au sujet des processus de mémorisation et d'intégration impliqués dans la
construction de nouvelles associations perceptuelles telles que: 1) leur robustesse au
cours du temps (mois ou années), c'est-à-dire les nouvelles associations sont-elle
ancrées dans la mémoire à long terme?; 2) la période optimale de co-exposition
(durée de la phase de co-exposition, fréquence et durée des séances pendant cette
phase) permettant de mémoriser de manière robuste la nouvelle association de façon
à induire à cours terme (immédiatement après la co-exposition) et long terme (plus
d'une semaine après la co-exposition) une augmentation de la sucrosité par ajout de
l'odorant à un niveau infraliminaire? La mesure de l'activité cérébrale par Imagerie par
Résonance Magnétique fonctionnelle (IRMf) en réponse à une exposition à un nouvel
arôme en combinaison avec du saccharose avant, durant et de façon répété au cours
du temps après l'apprentissage par co-exposition pourrait apporter des informations
pertinentes sur la plasticité des aires cérébrales impliquées dans les processus
d'intégration sensorielles.
L'étude des interactions perceptuelles entre olfaction, gustation et perception tactile a
mis en évidence de nombreuses interactions soit entre deux modalités sensorielles,
par exemple entre olfaction et perceptions sucré, amère et acide, soit entre trois
modalités sensorielles notament entre olfaction, amertume et perception froide. Ces
résultats démontrent que l'ajout d'un odorant dans un milieu sensoriellement complexe
a un impact large et difficilement maîtrisable sur la perception globale.
Notre investigation des déterminants sensoriels de la perception rafraîchissante a
permis d'identifier que la perception sucrée est pour la majorité des consommateurs
de notre étude, négativement corrélée à la perception rafraîchissante. Ensuite trois
caractéristiques sensorielles, une forte intensité froide, une forte acidité et une faible
épaisseur, ont été identifiées comme positivement associées à la perception
rafraichissante mais avec une importance variant entre consommateurs. Nous
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pensons que ces différences entre consommateurs sont explicables dues à des
différences d'habitudes alimentaires. En effet les consommateurs pour lesquels
l'intensité froide est le principal déterminant sensoriel de la perception rafraîchissante
sont consommateurs de chewing-gums à la menthe qu'ils consomment pour se
rafraîchir l'haleine.
Il est connu que les composé acides stimulent le flux salivaire et induisent par
conséquent une sensation de bouche hydratée (à opposer à une sensation de bouche
sèche ou pâteuse) et que les agents cooling peuvent améliorer la capacité d'attention
et stimuler l'éveil ressenti. Les résultats montrant que le produit surgelé perçu
rafraîchissant grâce à l'ajout d'acide citrique et d'agent cooling sont donc en
adéquations avec ces connaissances et démontrent pour la première fois qu'une
augmentation de l'activité cérébrale impliquée dans les processus attentionnels
(fréquence alpha) et d'intégration sensorimotrice (fréquence beta) est probablement à
l'origine des meilleures performances obtenues lors de la tâche attentionnelle.
D'autres investigations restent cependant nécessaires afin de valider si, pour être
perçu rafraîchissant, un produit doit obligatoirement avoir un impact
psychophysiologique.
Conclusion
Nos études nous ont permis d'étendre le champ des connaissances relatives aux
mécanismes psychophysiques sous-jacents aux interactions perceptuelles en nous
plaçant dans des conditions expérimentales proches de la réalité. Nos résultats nous
ont permis d'émettre des hypothèses concernant les mécanismes de mémorisation et
neuronaux sous-tendant nos interprétations. De plus pour la première fois nous avons
appréhendé d'un point de vue sensoriel et psychophysiologique les déterminants
d'une perception complexe. Ces nouvelles informations s'avèrent importantes dans le
cadre du développement de nouveaux produits répondant mieux aux attentes du
consommateur.
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Introduction
Food acceptance is greatly affected by sensory properties perceived by olfactory, taste
and tactile senses during consumption.
Olfactory perception results from the stimulation of the olfactory system by volatile
molecules. Air-borne molecules enter the nasal cavity either from outside by sniffing
through the nose which leads to orthonasal olfactory perception or during eating or
drinking via the oropharyngeal cavity at the back of the mouth and throat inducing
retronasal olfactory perception. Olfactory perceptions following orthonasal and
retronasal stimulations are called "odour" and "aroma", respectively, according to the
international standards (ISO 5492, 1995). But "flavour" instead of "aroma" is also
currently used for describing the retronasal olfactory perception.
Regarding gustatory perception, the scientific community now agrees that at least five
"basic” tastes exist: sweet, salty, sour, bitter and umami. These are perceived through
taste receptor cells found in tongue taste buds immersed in the epithelium of taste
papillae.
Tactile perception is classified as a part of the somatosensory system, which is
concerned with four major modalities: 1) discriminative touch related to detection of
pressure or vibration of objects in contact with the surface of the body, which allows
sensing size, shape and microstructure; 2) thermosensation related to temperature
detection; 3) proprioception related to detection of static position and movement of the
jaws, tongue, hands, fingers, etc; and 4) nociception (pain) related to detection of
tissue damage or events that could directly damage tissues and are perceived as
painful.
Trigeminal perception corresponds to both thermosensation and nociception
modalities and is caused 1) by chemical compounds leading for instance to hotness
and irritation (induced by chilli pepper, capsaicin, black pepper piperine, etc), fizziness
(soft drink carbonation) or coldness (menthol); and 2) by temperature.
To study the impact of ingredients and food processes on food perception, specific
methodologies were developed in the field of sensory evaluation to qualify and quantify
sensory dimensions perceived during consumption. Surprisingly, during sensory
evaluation of diverse foods, correlations were highlighted between subjects’ replies to
stimulation of different sensory systems. For example, it was found that the sweetness
rating of a fruity flavoured solution is positively correlated to the rating of the aroma
intensity (Frank and Byram, 1988). This phenomenon was named sensory interaction
and was historically found in bimodal food systems involving olfactory and taste stimuli.
Later it was demonstrated that other perceptions such as touch can be similarly
affected by sensory interactions. Multi sensory interactions were then highlighted in
complex food system involving odorant, tastant and tactile stimuli. Because of their
complexity, food systems may also induce characteristics resulting from simultaneous
multi-sensory stimulations and modulate by cognitive factors such as familiarity and
liking. Creaminess and freshness are two examples of complex perception widely used
for a long time by food marketing since perceived as a product benefit by consumers
but only recently scientifically investigated.
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Sensory interactions can result in a modulation of the stimuli amount reaching sensory
receptor mainly because of: 1) chemical interactions between sensory stimuli; and/or 2)
physical or chemical interactions between a sensory stimulus and the food matrix. This
type of interaction is named physicochemical interaction. Sensory interactions can also
be the consequence of associations between different sensory modalities which are
constructed during every day food experience. This type of interaction is named
perceptual interaction.
To formulate food with sensory properties satisfying consumers, it is essential to
acquire a better understanding of factors influencing food perception and more
specifically of mechanisms underlying perceptual interactions in simple but also
complex food as well as the mechanisms involved in complex perception, i.e.
perception not easily described by simple attributes.
The scientific approach of this PhD is as follows:
PART 1: Literature review focussing on interactions between olfaction, taste, tactile
perceptions and complex perceptions involved during food consumption with the
following objectives:
- Detail the studies that provide substantial advanced learning in the field rather than
proposing an exhaustive list of published works.
- Define work objectives accordingly.
PART 2: Exploration of bimodal perceptual interactions between olfaction and taste
and then widening of the approach to multi-modal interactions involving tactile
perception with a focus on:
- The role of familiarity and attentional strategy during exposure on the sensory
characterization of commercial beverages with olfactory and taste stimulus at
suprathreshold level;
- The impact of subthreshold olfactory stimuli on taste perception;
- The impact of olfaction on taste, trigeminal and texture perception.
PART 3: Extend the exploration of mechanisms underlying perceptual interactions to
the investigation of refreshing complex perception in terms of:
- Sensory drivers
- Associated psychophysiological factors
PART 4: General discussion and perspectives
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1
PART 1: Literature review
In the following review, results of the key studies in the field of sensory interactions are
first presented (§1.1) by separating interactions observed between olfactory and taste
stimuli (§1.1.1) and then involving tactile stimuli (§ 1.1.2). Second I focus on mechanisms
underlying sensory interactions (§ 1.2). Finally the concept of complex perception is
introduced (§ 1.3).
1.1 Examples of observed sensory interactions
1.1.1 Sensory interactions between olfaction and taste
1.1.1.1 Olfactory stimuli at a suprathreshold level
Pioneering studies reported that people describe a retronasal olfactory perception as a
taste. More recently, in the 1970's, Murphy et al. (1977) showed that subjects attribute a
sweet taste to ethyl butyrate, a volatile compound smelling of strawberry. This effect
disappears when the retronasal olfaction is blocked by closing the nostrils.
The influence of strawberry odorant on sweetness was subsequently confirmed in a
sucrose solution (Frank et al., 1989) and in whipped cream (Frank and Byram, 1988). But
in the latter study, no impact of peanut butter odorant on sweetness ratings of the
whipped cream is showed. Peach odorant is also highlighted as a sweetness enhancer
(Cliff and Noble, 1990). An increase in concentration of peach odorant leads to an
increase in perceived maximum intensity and total duration of sweetness. Other odorants
related to fruit were also found to enhance sweetness of sucrose solutions, for example
pineapple and raspberry (Prescott, 1999a), maracuja and caramel (Stevenson et al.,
1999). In the same latter study, Stevenson et al (1999) showed that non-food-related
odorants such as damascone or eucalyptol do not impact sweetness. They asked
subjects to score the "smelled sweetness" (which corresponded to the sweetness evoked
by the odour) and the in-mouth sweetness of several solutions flavoured with food and
non-food related odorants. Critically, they observed that "smelled sweetness" panel rating
is a good predictor of the odorant ability to change perceived in-mouth sweetness.
Regarding other tastes, a study by Djordjevic and colleagues (2004b) highlighted that soy
sauce odour can increase perceived saltiness, but strawberry odour does not. Conversely,
the same authors showed that strawberry odour appears to enhance sweetness but soy
odour does not. A study demonstrated that bitterness in olive oil can be enhance by cis-3hexenol, a volatile generally described as having a cut grass note (Caporale et al., 2004).
Finally, odours can also reduce perceived taste intensity. Specifically, a caramel odour
produces a decrease in sour taste intensity (Stevenson et al., 1999).
Many studies have highlighted that an odorant can modulate taste perception, but
relatively few reported that a tastant can modulate aroma perception. A time-intensity
study conducted in orange juices showed that an increase in sweetener concentration or
citric acid concentration induces an enhancement of in-mouth fruitiness intensity
(Bonnans and Noble, 1993). Similarly, an increase in sucrose concentration of a banana
odorant solution enhances fruity aroma intensity (Hort and Hollowood, 2004).
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1.1.1.2 Olfactory stimuli at a subthreshold level
Two studies focused on olfactory-taste interactions, when both odorant and tastant are
presented at a subthreshold level (Dalton et al., 2000; Pfeiffer et al., 2005). They explored
the impact of in mouth saccharine solution (sweet tastant) at a subthreshold
concentration on the benzaldehyde threshold determination (a volatile compound
generally described as having an almond note). Benzaldehyde was delivered
orthonasally using an olfactometer in two conditions: with and without having saccharine
in mouth. The olfactory threshold of benzaldehyde significantly decreases with the
presence of the saccharin solution in mouth. In a second experiment, saccharin was
replaced by monosodium glutamate (umami taste) at a subthreshold concentration. In
that case, the olfactory threshold of benzaldehyde remains unchanged. More recently, a
study showed that subthreshold concentrations of acetic acid can increase the perceived
retronasal olfactory intensity of three volatile coffee aroma compounds presented at
suprathreshold level. Such olfactory intensity enhancement was not observed with
subthreshold concentrations of butyric acid (Miyazawa et al., 2008).
In addition, Delwich and Heffelfinger (2005) showed that a sweetener, monosodium
glutamate (MSG) and a pineapple odorant, which are not perceived individually at
subthreshold level in water, are perceived in mixtures combining either: 1) the sweetener
and the odorant; or 2) MSG and the odorant.
To summarize, many studies show that an odorant can enhance taste perception and
that a tastant can modulate the retronasal olfactory intensity of an odorant. Such
phenomena are observed with stimulus at suprathreshold level. In subthreshold
conditions, sensory interactions were also highlighted
1.1.2 Sensory interactions involving tactile perception
Since in my PhD, I focus on cold trigeminal perception (thermosensation/nociception) and
in mouth thickness (proprioception), other dimensions related to tactile perceptions are
not detailed.
Regarding interactions involving thickness, several studies have been conducted in dairy
products. It was reported that the increase of a low-fat stirred yoghurt thickness using a
thickening agent decreases the intensity of perceived green-apple olfactory perception
and reduces perceived sweetness (Paçi-Kora et al., 2003). In the same matrix, results of
two other studies showed that: 1) the addition of odorant related to fatty attributes
(coconut, butter) enhances the yoghurt's perceived thickness (Saint-Eve et al., 2004); and
2) an increase of yoghurt physical viscosity (using thickening agents) leads to a
sweetness decrease (Paçi-Kora et al., 2003). In a dairy dessert, it was found that,
depending on the nature of the texture agent, the intensity of the fruity aroma changes
(Lethuaut et al., 2005). The intensity of the aroma is greater in soft dairy desserts
composed of lambda-carrageenan than in dessert composed of iota or kappa
carrageenan types. In another study, Lethuaut et al. (2003) showed that desserts with
lambda-carrageenan are perceived as sweeter than desserts with iota-carrageenan a for
similar sucrose content.
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Studies related to sensory interactions involving the trigeminal perceptions were mainly
carried out on pungency/hot perception induced by chemical compounds such as
capsaicin. In a key observation, it was shown that pungency masks olfactory and taste
perceptions (Prescott, 1999b; Reinbach et al., 2007). Very few studies have explored
interactions involving cold trigeminal perception. Recently it was demonstrated that an
increase in cooling intensity using cooling agent enhances the melon olfactory intensity of
a mixture containing a green colouring whereas cooling intensity increase does not
change the pineapple odour intensity of a mixture containing a purple colouring (Petit et
al., 2007).
To conclude about observed sensory interactions, a large number of studies reports
bimodal sensory interactions (mainly between olfactory and taste perception) in model
solutions. Although olfactory, taste and tactile dimensions are involved during food
consumption, few researchers investigated multi-sensory interactions
In this context, the first part of my PhD was organized as detailed below:
- First, a study of interactions between olfactory and taste modalities in existing
products and further exploration of the odorant impact at subthreshold level on
sweetness of sucrose solution;
- Second, widening of this approach to multi-sensory interactions in more complex
products involving olfactory, taste and tactile perceptions. Tactile perception was
apprehended in terms of trigeminal perception (coldness) and proprioception
(thickness).
1.2 Mechanisms underlying sensory interactions
Observed sensory interactions can be explained by physicochemical or perceptual
mechanisms. Physicochemical mechanisms are briefly reviewed (PART 1.2.1) and then
perceptual mechanisms reported in the literature are reviewed in more detail (1.2.2).
1.2.1 Sensory interactions induced by physicochemical interactions
Chemical or physical interactions between 1) sensory stimuli (e.g. odorants, tastants),
and/or 2) sensory stimuli and the food matrix ingredients (e.g. lipids, carbohydrates) lead
to a modification of stimuli released from food and reaching receptors during consumption.
Consequently, the perception related to the stimulus can be modified. Firstly the different
physicochemical mechanisms occurring in food are presented (PART 1.2.1.1 and
secondly the potential consequences on perception are discussed (PART 1.2.1.2).
1.2.1.1 Physicochemical mechanisms and their impact on release of sensory stimuli
The scientific community generally agrees that chemical bonds are mainly involved in
physicochemical interactions between volatiles and components of the food matrix.
Chemical bonds are classified into two categories: 1) high-energy bonds (covalent and
ionic); and 2) weak-energy bonds (hydrogen bonds and Van Der Waals forces). Chemical
bonds between aroma compounds and food ingredients reduce the volatile concentration
in headspace because of a decrease in the air/food partition coefficient (ratio of the
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concentration of the aroma compound in the air phase to its concentration in the food).
Volatile release is commonly measured in vitro by static-headspace analysis coupled with
a gas chromatograph and mass spectrometer (GC-MS) for quantifying and identifying the
volatiles. For example pectin addition in apple juice decreases isopropylalcohol release
(Walker and Prescott, 2000). Isoamyl acetate release from dairy dessert is reduced by
starch addition (Cayot et al., 1998), one of the factors influencing volatile release being
interactions with amylose. In addition a complementary study showed that release of
isoamyl acetate, ethyl hexanoate, and linalool depend not only on amylose content but on
amylose-amylopectin content ratio (Arvisenet et al., 2002). The same type of chemical
reactions can also account for a reduction in volatile compound release when a texturing
agent such as xanthan is added to a model system (Bylaite et al., 2005).
Physicochemical interactions between odorants and tastants have also been reported.
For example, a study conducted in orange juice showed that increasing sucrose from 0 to
60 W/V% modulates release of 15 orange juice volatiles according to their hydrophobicity
constant. Indeed release of volatiles with lower hydrophobicity constant (e.g. ethyl
acetate) is increased whereas release of volatiles with higher hydrophobicity constant
(e.g. octanal) is decreased (Nahon et al., 1998). Similarly, in sweetened strawberryflavoured yogurt, release of volatiles (ethyl butanoate, ethyl 3-methylbutanoate and (Z)hex-3-enol) is significantly and differently impacted by the nature and the concentration of
the sweet tastants (Mei et al., 2004).
Modification of food rheological properties is another key factor affecting odorant and
tastant release. A significant number of studies conducted in hydrocolloids gels or in
aqueous solutions investigated the impact of rheological properties, mainly viscosity, on
stimuli release by adding thickeners. According to Baines and Morris (1987) the
modification of odorant and tastant release occurs at concentrations above the coil
overlap concentration (C*) for a range of hydrocolloids. The main hypothesis is that an
increase in viscosity reduces the odorant and tastant transfer from the matrix to the
receptor, which is caused by a decrease in compound mobility. Consequently, the
increase of viscosity in hydrocolloid gel and solutions is generally reported as decreasing
odorant and tastant release (Baines and Morris, 1987; Hollowood et al., 2002). However,
more recent studies have shown that aroma release is not systematically related to C*
concentration (Hollowood et al., 2002; Cook et al., 2003; Bylaite et al., 2005).
The reduction of the water mobility due to the viscosity enhancement may also explain
the decrease in perceptual intensity. This has been shown for a sweet tastant (Mathlouthi,
1984; Mathlouthi and Seuvre, 2008). The authors assumed that water mobility facilitates
tastant transportation and detection.
Finally in-mouth food manipulation can also impact the release of volatile compounds.
Blisset et al. (2006) showed that during mastication of a lemon-flavoured confectionery
chew, differences between subjects in terms of chewing behaviour parameters such as
chewing rate and force influenced volatile release and consequently aroma perception.
1.2.1.2 Impact of physicochemical interactions in food on perception
Some in vitro instrumental analyses conducted in parallel with sensory measurement
showed that physicochemical interactions could be linked to differences in perception
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(Walker and Prescott, 2000; Guinard and Marty, 1995; Relkin et al., 2004). But other
studies also showed differences in perception not related to differences in
physicochemical measurements. For example, in model dairy desserts, changes in
physical textural characteristics and in sucrose concentration modulated olfactory
retronasal perception but not volatile release measured in vitro by static headspace gas
chromatography (Lethuaut et al., 2005). Therefore, the authors concluded that such
perceptual change is not caused by physicochemical interactions.
More recently in vivo instrumental measurement was used to better understand the
kinetics of stimuli release (mainly odorants) in in vivo conditions. Odorant concentration
was monitored in exhaled breath by on-line instrumental measurement such as
atmospheric pressure chemical ionisation mass spectrometry (APCI-MS), (Hewson et al.,
2008) or proton transfer reaction-mass spectrometry (PTR-MS), (Boland et al., 2006;
Lindinger et al., 2008). But the limitation of these studies is that physicochemical
measurements were conducted independently from sensory evaluation.
Sensory evaluation coupled with on-line in vivo instrumental measurement such as APCIMS (Davidson et al., 1999; Weel et al., 2002; Lethuaut et al., 2004; Pfeiffer et al., 2006;
Saint-Eve et al., 2006); or PTR-MS (Buettner et al., 2008) were carried out to more
efficiently identify the respective contribution of physicochemical and sensory interactions
to perception. Studies highlighted that: 1) physicochemical interactions have no impact on
perception (Weel et al., 2002); 2) physicochemical interactions explain some of the
observed changes in perception (Saint-Eve et al., 2004; Saint-Eve et al., 2006); and 3)
some of the reported perceptual modifications are not explained by physicochemical
interactions and are considered as perceptual (Davidson et al., 1999; Lethuaut et al.,
2004; Saint-Eve et al., 2004; Saint-Eve et al., 2006).
To summarize, some of the sensory interactions reported in the literature can be
explained by physicochemical interactions but this is not systematic. It may be because
physicochemical interactions cannot be identified for technical reasons (e.g. instrumental
measurement is not sufficiently sensitive) or because the origin of sensory interaction is
perceptual.
1.2.2 Role of perceptual mechanisms on sensory interactions
Cognitive factors are reported in the literature as being involved during the construction of
perceptual interactions: food experience, attentional strategy during exposure, spatial and
temporal co-occurrence during olfactory and taste perception and liking. The role of such
factors in perceptual interaction is detailed below
1.2.2.1 Role of food experience
- Stimuli at suprathreshold level
The scientific community agrees that food experience plays an essential role in the
construction of perceptual interactions (Prescott and Stevenson, 1995). The notion of
congruency between sensory qualities is essential to our understanding of the perceptual
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interaction between senses. Congruency is the extent to which two stimuli are
appropriate for combination in a food product (Schifferstein and Verlegh, 1996). Indeed
many studies reported that olfaction can lead to an enhancement of taste intensity only
when olfactory and taste stimuli are congruent, (Frank and Byram, 1988; Prescott, 1999a;
Hort and Hollowood, 2004; Djordjevic et al., 2004b).
Conversely, an odour can also decrease taste intensity when related to another taste, i.e.
the sourness suppression by caramel odorant (Stevenson et al., 1999) can probably be
explained by sweetness evoked by caramel odorant since caramel aroma and sweetness
are commonly associated in food. In addition, the symmetrically suppressive interactions
observed between tastants, for instance sweet tastant and acid tastants (Keast and
Breslin, 2003), could occur between a tastant and a taste induced by an odorant. Even
though congruency between two stimuli is a mandatory condition to observe perceptual
interactions, it has been shown that the level of congruency between sweetness and an
odorant (ham, lemon and strawberry) can not predict the degree of sweetness
enhancement by the odorant (Schifferstein and Verlegh, 1996).
Perceptual interactions between olfaction and taste can result from an association that is
formed through food exposure, without any explicit attention or learning (Koster et al.,
2004; Koster, 2005a). This was demonstrated through repeated exposure to an unfamiliar
odorant in solution (Petit et al., 2007) with a tastant. This induced olfactory-taste
association has been described as ‘learned synesthesia’ (Stevenson and Boakes, 1998).
This conclusion is supported by another study (Prescott et al., 2004). The authors
showed that a single co-exposure of a sweet tastant and an unfamiliar odorant (prune) is
sufficient to enhance the "smelled sweetness" of the odorant (prune). This result has
been confirmed by a study showing that pairing a new odorant with sucrose leads to an
enhancement of sweetness evoked by odorant sniffing (Yeomans et al., 2006). In
addition, saltiness rating decreases after odorant-sucrose pairing procedure.
Regarding a more complex system involving olfactory, trigeminal, and visual stimuli, a
repeated exposure to an incongruent mixture combining pineapple odorant, cooling agent,
and purple colouring, promotes perceptual interactions between olfaction and trigeminal
perceptions whereas before the co-exposure such interactions do not exist (Petit et al.,
2007).
To summarize, food experience modulates perceptual interactions according to studies
performed with sensory stimuli in model solutions. But the role of experience built through
repeated exposure to every day commercial food on perceptual interactions remains
unclear. Indeed, to our knowledge, the impact of the familiarity induced by a commercial
product, which is built over time by repeated purchase and consumption, on perceptual
interactions has never been investigated. This is critical since findings from experiments
carried out with model solutions could differ to those obtained in familiar products.
In this context the role of familiarity of drinks on olfactory and taste
interactions was explored during my PhD in common and less common bitter
drinks (coffee drink, cocoa drink and caffeinated milk).
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- Stimuli at subthreshold level
Consistently with findings obtained when stimuli are at suprathreshold level, perceptual
interactions and the role of congruency have been evidenced with subthreshold stimuli.
Indeed a subthreshold concentration of olfactory stimuli can reduce the detection
threshold of a congruent taste stimuli (Dalton et al., 2000; Pfeiffer et al., 2005). This
means that a mixture of subthreshold olfactory and taste congruent stimuli that are not
perceived individually can lead to a perceptible stimulus. One study produced
contradictory findings (Delwiche and Heffelfinger, 2005). The authors explored the impact
on perception of subthreshold concentrations of pineapple odorant in mixture either with
subthreshold concentrations of sweet tastant (congruent with pineapple aroma) or with
subthreshold concentrations of umami tastant (incongruent with pineapple aroma). They
showed that during discrimination tasks both mixtures (pineapple odorant-sweet tastant
and pineapple odorant-umami tastant) are significantly perceived as different from water
whereas individually, each of the three compounds are not significantly perceived as
different from water. The authors argued that an additivity effect of subthreshold odorant
and tastant concentrations reaching the threshold when mixed together and consequently
congruency does not seem to play a role here.
To conclude, congruency between odorant and tastant stimuli seems to play a role, even
if not systematic, in perceptual interactions produced by subthreshold concentrations of
either one or both stimuli. Sensory measurements used in studies dealing with
subthreshold odorant and sweet tastant stimuli are discriminative tests: threshold
detection (Dalton et al., 2000; Pfeiffer et al., 2005) and triangular tests (Delwiche and
Heffelfinger, 2005). These latter tests do not allow to describe the nature of the
perception induced by the combination of olfactory and taste subthreshold stimuli. Only
one study showed by descriptive tests that subthreshold carboxylic acids can increase
olfactory intensity of suprathreshold concentrations of coffee volatiles (Miyazawa et al.,
2008).
Today it is not possible to confirm that perceptual interaction with stimuli at subthreshold
concentrations can lead to an enhancement of sweet taste perception as observed in the
literature for perceptual interactions with stimuli at suprathreshold concentrations. Such a
confirmation would allow use of odorants related to sweetness at subthreshold level to
reduce sucrose content, while keeping the same sweetness without modifying the
product's olfactory characteristics.
In this context, the impact of subthreshold concentrations of an odorant
congruent with sweet taste on sweetness rating was explored in a sucrose
solution. Firstly five odorants were evaluated at suprathreshold level in a sucrose
solution. The odorant having the highest and the lowest sweetness enhancement
properties were selected. Secondly we explored at subthreshold level the impact of
both odorants on sweetness.
1.2.2.2 Role of attentional strategy during exposure
The observed modulation of the intensity of a sensory quality by another independent one
can be caused by experimental conditions which influence assessor attentional strategy
during attribute rating. This has been suggested and named the "dumping effect" by Clark
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& Lawless (1994) when applied to olfactory and taste perceptions. The authors showed
that the dumping effect can lead to a transfer of the perceived olfactory intensity onto the
taste scale when no olfactory intensity scale is available. This theory is supported by
findings showing that taste ratings can be modulated by both odour perception and by
attributes proposed by the experimenter (Frank et al., 1993; Frank, 2002). For example,
the sweetness of a sucrose solution is increased by a strawberry odorant. However, this
effect disappears when a fruity aroma scale is made available in addition to a sweetness
scale (van der Klaauw and Frank, 1996). But other paradigms different from attribute
ratings highlighted perceptual interactions between sweetness and congruent odorants
(Nguyen et al., 2000; Djordjevic et al., 2004a; White and Prescott, 2007). Consequently,
increase of sweetness rating by odorant is not solely due to a response bias but involves
perceptual interactions (Valentin et al., 2006).
The attentional strategy applied during exposure to a new food can impact the
construction of perceptual interactions. That was demonstrated by Prescott et al. (2004)
who compared the impact of two different attentional strategies on construction of
olfactory-taste interaction during co-exposure to a sucrose solution flavoured with an
unfamiliar prune odorant. Two groups of assessors were co-exposed by triangle tests but
the instructions varied between groups. The first group was asked to pick the sample with
the strongest overall flavour. The second group was asked to pick the sweetest sample or
the most intense in aroma (retronasal olfactory perception). The instructions received by
the first group encouraged a synthetical attentional strategy, i.e. the subjects evaluated
olfactory and taste stimuli as a whole, thereby promoting olfactory-taste interactions. In
contrast, the instructions received by the second group led subjects to adopt an analytical
attentional strategy, i.e. subjects considered olfactory and taste stimuli independently,
which limited olfactory-taste interactions. As hypothesized, results of a post-exposure
sweetness evaluation showed that the first group rated the flavoured sucrose solution
sweeter than the unflavoured sucrose solution but not the second group.
To summarize, attentional strategy applied during exposure seems to play a key role in
perception. However, the importance of attentional strategy during exposure, i.e.
analytical vs. synthetical, on the construction of perceptual interactions has been only
addressed in one study and it is worth validating these findings and further exploring this
factor.
Sensory information provided by a trained panel and by consumers may differ.
Indeed trained panels follow an analytical attentional process and consumers a
synthetical attentional process that may favour perceptual interactions.
This hypothesis was checked by exploring the impact of attentional strategy
comparing coffee aroma perception obtained: 1) by trained assessors using
sensory profiling; and 2) by consumers using a holistic approach: a sorting task
with verbalisation.
In addition during the set up of all different thesis protocols, the dumping effect
was limited by using as many scales as sensory characteristics describing the
products.
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1.2.2.3 Role of olfactory stimulation pathway on perceptual interactions
Recently, an orthonasal and retronasal delivery odorant device has been developed for
specifically investigating the role of the olfactory stimulation pathway on perceptual
interactions (Heilmann and Hummel, 2004). The device consists in a olfactometer
coupled to a two-tube system inserted into the nasal cavity, one being positioned into the
epipharynx for stimulating the olfactory receptors orthonasally and the second one being
placed in the antrum of the nasal cavity for simulating the olfactory receptors retronasally.
This system allowed researchers to compare the direct impact of the spatial localisation
of odorant on perception preventing any potential interactions for instance between
odorant and saliva. Using this system, a recent study showed that an odorant can
increase the intensities of thickness and creaminess, but only when the odour is
presented retronasally (Bult et al., 2007). According to the authors, spatial co-occurrence
can facilitate perceptual interactions since these conditions naturally occur during
everyday food consumption. However, another study using the same device
demonstrated that differences in viscosities within a range of semisolid food products can
modulate both perceived olfactory intensity of retronasal and orthonasal stimulations
(Negoias et al., 2008).
1.2.2.4 Role of temporal co-occurrence of stimulus delivery on perceptual interactions
A simultaneous presentation of odorant and tastant, i.e. temporal synchrony was shown
to impact perceptual interactions between olfaction and taste (Pfeiffer et al., 2005).
Indeed sweetness of saccharine is increased when presented simultaneously in
combination with benzaldehyde odorant. The effect is no longer observed when
presented with temporal asynchrony, i.e., subjects taste and then spit out the solution of
saccharine before sniffing the benzaldehyde sample. The same conclusions are valid for
olfactory and texture interactions. A creamy odorant enhances the perceived creaminess
of milk based food, but only when odorant and texture stimuli are delivered
simultaneously (Bult et al., 2007).
To summarize, respecting ecological conditions such as temporal and spatial cooccurrence of stimulus delivery during experiments can more efficiently promote
perceptual interactions.
Such conditions (temporal and spatial co-occurrence of stimulus delivery)
were therefore applied in the different experiments conducted during my PhD.
1.2.2.5 Impact of liking on perceptual interactions
In addition to taste acquisition, an odorant can acquire the hedonic valence, positive or
negative of the paired tastant during a co-exposure (or learning phase). For instance
pairing a tea drink with quinine during the exposure phase increases bitterness but also
decreases the liking for the tea drink when tasted alone (Yeomans et al., 2007). It is well
known that bitterness is generally perceived as unpleasant, being genetically coded as a
signaling system against potentially poisonous materials (Scott and Verhagen, 2000).
Similarly, it has been previously shown that odorant pairing with sweetness during a
conditioning period enhances the odorant liking when tasted alone (Zellner et al., 1983;
Brunstrom and Fletcher, 2008; Barkat et al., 2008). In addition Barkat et al. (2008)
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showed that the number of odorant and tastant co-exposures repetitions was more critical
than the total co-exposure duration for enhancing odour pleasantness. Conversely to
bitter taste, humans may be genetically pre-determined to like sweetness since sweet
food are generally safe sources of energy and nutrients (Beidler, 1982). Learning,
perhaps starting from in utero development, might also explain this phenomenon since
sweet foods are generally more often experienced than bitter food during every day life
(Reed et al., 2006). Similarly, congruency between sensory stimuli is the consequence of
life-time learning during food consumption. This may explain why congruency rating of
sweetness in combination with several odorants was shown as positively correlated to
pleasantness rating (Schifferstein and Verlegh, 1996) whereas experience of
new/unfamiliar foods generally causes a neophobic reaction (Birch, 1999).
1.2.3 Neural correlate of perceptual interactions
The mammalian nervous system has the ability to integrate signals generated by
physiological structures that are anatomically separated (Gibson, 1966; Marks, 1991)
such as olfactory receptors and taste buds. According to the latter authors, the aim of
such multi-sensory integration may be to enhance our efficiency for detecting and/or
identifying stimuli. Resulting from the evolution process, neurons respond firstly
independently to each of the five senses, then evolved to multi-sensory neurons (found in
the superior colliculi of cats and primates) that integrate cues from three sensory
modalities, vision, audition and somatosensation (Stein et al., 1993; Stein et al., 1993;
Wallace et al., 1996). By recording the electrophysiological activity of neurons, it has
been shown that taste and smell information converge onto a single neuron in the primate
caudal orbitofrontal cortex (Rolls and Baylis, 1994). On this basis such bimodal neurons
might exist in humans and might be at the origin of flavour processing and of the
representation of flavour (Rolls and Baylis, 1994; Critchley and Rolls, 1996).
1.2.3.1 Neural correlate of perceptual olfactory and taste interaction and impact of
congruency
Neural correlate of olfactory and taste interactions in humans can be obtained by
functional Magnetic Resonance Imagery (fMRI), a technique for mapping the functional
activities of the brain. The principle consists in measuring the oxygenation
(oxyhemoglobine/desoxyhemoglobine) which increases locally in areas activated due to
increased fresh oxygenated blood supply. Activity measurement is compared to the
baseline state.
fMRI imaging showed the convergence of taste and olfactory stimuli in the lateral anterior
part of the orbitofrontal human cortex (de Araujo et al., 2003). The authors showed that
activation induced by tasting a sucrose solution flavoured with strawberry is higher than
the sum of activation induced independently by each olfactory and taste stimuli in several
area of the orbitofrontal cortex. Super-additivity was similarly highlighted with vanilla and
sucrose (Small et al., 2004). But, the latter findings showed that super-additivity does not
occur with an incongruent salty solution flavoured with the same vanilla odorant. Small et
al. (2004) also demonstrated that the key brain areas underlying perceptual olfactory
taste interaction and the role of food experience are: insula, orbitofrontal cortex and
anterior cingulate cortex.
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The transfer of hedonic valence to odorant resulting from food experience can be
explored at a neural level since neurons from the orbitofrontal cortex, which have the
property to integrate different sensory stimuli, can also respond to pleasantness (de
Araujo et al., 2003).
1.2.3.2 Neural correlate of the impact of orthonasal vs. retronasal odorant presentation on
perception
I previously reported that stimulation of the olfactory pathway can modify the perception
of both odorants and the perceptual interactions involving taste and texture perception.
By coupling the odorant delivery system developed by (Heilmann and Hummel, 2004)
with the fMRI technique, Small et al. (2005) compared brain activations induced by food
and non-food related odorants (chocolate, lavender, butanol and farnesol) when delivered
orthonasally or retronasally. Chocolate aroma induces the highest difference in brain
activation between both stimulation pathways. But scoring of olfactory intensity and
pleasantness induced by chocolate does not differ between both olfactory stimulation
pathways. Comparing activated brain areas in both olfactory conditions: the neural area
involved in reward encoding responds more intensively when chocolate aroma is
perceived retronasally than orthonasally. The authors suggested that this difference may
be explained by the fact that chocolate, being a food-related odorant, is perceptually
more associated with the mouth than the nose.
1.2.3.3 Neural correlate of perceptual interactions involving tactile perception
Electrophysiological neuron recordings in macaque orbitofrontal cortex using water
stimuli at different temperatures and containing different concentrations of carboxymethylcellulose highlighted that the same neuron can either represent taste or temperature or
viscosity changes but that other neurons can respond to both temperature and viscosity
changes (Kadohisa et al., 2004). Neurons from the orbitofrontal cortex can also react to
trigeminal stimulation induced by capsaicin. More recently, neural electrophysiological
recordings in macaques showed that some single neurons from insula, orbitofrontal
cortex and amygdala respond to taste, temperature, viscosity and fat perception
(Kadohisa et al., 2005a; Kadohisa et al., 2005b).
Using fMRI, it has been demonstrated that the same human neuron assembly can
respond to both gustatory and tactile stimuli, e.g. astringency induced by aluminium
potassium sulfate (Cerf-Ducastel et al., 2001). A brain activation map was built by the
authors showing a wide overlap of taste and tactile representations especially in
subinsular and opercular regions according to fMRI results. In another study, combination
of neurophysiological recordings in rhesus macaques and fMRI study in human subjects
showed that human primary taste cortex and frontal operculum provide combined
representations of the taste, temperature, viscosity and texture (Rolls, 2007).
To summarize, perceptual sensory interactions and the role of congruency evidenced at a
psychophysical level have also been demonstrated at the brain level: 1) by fMRI in
human highlighting an increase of activation in specific brain areas higher during
simultaneous perception of congruent olfactory and taste stimulus compared with
independent presentations of identical stimuli; and 2) by electrophysiological neuron
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recording in animal showing that different sensory stimuli can be integrated by single
neurons. But similarly to studies conducted at psychophysical level, the role of
congruency in perceptual interactions was widely investigated in model solutions whereas
the impact of familiarity induced by realistic everyday food on neural representation of
perception has not yet been explored.
In this context a study was designed in order to explore at a neural level
through fMRI the potential impact of product familiarity on olfactory and taste
interactions. This work represented the neural correlate of the study conducted at
a psychophysical level in PART 2.1.1 with coffee, cocoa and caffeinated milk
drinks.
1.3 Complex perception
We define a complex perception as a perception that is not easily described with simple
attributes such as sweet or thick but that result from an integration of unitary percepts.
Consequently product development delivering such perceptions remains challenging.
Some studies have focused on complex perceptions such as creaminess (Richardson
Harman et al., 2000; Tournier et al., 2007) and freshness ((Peneau et al., 2006; Peneau
et al., 2007). Findings of these studies showed that olfaction, taste and tactile sensory
dimensions are involved in creaminess and freshness. Moreover, food habits,
demographic characteristics and liking contribute to the construction of these complex
perceptions as detailed below:
1.3.1 Creaminess
Creamy (or unctuous in French) is a term commonly used by consumers for describing
semi-solid products but is only very briefly defined in AFNOR as moderate level of
viscosity, example double cream (ISO 5492, 1995). Merriam–Webster’s Dictionary (1999)
proposes the following definition: which has the consistency of cream. Richardson
Harmann et al. (2000) and Tournier et al. (2007) enriched the understanding of
creaminess by combining sensory profiling data from a trained panel and creaminess
scoring from a group of consumers following preference mapping methodology on a
range of dairy products. The first study highlighted that differences in creaminess scores
are mainly explained by differences in consumer demographics and preferences but not
by differences in product sensory attributes. The second study showed that creaminess
perception results from a combination of several sensory characteristics such as
homogenous intensity of the texture, fattiness or sweetness. However, sensory drivers
positively associated with creaminess vary among consumers probably due to individual
consumer's different food experience (Koster, 2005b; Koster et al., 2004). Finally, the
authors also noted a positive correlation between creaminess and liking.
1.3.2 Freshness
Freshness is a term not fully understood and briefly defined by AFNOR for fruits and
vegetables as a turgescent product with no signs of withering or ageing, the cells of which
have not deteriorated (ISO 7563, 1998).
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An investigation conducted by Péneau et al. (2006) aimed at better understanding the
term freshness from a consumer point of view. Six apples stored under different
conditions were tested. Experimenters asked consumers during a local exhibition to score
the importance of different sensory items and of other characteristics such as nutritional
value, organic, cultivar. Freshness and liking were also scored. A key finding is that the
perception of freshness is strongly related to taste, crispness, juiciness and liking. The
concept of freshness seems to be interpreted differently according to demographic
characteristics (e.g. gender) and consumption habits of consumers. A second study
having the objective to characterize the apples from a sensory point of view highlighted
that texture properties are key drivers of freshness (Peneau et al., 2007).
Within the framework of my PhD, i.e. starting from simple bimodal perception
to complex multi-sensory perceptions, the last objective is to explore a complex
perception. During our work dedicated to the investigation of perceptual
interactions between olfactory, taste and tactile perceptions, a range of viscous
liquid products were formulated. Tasting conducted for validating sensory
differences among products revealed that refreshing attribute was elicited by a
significant number of assessors. According to the limited literature, refreshing
perception seemed to be consistent with our concept of complex perception and
was therefore explored in PART 3.
1.3.3 Refreshing
Refreshing perception has been poorly investigated but nevertheless is widely used by
consumers and food marketing. The Merriam-Webster Dictionary & Thesaurus (2006)
defines refreshing as Serving to restore strength and animation, to revive, to arouse, to
stimulate, to run water over or restore water to, with thirst quenching properties,
suggesting that refreshing is linked to physiological factors such as thirst-quenching and
arousal. Some sensory and consumer studies have explored refreshing with regards to: 1)
perceived food characteristics (Zellner and Durlach, 2003); and 2) expected food
characteristics (Clydesdale et al., 1992; Zellner and Durlach, 2002; Zellner and Durlach,
2003). From these data and in line with the dictionary definitions, refreshing seems to be
associated with specific sensory characteristics (e.g. cold, liquid) related to water drinking
and in relation with physiological states such as thirst
The sensory determinant of refreshing and the role of physiological parameters
in refreshing perception were explored in my work. First, a systematic study was
conducted combining a conventional sensory profiling with trained assessors and
refreshing intensity scoring by consumers to quantify the respective contribution
of sensory attributes to refreshing perception. Sensory dimensions involved in
refreshing perception over time were also investigated using an innovative
temporal methodology, Temporal Dominance of Sensations (TDS). Finally, we
related refreshing perception and physiological parameters: mental energy and
saliva properties.
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Scientific approach
This literature review enabled us to highlight different gaps and questions related to
perceptual interactions which were then investigated. We organized our work into two
main research parts (Parts 2 and 3) as illustrated in Fig.3
Fig.3: Overall representation of the PhD scientific approach
Based on the literature review two research axes were defined with the overall objective to
explore mechanisms underlying perceptual interactions in realistic products. Firstly perceptual
interactions were investigated starting from bimodal olfactory and taste interactions and the
approach was widened to tactile perception. Secondly, perceptual and psychophysiological
factors impacting refreshing perception were explored.
The two research parts are introduced below with the different experiments in terms of
objectives and main findings. The details of material & methods, resultats and discussion
of each experiment are then presented in each corresponding article.
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PART 2: Perceptual multi-sensory interactions
The role of congruency on perceptual interaction has been widely investigated in solution
between sensory stimuli and mainly odorant and tastant. Congruency between two stimuli
results from repeated consumption of food containing these stimuli in combination.
Congruency is therefore strongly related to the level of familiarity for these paired stimuli.
To our knowledge, the impact of familiarity at an upper level, i.e. induced by the overall
product, has never been investigated. Similarly, the exploration of perceptual interactions
on product description was widely studied with experienced assessors using an analytical
approach, generally a conventional sensory profiling where subjects are trained to
evaluate independently each sensory attribute. Such a strategy can reduce the impact of
perceptual interactions on perception. Results from a trained panel were never compared
with a more synthetical procedure closer to how consumers apprehend foods during
everyday consumption, i.e. considering perception as a whole without dissociating each
sensory dimension. For improving basic knowledge about perceptual interaction
mechanisms but also to explore perceptual interactions in a more realistic environment in
terms of product and evaluation procedure, we investigated in PART 2.1.1: 1) the impact
of existing products differing in familiarity on olfactory and taste interactions at
psychophysics level and neural level by functional Magnetic Resonance Imagery (fMRI);
and 2) the effect of two evaluation methodologies encouraging attentional strategy during
tasting (sensory profiling using trained assessors) or synthetical strategy (sorting task
with consumers) on description of coffee aroma.
The role of subthreshold stimuli was poorly investigated and no study has been designed
to answer the following questions: 1) can subthreshold level of odorants increase taste;
and 2) after co-exposure between a new odorant and a tastant, can this odorant at
subthreshold level enhance perceived taste (as shown in the literature when presented at
suprathreshold level)? These remaining questions are crucial regarding the overall
understanding of perceptual interaction origin and mechanisms. In addition, the outcome
of such a study could be applied to reduce sucrose content in food by adding
subthreshold odorant concentration to keep similar sweetness and without perceived
olfactory changes. Indeed health benefits are today key during consumer food choice, but
without compromising pleasure. In the PART 2.1.2 the impact of subthreshold odorant
stimuli on sweetness was explored together with the impact of co-exposure on the
construction of perceptual interactions between sweet taste and an unfamiliar odorant.
Investigations about perceptual interactions have mainly been carried out between
olfactory and taste interactions. However few are known where olfactory, taste and tactile
perceptions (including trigeminal and texture perception) are involved although this
occurs during consumption of many foods. Cold trigeminal stimulation generated by
cooling agents is also more and more used for product formulation, mainly in
confectionary but also in hot beverages in order to create new sensory experiences for
consumers. But few studies have explored perceptual interactions involving coldness. We
therefore extended our approach to a more complex system combining olfactory, taste,
cold trigeminal and proprioceptive (thickness) perceptions (PART 2.2). To reach this
objective, ingredients related to olfactory (mint and peach odorant), taste (citric acid) and
trigeminal (cooling agent) perceptions were added to a sweet viscous liquid (also termed
"gel" in this work) basis according to an experimental design. Two formulation designs
were built by adding for the first set of products a peach odorant at two concentrations,
for the second set a mint odorant at two concentrations and for both sets a cooling agent
and citric acid at two different concentrations. A total of eight mint-flavoured samples and
eight peach-flavoured samples were therefore developed. After validating the range of
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samples in terms of sensory diversity (PART 2.2.1), we explored perceptual interactions
according to a sensory profiling method (PART 2.2.2).
PART 2.1 Olfactory and taste perceptual interactions
PART 2.1.1 Role of product familiarity and exposure strategy with stimuli at
suprathreshold level
The objective of this part was to better understand from both psychophysical and neural
point of view how familiarity for the product modulates perceptual interactions between
taste and olfaction. To achieve this objective I explored the impact of different odorants
on a familiar black coffee drink, a cocoa-based beverage reconstituted with water
(somewhat less familiar than the coffee drink) and an unfamiliar caffeinated milk
beverage. The three products were profiled with and without odorant addition. We also
planned to investigate the role of familiarity on perception at a neural level by functional
Magnetic Resonance Imagery (fMRI) with the objective to highlight brain areas involved
in taste modulation when product familiarity changed.
Key results are presented below.
Product familiarity modulates perceptual interactions between olfaction and taste. Vanilla
odorant increases sweetness of the familiar cocoa drink. But, in the unfamiliar caffeinated
milk, vanilla odorant did not impact sweetness but enhanced bitterness.
The fMRI study is planned for October 08, consequently the findings are not here.
Details can be found in the article pp. 32-42:
Labbe,D., Damevin,L., Vaccher,C., Morgenegg,C., & Martin,N. (2006) Modulation of
perceived taste by olfaction in familiar and unfamiliar beverages. Food Quality and
Preference, 17, 582-589.
The second objective was to explore the impact of two different attentional strategies on
perception: analytical and synthetical attentional strategies. To reach this objective,
coffee aroma description done by sensory profiling with a trained panel (analytical
strategy) was compared to coffee aroma description obtained by an holistic approach
according to a sorting task with verbal description (synthetical strategy) carried out by a
group of coffee consumers.
Key results are presented below:
Attentional strategy during exposure strongly affects perception of coffee aroma. Indeed
consumers group the coffees consensually but differently from the trained panel. The
consumer holistic approach considering the product as a whole may promote the impact
of previous food experience on perception, such as odour and taste association
constructed during every day coffee exposure. On the contrary, the analytical approach
followed by trained panelists probably reduces the impact of food experience on
perception and consequently the role of interaction between sensory modalities.
Details can be found in the article pp. 43-51:
Labbe,D., Rytz,A. & Martin,N. (2006). Coffee aroma is perceived differently by
consumers and by trained panelists. Proceedings of the 21st International Conference on
Coffee Science, Montpellier, France
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PART 2.1.2 Impact of subthreshold olfactory stimuli on taste perception
The impact of common odorants congruent with sweet taste on sweetness of a sucrose
solution was investigated with odorant at subthreshold level. In addition, I explored
through concomitant olfactory and taste exposure, whether an unfamiliar odorant can be
perceptually associated with sweetness when presented at a subthreshold concentration.
Moreover, the impact of the attentional strategy during exposure (analytical vs.
synthetical) on building of olfactory and taste association was investigated to complement
the part 2.1. Naive assessors were conditioned to sucrose and unfamiliar odorant paired
stimuli either following: 1) an analytical strategy by sensory profiling training where
subjects learnt to consider independently each sensory dimension; or 2) following a
synthetical strategy by triangle tests where subjects were encouraged to consider the
different sensory dimensions as a whole perception. Indeed since different sensory
methodologies can involved different attentional strategies, understanding the impact of
this factor on perception is key to be able to better select sensory tools according to the
problematic.
Key results are presented below:
The odorant having the highest boosting effect on sweetness when tasted at
suprathreshold level showed the highest boosting effect on sweetness when presented at
subthreshold level. Regarding the odorant having the lowest boosting effect on
sweetness at suprathreshold level, no odorant impact on sweetness is evidenced at
subthreshold concentration. Similarly to perceptual interactions with stimuli at
suprathreshold level, congruency may play a role on perceptual interaction when a
stimulus is at a subthreshold level.
Details can be found in the article pp. 52-62:
Labbe,D., Rytz,A., Morgenegg,C., Ali,S., & Martin,N. (2006) Subthreshold Olfactory
Stimulation Can Enhance Sweetness. Chemical Senses, 32, 205-214.
Unfamiliar odorants co-exposed with sweet tastant acquire the property to enhance
sweetness when presented at suprathreshold level, but only when co-exposure is done
according to a synthetical approach. As hypothesized, considering the different sensory
stimuli as a whole during exposure facilitates the construction of perceptual interactions
compared to an analytical approach where sensory stimuli are considered individually.
But when presented at subthreshold level, the odorant does not impact sweetness
whatever the applied attentional strategy during co-exposure. Co-exposure through
laboratory conditions could not reproduce perceptual associations as powerful as those
constructed during every day life experience.
Details can be found in the article pp.63 -77:
Labbe, D., & Martin, N. Impact of novel odorants at supra and subthreshold level on
sucrose perception further to an experimental implicit associative learning (Submitted to
Chemical Senses)
PART 2.2: Perceptual interactions between olfactory, taste and tactile
perceptions
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PART 2.2.1 Validation of the sensory diversity induced by formulated products
The objective of this work was to ensure that the ingredients added to the viscous fluid
products according to a factorial design deliver a wide sensory diversity mainly in terms of
olfaction, taste, and trigeminal perception. Each of the two set of products, i.e. the eight
mint flavoured products and the eight peach products were characterized by forty naive
internal assessors following a sorting task with free description. This methodology was
selected since our objective was to validate that products were different enough to be
discriminated not only by a trained panel but also by naive people.
Key results are presented below. Details can be found pp. 78-82.
Within each set, assessors consensually discriminate and describe samples validating
that the sensory difference induced by the product range is wide enough to conduct
PART
2.2.2. In addition, as refreshing attribute was elicited by a significant number of assessors,
we chose to explore further this complex perception in PART 3.
PART 2.2.2 Impact of olfaction on taste, trigeminal and texture perceptions
The aim of this work was to systematically explore by sensory profiling perceptual
interactions involving olfactory, taste and texture perception in the same two ranges of
viscous fluid products developed in PART 2.2.1.
Key results are presented below.
Olfaction strongly influences taste and cold trigeminal perception, but differently
according to the odorant quality. For instance, sweetness is increased by both mint and
peach odorant, however mint odorant enhances coldness but not peach odorant.
Olfaction also modulates perceptual taste-taste and taste-trigeminal interactions. These
findings highlight the large range of expected but also unexpected olfactory-trigeminaltaste perceptual interactions in complex food systems (for example bitterness
enhancement by coldness).
Details can be found in the article pp. 83-93:
Labbe,D., Gilbert,F. and Martin,N. (2008) Impact of olfaction on taste, trigeminal, and
texture perceptions. Chemosensory Perception, 1, 217-226.
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PART 3: Complex perception: refreshing
First a review considering the sensory, consumer and psychophysiological aspects of
refreshing perception was written (PART 3.1) to better identify gaps in this field. Then, we
systematically explored the sensory foundations of refreshing perception (PART 3.2)
using samples based on the liquid viscous solution used in PART 2.2. The sensory
drivers were investigated through two approaches: 1) a punctual approach where
perception was studied immediately after consumption (PART 3.2.1); and 2) a temporal
approach focused on the role of temporal olfactory, taste and trigeminal perception
perceived over a 5 min-period on refreshing perception (PART 3.2.2). The contribution of
specific physiological factors in refreshing perception, selected according to literature
review findings, was studied during two independent clinical studies (PART 3.3). The
objective was to assess the impact of two frozen snack differing in refreshing intensity on
mental energy (PART 3.3.1) and saliva properties (PART 3.3.2).
PART 3.1 Sensory basis of refreshing perception: role of psychophysiological factors and
food experience
Key results are presented below.
Based on a few articles dedicated to refreshing perception from sensory and consumer
aspects, the main unsolved question was how a refreshing value (perceived or expected)
can be attributed to foods or drinks. By covering in our review the field of
psychophysiologics related to hydration/dehydration, we proposed that a product is
refreshing when it shares some characteristics of water in terms of sensory properties
(clear, cold, liquid) and can alleviate psychophysiological symptoms (e.g. thirst, mental
fatigue) in a similar manner to water (e.g., acidic foods and drinks, cooling compounds).
Details can be found in the article pp. 96-115
Labbe,D., Almiron-Roig,E., Hudry,J., Leathwood,P., & Martin,N. Sensory basis of
refreshing perception: role of psychophysiological factors and food experience (submitted
to Physiology & Behavior).
PART 3.2 Sensory foundations of refreshing perception
First, a set of viscous fuid products (termed "gel" in this study) was described by sensory
profiling using an expert panel and then scored for refreshing intensity by consumers
(PART 3.2.1). Both data sets were related by the internal preference mapping
methodology using refreshing scores rather than liking score. The sensory properties
perceived over time were also investigated and linked to refreshing perception. Results
from sensory profiling and from Temporal Dominance of Sensation were compared
(PART 3.2.2).
Key results are presented below.
Consumers agree quite well on the least refreshing products which are the sweetest, but
they differ regarding the sensory drivers of the most refreshing products. Three clusters
of consumers are identified for which refreshing was driven mainly by 1) cold-mint
perception; 2) acidity; and 3) thickness. Food habits may partly explain the different key
sensory drivers among consumer clusters.
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Details can be found in the article pp. 116-126:
Labbe,D., Gilbert,F., Antille,N. and Martin,N. (2009) Sensory determinants of
refreshing. Food Quality and Preference, 20, 100-109.
TDS provides information on the dynamics of perception after product consumption
that is not available using a conventional profiling method and that can bring added value
for the understanding of refreshing complex perceptions.
Details can be found in the article pp. 127-133
Labbe,D., Schlich,P., Pineau,N., Gilbert,F. and Martin,N. (2009) Temporal Dominance
of Sensations and Sensory Profiling: A Comparative Study. Food Quality and Preference,
20, 216-221.
PART 3.3 Role of psychophysiological factors in refreshing perception: mental energy
and saliva
Mental energy was explored as a psychophysiological marker of refreshing perception in
terms of mood, cognitive performances and brain electrical activity (PART 3.3.1). The role
of saliva in refreshing perception was also investigated in terms of flow and friction
coefficient (PART 3.3.2). The two study proposals have been approved by the ethical
committee from the Centre Hospitalier Universitaire of Vaud at Lausanne. The impact of
the two frozen snacks differing in refreshing intensity and of a glass of water (positive
control for refreshing perception) was explored on: 1) mental energy in terms of mood,
attentional performance and brain oscillations and 2) saliva in terms of salivary flow and
saliva lubricating properties.
Key results are presented below:
Compared to the standard frozen snack and the glass of water, consumption of the
refreshing frozen snack increases: 1) brain electrical activity and mainly alpha waves that
are generally associated with alertness mental state; and 2) subject attentional
performances obtained during a visual detection task. Subjective alertness rating does
not differ between the two frozen snacks.
Details can be found in the article pp. 134-152:
Labbe,D., Martin,N., le Coutre,J. & Hudry,J. The impact of refreshing perception on
mental energy: changes in mood, cognitive performance and brain oscillations (submitted
to International Journal of Neuroscience)
After consumption of the refreshing product, saliva rate is higher and produced saliva
is more lubricating compared to results obtained after consumption of the standard frozen
snack and glass of water. This physical parameter change is associated with the intense
salivating intensity remaining after the refreshing frozen snack consumption according to
TDS measurement.
Details can be found in the article pp. 153-166:
Labbe,D., & Martin,N. Modulation of saliva flow, saliva lubricating properties and
related lingering perceptions by refreshing frozen snacks (in preparation for Physiology &
Behavior)
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List of original communications
This thesis is based on the following original communications:
Publications in peer-reviewed journals & Proceedings
PART 2: Perceptual multi-sensory interactions
PART 2.1 Olfactory and taste perceptual interactions
PART 2.1.1 Role of product familiarity and exposure strategy with stimuli at
suprathreshold level
- Labbe, D., Damevin,L., Vaccher,C., Morgenegg,C., & Martin,N. (2006). Modulation of
perceived taste by olfaction in familiar and unfamiliar beverages. Food Quality and
Preference, 17, 582-589.
- Labbe, D., Rytz, A. & Martin, N. (2006). Coffee aroma is perceived differently by
consumers and by trained panelists. Proceedings of the 21st International Conference on
Coffee Science, Montpellier, France
PART 2.1.2 Impact of subthreshold olfactory stimuli on taste perception
- Labbe, D., Rytz,A., Morgenegg,C., Ali,S., & Martin,N. (2006). Subthreshold Olfactory
Stimulation Can Enhance Sweetness. Chemical Senses, 32, 205-214.
- Labbe, D., & Martin, N. Impact of novel odorants at supra and subthreshold level on
sucrose perception further to an experimental implicit associative learning (Submitted to
Chemical Senses)
Part 2.2 olfaction, taste and tactile perceptual interactions
PART 3.2.2 Impact of olfaction on taste, trigeminal and texture perceptions
- Labbe,D., Gilbert,F. and Martin,N. (2008) Impact of olfaction on taste, trigeminal, and
texture perceptions. Chemosensory Perception, 1, 217-226.
Part 3: Complex perception: refreshing
Part 3.1 Sensory basis of refreshing perception: role of psychophysiological factors and
food experience
- Labbe,D.,Almiron-Roig,E.,Hudry,J., Martin,N., & Leathwood,P. Sensory basis of
refreshing perception: role of psychophysiological factors and food experience (Submitted
to Physiology & Behavior)
Part 3.2 Sensory foundations of refreshing perception
- Labbe,D., Gilbert,F., Antille,N. and Martin,N. (2009) Sensory determinants of refreshing.
Food Quality and Preference, 20, 100-109.
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- Labbe,D., Schlich,P., Pineau,N., Gilbert,F. and Martin,N. (2009) Temporal Dominance
of Sensations and Sensory Profiling: A Comparative Study. Food Quality and Preference,
20, 216-221.
PART 3.3 Impact on physiological factors on refreshing perception: mental energy and
saliva
- Labbe,D, Martin,N, le Coutre,J, &. Hudry,J. The impact of refreshing perception on
mental energy: changes in mood, cognitive performance and brain oscillations (submitted
to International Journal of Neuroscience)
- Labbe,D., & Martin,N. Modulation of saliva flow, saliva lubricating properties and related
lingering perceptions by refreshing frozen snacks (in preparation for Physiology &
Behavior)
International symposium oral communications
- Labbe,D., Rytz,A. & Martin,N. Coffee aroma is perceived differently by consumers and
by trained panelists. 21st International Conference on Coffee Science (ASIC), September
2006, Montpellier, France.
- Labbe,D., Rytz,A., Morgenegg,C., Ali,S. & Martin,N. Subthreshold Olfactory Stimulation
Can Enhance Sweetness. 17th conference of the European Chemoreception Research
Organisation (ECRO), September 2006, Grenada, Spain.
- Labbe,D., Gilbert,F., Antille,N., & Martin,N. Sensory determinants of refreshing. 2nd
European Conference on Sensory Consumer Science of Food and Beverage, September
2006, The Hague, the Netherlands.
- Labbe.D., & Martin,N. Multisensory Processing in Flavour Perception, invitation at a
satellite symposium in the frame of the International Multisensory Research Forum, July
2008, Hamburg, Germany
- Labbe,D., Martin,N., Rami,S., Le-Coutres,J. and Hudry,J. Mental energy enhancement
by refreshing frozen snack. 18th conference of the European Chemoreception Research
Organisation (ECRO), September 2008, Portoroz, Slovenia
First patent filling
- Labbe,D. & Martin,N. (2007). A sweet food composition with low sugar content. WO
110115
- Labbe,D., Puaud,M., & Lim,T. (2007). Production of food products with enhanced in
mouth and mental refreshment. EP n°07120824.3
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2 PART 2 Perceptual multi-sensory interactions
2.1 Olfactory and taste perceptual interactions
2.1.1 Role of product familiarity and exposure strategy with stimuli at suprathreshold level
2.1.1.1 Modulation of perceived taste by olfaction in familiar and unfamiliar beverages
Labbe,D., Damevin,L., Vaccher,C., Morgenegg,C. and Martin,N. (2006). Food Quality
and Preference, 17, 582-589.
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PART 2: Perceptual multi-sensory interactions
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PART 2: Perceptual multi-sensory interactions
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PART 2: Perceptual multi-sensory interactions
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PART 2: Perceptual multi-sensory interactions
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PART 2: Perceptual multi-sensory interactions
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PART 2: Perceptual multi-sensory interactions
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PART 2: Perceptual multi-sensory interactions
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Additional unpublished results
In this published work, the selected drinks were a coca based beverage reconstituted
with water assumed as familiar and a caffeinated milk assumed as unfamiliar. Latter,
another bitter drink was tested in this study, a black coffee brew beverage assumed as
the most familiar among the three drinks according to internal consumer insight.
Three products were therefore evaluated, a very familiar bitter coffee brew, a familiar
bitter cocoa drink and an unfamiliar bitter milk. As for the cocoa drink, two sets of three
coffee brews were prepared: 1) one set with a coffee brew reference, and two coffee
brews flavoured with a natural coffee flavouring (produced internally) at two
concentrations for boosting the intrinsic coffee odour and aroma; and 2) one set with a
coffee brew reference, and two coffee brews flavoured with the same vanilla flavouring as
the one used for flavoured cocoa and milk drinks. Product preparation, sensory
evaluation procedure and statistical analyses were the same as described in
Results of the evaluation conducted with noseclip validates that the coffee flavouring
does not taste by itself (Fig.1 a-b).
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a) Coffee brews with coffee flavouring
b) Coffee brews with vanilla flavouring
Fig.1 a-b: Flavouring impact on coffee brew taste and texture. Evaluation conducted with noseclip
for a) coffee brew with coffee flavouring; b) coffee brew with vanilla flavouring; flavouring.
According to the F-ratio of the product factor ANOVA: NS means “Non-Significant”.
Without noseclip, the results obtained with coffee brews are consistent to those obtained
with cocoa drink since: 1) coffee flavouring addition enhances bitterness and decreases
sweetness (Fig.2) and; 2) vanilla flavouring addition induces sweetness. The mechanistic
hypotheses underlying such taste modulation were detailed in the discussion part of the
published article. Actually, since vanilla odour/aroma is congruent with sweet taste;
sweetness probably reduces bitterness by symmetrical odour induced taste-taste
interactions.
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a) Coffee brews with coffee flavouring
b) Coffee brews with vanilla flavouring
Fig.2 a-b: Flavouring impact on beverage odour, aroma, taste and texture/Evaluation conducted
without noseclip. According to the F-ratio of the product factor ANOVA: NS means “Non
Significance”; *, **, *** indicate product effect at P<0.05, P<0.01, P<0.001, respectively.
According to the Dunett test, a white star indicates a significant difference between each sample
and the reference (P<0.05).
The main finding of this additional experiment is that the taste modulation induced by
vanilla flavouring follows the same direction as that obtained with the cocoa beverage in
terms of sweetness enhancement and bitterness decrease but with a more powerful
effect. Indeed the p-values of the sweetness increase induced by vanilla flavouring were
0.03 and 0.01 for the cocoa drink and coffee drink, respectively and the p-values of the
bitterness reduction were 0.34 and <0.0001 the coca drink and coffee brew, respectively.
These results reinforce our hypothesis that olfactory and taste integration is product
dependent and related to food experience.
article 1
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2.1.1.2 Coffee aroma is perceived differently by consumers and by trained panelists
.Labbe,D., Rytz,A. & Martin,N. (2006). Proceedings of the 21st International Conference
on Coffee Science, Montpellier, France
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2.1.2 Impact of subthreshold olfactory stimuli on taste perception
2.1.2.1 Subthreshold olfactory stimulation can enhance sweetness.
Labbe,D., Rytz,A., Morgenegg,C., Ali,S., & Martin,N. (2006). Chemical Senses, 32, 205214.
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3
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2.1.2.2 Impact of novel odorants at supra and subthreshold level on sucrose perception
further to an experimental implicit associative learning
Labbe,D., & Martin,N. to be submitted to Chemical Senses
Abstract
The impact of co-exposure to a novel odorant and sucrose on the construction of
perceptual interactions was explored. The first objective was to validate the absence of
associative learning when a group of subjects were co-exposed following a sensory
profiling training approach compared to a group of subjects exposed according to a
synthetical approach. The second objective was to explore the impact of the odorant at
subthreshold level on sweetness further to the implicit associative learning. Sweetness of
the sucrose solution was increased by the odorant only when scored by the group coexposed according to the synthetical approach. We confirm that co-exposure following a
sensory profiling training did not promote implicit learning likely because this approach
encouraged subjects to consider sensory dimensions analytically. At subthreshold level,
the odorant does not impact perception of the sucrose solution whatever the co-exposure
approach. The potential role of neural integration processes and plasticity in these results
is discussed.
Keywords: Olfaction, taste, Perceptual interaction, Associative learning, Suprathreshold,
Subthreshold,
neural
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Introduction
Flavor is defined a s a complex combination of the olfactory, gustatory and trigeminal
sensations perceived during tasting (ISO 5492, 1995). Regarding perceptual interactions
involved in flavor perception, the most commonly reported interactions are between
olfactory and sweet taste perceptions. First, the impact of strawberry odorant on
sweetness was highlighted in a sucrose solution (Frank et al., 1989) , then, other
odorants were also found to enhance sweetness of a sucrose solution, for example
pineapple and raspberry (Prescott, 1999), maracuja and caramel (Stevenson et al., 1999).
Such odorants are generally present in sweet food, these odorant stimuli are therefore
congruent with sweet taste stimulus. Congruency between sensory qualities is a crucial
factor for perceptual interaction between senses. Congruency is the extent to which two
stimuli are appropriate for combination in a food product (Schifferstein, 2006). Studies
showed that odorants incongruent with sweet taste do not increase sweetness contrary to
congruent odorants (Stevenson et al., 1999; Djordjevic et al., 2004). The scientific
community agrees that different sensory stimuli become congruent when conjointly and
repeatedly experienced through everyday food consumption. Perceptual interactions
result therefore from an implicit associative learning between sensory modalities (Koster,
2005; Koster et al., 2004).
The construction of perceptual interactions through implicit associative learning was also
experimentally demonstrated by repeated exposures to a novel odorant in solution with
sucrose or citric acid tastants (Stevenson et al., 1995). Further to the odorant and tastant
co-exposure, the odorant acquires the taste property of the co-exposed tastant, sweet or
sour. This implicit associative learning leading to olfactory-taste interactions was
described as ‘learned synthestesia’ (Stevenson and Boakes, 1998). Results from a
further work showed that: 1) pairing a new odorant with. sucrose leads to an
enhancement of sweetness evoked by odorant sniffing; and 2) saltiness of a MSG and
NaCl mixture decreases after odorant-sucrose paring procedure (Yeomans et al., 2006).
The attentional strategy during exposure is an additional important factor influencing
construction of perceptual associations. Prescott et al. (2004) compared the impact of two
different co-exposure procedures on the construction of perceptual associations between
a novel olfactory stimulus and sweet taste both at suprathreshold level. Each task
encouraged subjects to consider olfactory and taste sensory dimensions either
analytically or synthetically. Comparing post and pre-exposure results, the synthetical
exposure strategy group rated the flavored sucrose solution sweeter than the unflavored
sucrose solution but not the analytical exposure strategy group. However, another study
showed that the construction of perceptual association may occur further to a sensory
profiling training, a procedure still encouraging an analytical attentional strategy
(Stevenson and Case, 2003). In the latter study, training consisted in differentiating the
sensory dimensions during tasting of an odorant mixed with sucrose or citric acid in
solution. All subjects also rated their liking of the solutions and according to Prescott et al.
(2004), who discussed previous results from Stevenson and Case (2003), liking rating
may have encouraged subjects to consider synthetically the olfactory and taste sensory
dimensions what may explain why perceptual associations were built. The first objective
of this study was to validate that the construction of perceptual associations between a
novel odorant and sucrose is limited when co-exposure to the odorant and tastant mixture
follows a sensory profiling training procedure, without hedonic evaluation.
A few studies focused on olfactory and taste interactions, when both odorant and tastant
are presented at a subthreshold level (Dalton et al., 2000; Pfeiffer et al., 2005). Firstly, the
authors explored the impact of an in-mouth saccharine solution (sweet tastant) at a
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subthreshold concentration on benzaldehyde threshold determination (a volatile
compound generally described as having an almond-like odor). The olfactory threshold of
benzaldehyde significantly decreases with the presence of the saccharine solution in
mouth. In addition, Labbe et al. (2006) showed that sweetness rating of a sucrose
solution is increased by a simultaneous retronasal olfactory stimulation by subthreshold
ethyl butyrate odorant, commonly described as having a strawberry-like odor. More
recently, Miyazawa et al. (2008) demonstrated that subthreshold concentrations of acetic
acid increase the perceived retronasal olfactory intensity of three coffee aroma volatile
compounds presented at suprathreshold level. The second objective of the present study
was to explore whether after implicit associative learning between an odorant and a
sweet tastant, odorant had an impact on sweetness when presented at subthreshold level.
Experiment 1: impact of two implicit associative learning procedures on
construction of perceptual interaction with stimuli at suprathreshold level
Material and methods
Odorant selection
Two commercial odorants, elderflower (product code CD95904) from Givaudan S.A.
(Dübendorf, Switzerland) at 1200 ppm and cactus (product code 505898 A) from
Firmenich (Geneva, Switzerland) at 350 ppm were selected among twelve odorants
during a preliminary study conducted with eleven subjects. The subjects were asked to
taste and swallow the odorant solutions and to score familiarity induced by the odorants
on a 10cm scale anchored at the extremities from the left to the right with "Not at all
familiar" and "Extremely familiar". Odorants were evaluated in Vittel water solution with
and without 7% sucrose. Among the twelve odorants, elderflower and cactus odorants
were scored lower in familiarity (+/-standard error) in unsweetened water (3.8 +/-1.5 and
4.9 +/-1.5, respectively) and in a 7% sucrose solution (3.8 +/-1.6 and 3.7 +/-1.9,
respectively).
An unflavored sucrose solution at 7% was also prepared. All one-liter solutions were
prepared each morning prior to the test and stored at room temperature (22°C) until use.
Subjects and procedure
Twenty four naïve women between 40 and 45 years old took part in the study. Subjects
were previously selected for normal olfactory and taste acuity based on the procedure
ISO 8586-1 (1995).
The pre-exposure session (PRE) and the post-exposure session (POST) were conducted
by all assessors and consisted in scoring: 1) sweetness of the two flavored unsweetened
solutions, the two flavored sucrose solutions and the unflavored sucrose solution (which
was replicated); and 2) familiarity (by smelling) and retronasal olfactory intensity of the
two flavored unsweetened solutions.
The group of twenty four assessors was then split randomly into two groups of twelve,
each group being co-exposed to sucrose with one of the two odorants. Within each group
of twelve assessors: 1) six assessors were co-exposed following an analytical attentional
strategy (ANA), i.e. sensory profiling training; and 2) six assessors were co-exposed
following a synthetical attention strategy (SYN), i.e. triangle test.
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Three exposure sessions lasting one hour were carried out for ANA and SYN groups
using four solutions obtained by four successive dilutions of the flavored sucrose solution
evaluated in PRE and POST with a dilution step of 1.2. The aim was to limit boredom by
presenting solutions with different olfactory and taste intensities. However solutions had
the same ratio of odorant and sucrose concentration to limit changes in the sensory
olfactory and taste balance which could facilitate the dissection of the two sensory
dimensions. Each of the four solutions was presented three times during each exposure
session. Subjects were not informed about the replications of the three solutions. At the
end of the exposure phase, assessors from ANA or SYN groups were exposed to the
same number of times and volume of olfactory and taste stimuli, i.e. thirty-six flavored
sucrose solutions of 50 ml with a constant odorant-sucrose ratio.
The odorant evaluated in PRE and POST was named either: 1) TEST when co-exposed
with sucrose during the exposure phase; and 2) CONTROL when not co-exposed with
sucrose during the exposure phase. Each of the five sessions, i.e. the PRE, POST and
the three exposure sessions, were conducted on five separate and consecutive days
(See Table 1).
Table 1: Steps and duration of experiments 1 and 2
- Sensory profiling training exposure (ANA)
The training aimed at promoting analytical attentional strategy during the co-exposure
since assessors were encouraged by this procedure to consider independently olfactory
and taste related perceptions. The six assessors exposed to cactus odorant and the six
assessors exposed to elderflower odorant conducted the exposure sessions separately.
The first session of the exposure consisted in sniffing and tasting the twelve flavored
sucrose solutions and in describing with their own vocabulary the olfactory and taste
characteristics of the solutions. During the second session the attribute list was reduced
by removing redundant and confusing attributes (ISO 11035, 1995). The assessors
tasted again the twelve flavored sucrose solutions and selected for each solution the
attributes they considered as relevant. At the end of this session, sweetness and two
attributes related to olfactory perception were kept for the third session. Finally the last
training session consisted in a series of ranking tests. The four sucrose flavored solutions
were ranked from the least to the most intense for each of the three attributes.
- Triangle test exposure (SYN)
The aim of conducting triangle tests was that the subjects were encouraged to acquire a
synthetical attentional strategy so that they integrated olfactory and taste stimuli as a
whole perception, namely flavor perception. Indeed subjects were asked to pick the odd
sample based on overall perception and not to focus independently on each sensory
dimension. The twelve assessors conducted a series of four triangle tests during each of
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the three sessions. For each of the three sessions, the six assessors exposed either to
the cactus odorant or to the elderflower odorant carried out simultaneously the triangle
tests in booths. Within each triangular test, three identical flavored sucrose solutions were
presented. Each of the four triangle tests was conducted with one of the four flavored
solutions previously described. The same four triangle tests were repeated during each of
the three sessions.
Tasting conditions
Solutions were coded with three-digit random numbers and 50-ml portions were served in
100-ml plastic cups. Assessors were asked to sip and swallow the solutions. Rinsing was
done between products with water and unsalted crackers for the PRE and POST
evaluations and between each triangle test for the SYN group exposure sessions. For
PRE and POST evaluations, the six samples (the two flavored unsweetened solutions,
the two flavored sucrose solutions and the two unflavored sucrose solutions) were
presented according to a presentation design, based on Williams Latin squares,
balancing position and order effects. The design was identical for both evaluations. For
each SYN group exposure session, the four triangle tests were presented in the same
order within each group of six assessors being exposed to the same odorant. Data was
acquired on a computer screen with FIZZ software Version 2.20E (Biosystemes,
Couternon, France) for the PRE and POST evaluations and during the SYN group
exposure. The same 10 cm scales as those described for the odorant selection were
used for the PRE and POST exposure evaluations. Tests were conducted in an airconditioned room (22°C), under white light in individual booths.
Statistical analyses
- Unsweetened flavored solutions
The objective was to determine if odorant and sucrose co-exposure impacted 1) odorant
familiarity when the solution was sniffed; and 2) retronasal olfactory intensity and the
sweetness evoked by the odorant when the solution was tasted. For each attribute,
individual scores obtained in PRE were subtracted to individual scores obtained in POST
(POST-PRE) within each odorant category (TEST and CONTROL) and within each
exposure type (ANA and SYN). A positive value means that exposure induced an
increase of the attribute intensity, a negative value indicates that exposure induced a
decrease of the attribute intensity and a value close to zero means that exposure did not
change perception.
An Odorant (TEST, CONTROL) x Attentional strategy (ANA, SYN) analysis of variance
(ANOVA) with interactions was performed on POST-PRE scores to explore the impact of
both factors on familiarity, aroma intensity and sweetness.
- Sweetened flavored solutions
The objective was to investigate if further to the exposure stage, the odorant impacted
sweetness of the flavored sucrose solution. This was conducted in three steps. Step 1:
the sweetness mean of the two unflavored sucrose solutions was calculated per assessor
and for each stage (PRE, POST). Each value was used as a sweetness baseline for each
stage. Step 2: the ability of the odorant to modulate sweetness was measured for each
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period, each subject and each odorant by subtracting the sweetness of the unflavored
sucrose solution calculated in step 1 from the sweetness of the flavored sucrose solution
(called relative sweetness score). Step 3: the individual relative sweetness score
calculated in PRE was subtracted from the individual relative sweetness obtained in
POST (POST-PRE).
An odorant category (TEST, CONTROL) x attentional strategy (ANA, SYN) analysis of
variance (ANOVA) with interactions was calculated to explore the impact of both factors
on sweetness as defined in step 3.
Analyses of Variance (ANOVA) were calculated, using NCSS software version 2007
(Number Cruncher Statistical Systems, Karysville, Utah, USA.). Post-hoc pair
comparisons were conducted by a Student t-test. Confidence level was set to 95% for all
analyses.
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Results
Results of PRE confirmed that cactus and elderflower odorant did not differ according to
Student t-test (two-tailed paired): 1) in familiarity (p-value=0.23) with mean scores of 6.1
and 6.9; 2) in retronasal olfactory intensity (p-value=0.59) with mean scores of 6.8 and
6.6; and 3) in sweetness (p-value=0.51) with mean scores of 0.6 and 0.4, respectively.
Unsweetened flavored solutions
Evolutions of familiarity, retronasal olfactory intensity and sweetness between POST and
PRE were not affected by the attentional strategy according to ANOVA with [F(1,44)=0.4],
[F(1,44)=0.69] and [F(1,44)=0.10] for each of the three attributes, respectively.
Evolution of sweetness [F(1,44)=1.49] and retronasal olfactory intensity [F(1,44)=2.69]
between POST and PRE were not affected by the odorant category (TEST, CONTROL).
A trend existed regarding familiarity evolution [F(1,44)=3.27, p-value=0.07] according to
the odorant category. As expected, increase in familiarity for the unsweetened flavored
solution was higher when flavored with TEST than with CONTROL (See Figure 1).
5
Change in familiarity
4
3
2
1
0
-1
control
test
Fig.1: Panel mean score for change in familiarity (+/- SEM) of CONTROL and TEST
unsweetened solutions
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Sweetened flavored solutions
Change in sweetness between PRE and POST was not significantly impacted by
attentional strategy (ANA vs. SYN) [F(1,44)=0.74] and odorant category (TEST,
CONTROL) [F(1,44)=0.4]. But interaction between both factors (See Figure 2) was
significant [F(1,44)=5.92, p-value<0.05]. Pair comparisons by Student-t test revealed a
significant difference in sweetness for the TEST odorant between the two strategies (pvalue<0.05) (See Figure 2). In addition, sweetness change of the sucrose solution
flavored with TEST was significantly higher than zero for SYN (mean of 1.20 with a
confident interval of 0.84) and significantly lower than zero for ANA (mean of -1.80 with a
confident interval of 1.43).
Change in sweetness mean score
(post exposure -pre-exposure)
3
2
1
0
-1
-2
-3
CONTROL
TEST
ANA
SYN
Fig.2: Panel mean score for change in sweetness (+/- SEM) of sucrose solution solutions
flavoured with the control and test odorant according to the attentional strategy applied
during exposure. A significant change is observed for solution flavoured with odorant test
when evaluated by SYN group (sweetness increase) and by ANA group (sweetness
decrease).
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Experiment 2: Impact of olfactory stimuli at subthreshold level on sweetness after
implicit associative learning
The second experiment was conducted to investigate the impact of the co-exposed
odorant on sweetness of a sucrose solution when presented at subthreshold level. Two
sucrose solutions, with and without odorant, were compared by a 3-AFC discrimination
test. We supposed that a sucrose solution flavored at subthreshold level with the coexposed odorant should be perceived differently from the unflavored sucrose solution due
to sweetness change induced by perceptual interactions.
The experiment started after a two-days break (week-end) following the first experiment
and consisted in two sessions for the determination of individual detection threshold and
one session for the evaluation of the odorant impact at subthreshold concentration.
These three sessions were conducted on three separated and consecutive days (See
Table 1). The same twenty-four assessors as for the first experiment participated to the
second experiment.
Material and methods
Procedure and statistical analyses
- Determination of individual odorant subthreshold concentrations
Firstly, the detection threshold of each odorant in mouth was determined for each of the
twenty four subjects in Vittel mineral water using the Forced-Choice Ascending
Concentration Series Method of Limit (ASTM, 1991) during two sessions (one for each
odorant). Each subject performed for each odorant a series of fifteen 3-AFC tests with an
ascending concentration of odorant using a dilution factor of 2 as described in Labbe et al.
(2006). The ranges of concentrations used according to the supplier recommendations
and preliminary trials were from 6.7E-04 to 11 ppm for cactus and from 4.8 E-03 to 78
ppm for elderflower. For each subject and odorant, the concentration above which all 3AFC tests were correctly performed was considered as the individual detection threshold
concentration. Finally the subthreshold value was obtained by dividing the threshold
value by 64 to stand largely below the individual threshold.
- Criteria to conclude to an impact of subthreshold odorant concentration on sucrose
solution perception
For each odorant, assessors carried out a 3-AFC test in a 7% sucrose solution, one of
the three sucrose solutions being flavored with the subthreshold odorant concentration. A
minimum of five 3-AFC tests out of six had to be solved to consider the flavored and
unflavored sucrose samples as significantly different according to the binomial law with a
confidence level set at 95%.
Tasting conditions
Solutions were coded with three-digit random numbers and 50-ml was served in 100-ml
plastic cups. Assessors were asked to sip and swallow the solutions. Rinsing was done
between each 3-AFC test with water and unsalted cracker. Data was acquired on a
computer screen with FIZZ software Version 2.20E (Biosystemes, Couternon, France).
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Results
The lowest and highest threshold concentrations within the twenty-four subjects were 1)
1.34E-03 and 1.10 ppm with a panel geometric mean of 4.2E-02 ppm for cactus and 2)
7.6E-02 and 2.45 ppm with a panel geometric mean of 3.8E-01 ppm for elderflower.
Only one subject out of twenty-four significantly distinguished the flavored from the
unflavored sucrose solution for the TEST odorant (See Figure 3). Consequently we could
not conclude to significant differences between odorant category and applied attentional
strategy during exposure.
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a)
Number of solved 3-AFC tests (out of 6)
6
5
4
3
2
1
0
1 2 3 4 5 6 7 8 9 10 11 12
13 14 15 16 17 18 19 20 21 22 23 24
Assessors n°
b)
Number of solved 3-AFC tests (out of 6)
6
5
4
3
2
1
0
1 2 3 4 5 6 7 8 9 10 11 12
13 14 15 16 17 18 19 20 21 22 23 24
Assessors n°
Fig.3: Performance of assessors in solving sucrose solution based triangle tests
flavoured at subthreshold with a) the control odorant; and b) the test odorant. The 12
assessors from ANA group and the 12 assessors from SYN group are represented in
grey and black, respectively.
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Discussion
Impact of exposure approaches on implicit associative learning
After the odorant and sweet tastant co-exposure following a sensory profiling training
approach, the co-exposed odorant did not enhance in-mouth sweetness. This finding
validated that this approach encouraged subjects to consider perceptual dimensions
analytically as assumed by Prescott et al. (2004). This finding showed that a descriptive
profiling training where subjects learn to disconnect with reference solutions the different
sensations that will be further conjointly experienced in the product tested could reduce
the impact of perceptual interactions on product description (e.g. by tasting an odorant
and a tastant first in mixture and then independently). Furthermore, sweetness of the
flavored sucrose solution was perceived as being weaker after exposure than before
exposure. As assessors were trained not to over evaluate sweetness of the flavored
sucrose solution by dissociating olfactory and taste perceptions, this may have led them
to under evaluate sweetness when the co-exposed odorant was present. Prescott et al.
(2004) showed the same pattern but the authors do not suggest any interpretation.
Odorant and tastant co-exposure according to an approach encouraging a synthetical
attentional strategy led to the construction of perceptual olfactory-taste association as
previously shown by Stevenson and Case (2003), Prescott et al. (2004) and Yeomans
(2006). Indeed when olfactory and taste stimuli were considered as a whole during inmouth experience, the integration of both sensory dimensions was facilitated.
Whatever the exposure approach, when the unsweetened solution was in mouth, the
odorant did not enhance sweetness. However Prescott et al. (2004) highlighted that after
exposure, the smelled odor was perceptually associated with sweetness. In the present
study, where we focused on in-mouth perception, a minimum amount of sweet tastant
seemed to be required to induce the in-mouth perceptual association between odorant
and sweetness.
Impact of implicit associative learning on sweetness with olfactory stimulus at
subthreshold level
The impact of subthreshold olfactory stimulus on sweetness has already been
demonstrated with common odors congruent with sweetness such as almond (Dalton et
al., 2000; Pfeiffer et al., 2005) and strawberry (Labbe et al., 2006). In the present study,
the expected enhancing effect of the odorant co-exposed with sucrose on sweetness was
not obtained at subthreshold level whatever the attentional strategy applied during
exposure. Indeed flavored and unflavored sucrose solutions were not discriminated
through a 3-AFC procedure suggesting that sweetness of the flavored sucrose solution
was not increased by perceptual interaction. The perceptual association built
experimentally was probably not as strong as those constructed over life with familiar
odors. This may explain why sweetness could not be modulated by a subthreshold
concentration of a novel odor experimentally co-exposed with sucrose contrary to what
shown with common odorants congruent with sweetness.
Role of neural integration processes in the construction of perceptual associations
Perceptual interactions between different sensory dimensions are underlain by neural
processes as shown with suprathreshold olfactory and taste stimuli by Small et al. (2004).
Using functional Magnetic Resonance Imagery (fMRI), the authors highlighted a supra--M
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additive effect of a congruent vanilla odorant and sucrose mixture on neuron activity from
insula/orbitofrontal cortex compared to the activation induced by each ingredient
independently. The authors suggested that these observations may be explained by the
presence of specific neurons from insula/orbitofrontal cortex which may integrate both
olfactory and taste stimuli when congruent. Such bimodal neurons have been highlighted
in macaque orbitofrontal cortex (Rolls and Baylis, 1994), and amygdala (Kadohisa et al.,
2005) and may result from repeated and simultaneous exposure to a given olfactory and
taste combination during life time. Other key structures, referred as to key nodes of the
flavor network, have been recently proposed as candidates having a role in olfactory and
taste integration processes such as the frontal operculum and the anterior cingular cortex
(Small and Prescott, 2005).
Different studies also report olfactory and taste interactions with common odors
congruent with sweet taste at a subthreshold concentration (Pfeiffer et al., 2005; Dalton et
al., 2000; Labbe et al., 2006). We may argue that repeated exposure over life to odorants
might lower the activation threshold of olfactory neuron receptors below the perceptual
threshold level and may explain the perceptual association between odor at subthreshold
level and sweetness. A few studies support the existence of odorant-specific plasticity in
the peripheral olfactory system further exposure (Wysocki et al., 1989; Wang et al., 2004).
Indeed authors showed that subjects anosmic to androstenon can acquire sensitivity to
odorant through exposure. However, in our study, we did not observe sweetness
modulation by the co-exposed odorant at subthreshold level. The short number of
experimental exposure sessions to the novel odorant may not have been sufficient
enough to lower the activation threshold of olfactory neuron receptors as we previously
argued for familiar odorants.
Perspectives
As perspective, additional psychophysics and neuroimaging works could extend our
understanding about the plasticity of the flavour network during associative learning and
on how unitary perceptions are generated over time. The combined measurement, in
response to stimulation by a sucrose solution containing a new odorant at subthreshold
level: 1) of neural activity by fMRI of orbitofrontal cortex, amygdala, operculum and cortex
cingular anterior structures; and 2) of perceptual impact of such odorant on sweetness,
before and repeatedly after several exposure periods may bring relevant insights.
Acknowledgements
We gratefully acknowledge Givaudan SA Switzerland and Firmenich SA Switzerland who
kindly provided flavorings and Elizabeth Prior for reviewing the English manuscript.
References
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2.2 Olfaction, taste and tactile perceptual interactions
2.2.1 Validation of the sensory diversity induced by formulated products
Introduction
The second main objective of my PhD work was to explore perceptual interactions in a
model involving olfactory, taste and tactile perceptions. To our knowledge, the perceptual
impact of cold trigeminal perception on other perceptions was so far little investigated.
Since cooling agents are widely used by food companies in confectionary products and
beverages, there is a clear need to better understand how this perception impacts the
overall product characteristics. Based on this status, our exploration of perceptual
interactions was extended to cold perception using a cooling agent. Two different
flavourings and citric acid were selected as odorants and tastant. Finally a sweet viscous
fluid was used as a model to increase perceptual complexity bringing sweet taste and
thickness proprioception. Regarding tactile perception, two aspects were therefore
studied: trigeminal perception (coldness) and proprioception (thickness).
Before exploring sensory differences between products and the role of perceptual
interactions in product perception according to a sensory profiling approach, the first
objective of this PART 2.2 was to ensure that formulated samples prepared according to
a factorial design were different enough to be discriminated not only by a trained panel
but also by naive people. To reach this objective, a sorting task with verbalisation was
carried out by forty naive internal assessors for each of the two sets of products, i.e. the
eight mint flavoured products and the eight peach-flavoured products.
Methods
Subjects
Forty naive assessors with an average age of 30 (twenty two women and eighteen men)
from the Nestlé Research Center, were recruited for the study. They had never previously
participated in sensory panels.
Products
A viscous solution containing fructose (16%), sucrose (32%), dextrose (32%), xanthan
(0.5%), and water (19.5%) was used as a model for this study. With each type of odorant,
a formulation design was built by adding for the first set a peach odorant (supplier
reference 78130-33) at 0.10% and 0.20%, for the second set a mint odorant (supplier
reference 11606) at 0.01% and 0.03%, and for both sets a cooling agent (WS-3) at 0.10%
and 0.20%, and citric acid at 0.20% and 0.60%. The two odorants and the cooling agent
were provided by Givaudan SA (Geneva, Dübendorf). The two sets of eight products
were therefore based on a 23 formulation design and only differed by odorant type.
Tasting conditions
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During a first session, the panelists were informed about the principle of the sorting
procedure. The second and third sessions were dedicated to the evaluation. For both
evaluation sessions, the panelists received the eight samples at once within a same
odorant. They were asked to smell and taste the products and then to sort them into
groups having similar sensory properties. No other recommendation was given, except
that they had to make at least two groups. Then they were invited to describe their groups
with their own vocabulary. The two flavoured sample sets, i.e. mint and peach, were
randomized between subjects and the two sessions.
Evaluation was conducted in an air-conditioned room (20 °C) and under white light in
separate booths.
Statistical analyses
Sorting data was analysed in three steps (Cartier et al., 2006)
Product map using multidimensional scaling (MDS)
An individual binary dissimilarity matrix indicating whether two samples were grouped
together was constructed for each panelist. The 40 individual matrices were summed and
the resulting dissimilarity matrix was submitted to classical metric multidimensional
scaling MDS (Togenson, 1952) using NCSS software version 2007 (Number Cruncher
Statistical Systems, Karysville, Utah, USA).
Product description using the vocabulary elicited to describe groups
The vocabulary generated to describe the groups was used to build a contingency table
for the products. Each term used to describe a group of samples was reallocated to each
product of the group. We therefore assumed that all the products belonging to the same
group could be described by the same terms. The resulting contingency table was then
reduced and simplified: 1) terms having similar meanings were grouped by the Sensory
Analyst; and 2) only terms having a quotation frequency higher or equal to 10% for the
sample with the highest elicitation rate were considered.
Projection of product descriptions on MDS-map
The items describing products were projected onto the MDS map using the correlation
structure of product coordinates on the map and product descriptions in the contingency
table.
Results
Regarding the mint and peach flavoured products (Fig.1a-b), both maps represent well
the initial data, as shown by low stress (= 0.23 and 0.18, for mint- and peach-flavoured
products, respectively), and high Pearson's correlation between Euclidian distance
between products represented on the two dimension map and MDS dissimilarities (r=
0.93 and 0.96, for mint and peach flavoured products, respectively).
For both set of products, assessors grouped consistently samples according to the level
of citric acid (axis 1). Samples with the highest citric acid level are represented on the
right hand side of the map and samples with the lowest citric acid level are represented
on the left hand side of the map. Elicited terms were coherent with product formulation
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since samples with the highest citric acid level were described as acid whereas samples
with the lowest citric acid level as sweet.
The cooling agent level also impacted grouping of the mint flavoured samples. The four
samples with the highest cooling agent level are opposed to the four samples with the
lowest cooling agent level (Fig.1). For the peach flavoured samples (Fig.2a), the cooling
agent level also impacted the grouping of samples (axis 2) but with a citric acid level
interaction. Indeed the four products within each citric acid level are grouped into two
pairs (based on the level of cooling agent) which are positioned differently according to
the citric acid level. For both sample sets, products with the highest cooling agent and
citric acid levels were described as refreshing with a frequency of 15% and 19% for the
mint and peach flavoured range of samples, respectively.
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a)
b)
Fig.1: First factorial maps of MDS issued from sorting with assessors with projection of consumer
items describing the products for:
a) mint flavoured samples containing the ingredients symbolized below with the respective
concentrations
b) peach flavoured samples containing the ingredients symbolized below with the respective
concentrations
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Discussion
The ingredients added into the viscous liquid model according to a factorial design
allowed to formulate eight products with a wide sensory diversity within a same odorant.
Indeed assessors were able to 1) discriminate samples according to the level of cooling
agent and citric acid; and 2) described products consensually and relevantly, i.e. with
vocabulary corresponding to product composition.
The last step of the PhD work was to explore a complex conception, i.e. a perception not
described with simple attributes such as sweet or thick but that result from an integration
of unitary percepts. In the present study, within each odorant type, samples with both a
high cooling and acid levels were described as refreshing. In addition samples refreshing
and sweet descriptor were negatively correlated. Consequently, we considered refreshing
perception as potentially a complex perception since driven by different sensory
dimensions. The sensory foundations of refreshing perception were systematically and
deeply studied in the PART 3.2 of the PhD work using a similar liquid viscous model
system.
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2.2.2 Impact of olfaction on taste, trigeminal and texture perceptions
Labbe,D., Gilbert,F. and Martin,N. (2008) Impact of olfaction on taste, trigeminal, and
texture perceptions. Chemosensory Perception, 1, 217-226.
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2.2.3 Setting up of the range of liquid viscous products for the investigation of refreshing
complex perception
As shown in the PART 2.2.1, refreshing term was spontaneously elicited by naive
assessors during sorting task with free vocabulary generation conducted with liquid
viscous solutions. In addition refreshing seems a complex perception, i.e. related to more
than one sensory dimension since products spontaneously described as refreshing
during the sorting task: 1) were opposed to products described as sweet; and 2)
contained a high citric acid and cooling agent levels. The range of viscous liquid products
was therefore used in the next part with the objective to investigate systematically the
sensory drivers of refreshing after a re-formulation work to widener the sensory diversity
as described below (Fig.1):
- Step 1: selection of the most different peach and mint flavoured products in terms of
sensory perception among the 16 products used in the PART 2.2.
This was done on each set of products within the odorant type by performing a Principal
Component Analysis on sensory profiling data obtained PART 2.2.1 with the sensory
attributes as variables and the products as observations; and 2) selecting the most
extreme products of the first biplot.
- Step 2: widening the sensory diversity in terms of in mouth texture perception. Indeed
according to literature review on sensory foundations of refreshing perception presented
in PART 3.1, thickness could be a driver of refreshing perception. The nine most extreme
samples were therefore formulated with two levels of thickener (xanthan), two sensory
profiling were carried out on samples within a same odorant type and a Principal
Component Analysis was run. Again, the most extreme samples represented on the first
biplot were selected. Five mint and four peach flavoured products were chosen for the
PART 3.2.
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Fig.1: Schematic representation of performed formulation and evaluation steps to obtained
samples used in PART 3.2.
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PART 3: Complex perception: refreshing
3.1 Sensory basis of refreshing perception: role of psychophysiological factors
and food experience
Labbe,D., Almiron-Roig,E., Hudry,J., Leathwood,P., & Martin,N. submitted to Physiology
& Behavior
Introduction
The Merriam-Webster Dictionary & Thesaurus (2006) defines “refreshing” as “serving to
restore strength and animation, to revive, to arouse, to stimulate, to run water over or
restore water to, with thirst quenching properties”, suggesting that “refreshing” is linked to
physiological factors such as thirst-quenching and arousal. To our knowledge, no peerreviewed study has explored the relationship between the refreshing value of foods and
drinks and these physiological factors. In contrast, a number of sensory and consumer
studies have explored the refreshing attribute with regard to both expected and perceived
characteristics of foods and drinks (Zellner and Durlach, 2003; Labbe et al., 2007). From
these studies and in line with the dictionary definitions, we find that the perception of a
food or drink as being refreshing is often associated with specific sensory characteristics
related to psychophysiological states linked with water drinking and modulated by
experience. This review explores the concept of refreshing following three axes. First, at
the perceptual level, we examine the sensory drivers of refreshing perception. Second,
studies of psychophysiological states in relation to water drinking are examined and
potential links between psychophysiological factors and specific sensory characteristics in
refreshing perception are reviewed. Third, studies on the influence of food experiences
on perception of refreshing foods and drinks are explored. Finally, based on the reviewed
studies, a model of the construction of refreshing perception is proposed.
Keywords: Refreshing, Sensory, Perception, Psychophysiology, Food experience.
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Sensory drivers of refreshing perception
Sensory attributes related to food low temperature and cooling perception are among the
most noteworthy drivers of refreshing perception (Zellner and Durlach, 2002). Indeed,
cool drinks are typically described by consumers as being “refreshing”, especially during
hot weather (Scriven et al., 1989). In addition, cold solid foods, such as ice cream, are
also considered to be “refreshing” by many people (Zellner and Durlach, 2002).
Labbe and co-workers explored the sensory determinants of refreshing perception
produced by a range of viscous fluid products varying in cooling agent, citric acid and
thickener levels, and flavoured either with peach or with mint aroma (Labbe et al., 2007).
A Swiss panel of 12 trained assessors described the nine products using a list of sensory
attributes. Another group of 160 French consumers scored perceived refreshing intensity
of the same products. The intensity of the attributes “cold” and “acid” (as defined by the
trained assessors) is positively correlated with perceived refreshing intensity (rated by
consumers). In contrast, increases in “sweetness” and “thickness” are negatively
correlated with refreshing intensity. This latter finding is supported by two earlier studies.
In the first, carried out in the UK with a range of non-alcoholic beverages (carbonated
lemon, orange beverages, sparkling water, isotonic drinks, cola drinks and strawberry
milk), thickness and sweetness are identified as negative drivers of thirst-quenching
(McEwan and Colwill, 1996). In the second, a trained panel of American beer drinkers
rated 18 different beers for various sensory attributes. Refreshing intensity correlates
negatively with the intensity of the following attributes: “sweetness”, “thickness”, “flavour”,
“astringency” and “after-taste”. In addition, the attribute “thirst-quenching” is also strongly
correlated (r = 0.95) with perceived refreshing intensity (Guinard et al., 1998). These two
studies demonstrate that perceived thirst-quenching and refreshing are closely linked and
seem to be influenced by common sensory properties of drinks.
Non-oral sensory properties, such as colour, can also influence the perception of
refreshing sensation induced by foods and beverages as shown by Zellner and Durlach
(2003). In this study, three differently flavoured beverages (mint, lemon, and vanilla) were
prepared in eight different colouring versions (clear, red, blue, green, yellow, purple,
orange and brown). Three groups of American students tasted one set of eight beverages
among the three flavours and rated perceived refreshing intensity. For all three flavours,
colour significantly influences perceived refreshing intensity: for lemon, students rate the
brown beverage as less refreshing than the clear, purple and yellow versions; for the mint,
the brown and red versions receive the lowest refreshing intensity scores, and for vanilla,
all colours are rated as about equally refreshing. Zellner and Durlach suggested that the
clear beverages are often rated as most refreshing because of the association between
clarity and water, while the low refreshing ratings for brown coloured lemon or mint
flavoured drinks may have occurred because the colour is inappropriate (Zellner and
Durlach, 2003).
Another study combined structured interviews on constructs of what constitutes oral
freshness, with time-intensity measures of perceived oral freshness. The products
investigated were different candies menthol and fruit flavoured, and different beverages
(mineral water, cold water, apple juice and menthol water) [8]. This study was conducted
in 12 European native English speakers. From the interviews, 42 individual factors are
compiled and grouped according to six major clusters: in mouth cleanness, energy
(including “bubble” factor), waterness (including factors related to the experience of
“having water in mouth”) , coldness, taste (including mint and menthol factors) and smell.
In the time-intensity study, cold water, menthol water and menthol candy produce the
highest peak and mean scores for perceived freshness intensities. From this combination
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of approaches, the authors concluded that oral freshness is a multidimensional and
dynamic concept and that the importance of each factor in the construction of this
concept varies among subjects.
In summary, there is scientific evidence that olfaction, taste, texture and trigeminal
stimulations can influence refreshing perception. The concepts of refreshing, thirstquenching and freshness seem to be closely linked since they have common sensory
drivers. This statement is reinforced by the strong positive correlations reported in the
subjective ratings of refreshing and thirst-quenching intensities.
Psychophysiological origins of the sensory drivers of refreshing perception
The first part of this review highlighted the links between refreshing and sensory
perceptions linked to water consumption (coldness, clarity, i.e. transparency, liquid
texture, low sweetness, etc.). We now examine psychophysiological factors leading to
water consumption in an attempt to better understand how and why refreshing value is
attributed to these specific sensory characteristics.
Thirst
Thirst is the sensation of needing and/or wanting to drink. It has been variously described
as “the perception of one’s need for drink” (French et al., 1995), as “the consequence of
the need to moisten the mouth” (Brunstrom et al., 2000) and as “a physiological state
linked to fluid deficit” (Saltmarsh, 2001). Physiologically, thirst is part of the mechanism
that controls osmolality or volume of the extracellular liquid, and is regulated by the brain
in the hypothalamus (Lahera and Tresguerres, 1992). Two types of physiological thirst
are identified: “osmotic thirst”, the desire to drink related to an increase in solute
concentrations in the extracellular fluid, and “volumetric thirst” linked to a drop in vascular
volume such as during hemorrhage. These aspects of thirst do not however account for
all “regular” drinking which seems to be driven by a combination of: 1) unpleasant drymouth sensations (Brunstrom et al., 2000); and/or 2) habits or social cues (French et al.,
1995); and/or 3) the act of eating, possibly as an aid to the formation of a bolus that will
be easier to swallow (Brunstrom, 2002).
Brunstrom et al. have proposed three stages for a drinking episode: initiation,
maintenance and termination (Brunstrom et al., 2000). Initiation usually starts before
physiological fluid need, due to in-mouth sensations and/or social cues (e.g., a meal).
Initiation is followed by maintenance, corresponding to the period while thirst still persists
at a high enough level to induce continued drinking. Drinking termination, or satiation, is
triggered by oropharyngeal perceptions, gastric distension and/or post-absorptive
mechanisms or cognitive factors. During dehydration, a 1-1.5 % increase in extracellular
fluid osmolality is enough to induce a sensation of thirst (Lahera and Tresguerres, 1992).
Even during rapid dehydration, as may occur during marathon running, thirst appears to
provide an adequate indication of physiological water needs and stimulation to drink
(Noakes, 2007). Termination of drinking might be expected to occur when extracellular
fluid osmolality is normalized. However this does not seem to be the case. Even in
dehydrated subjects, alleviation of thirst occurs before plasma dilution is significant.
Osmolality usually starts to decrease about 10 minutes after cessation of drinking
(Saltmarsh, 2001). This delay probably represents the time needed for the water to cross
the intestinal wall and, via the blood, influence osmoreceptors in the brain. So if
alleviation of thirst occurs before osmolality is restored, other pre-absorptive mechanisms
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need to be involved. Stretch receptors present in the mouth, throat and stomach have
been postulated as mediators of the transmission of information to the brain on how much
liquid has been ingested (Saltmarsh, 2001), but additional factors may be implicated.
Sensations leading to termination of drinking may be based on a combination of
physiological responses to liquid passage through the oral, pharyngeal and gastric
spaces. Psychological factors such as learning are also likely to play a role, and these
are discussed in the last section of this review.
Under free-living conditions and with unrestricted access to water, young men become
thirsty and drank before fluid deficit is developed (Phillips et al., 1984). This phenomenon
is termed “anticipatory thirst” (Phillips et al., 1984). The mechanisms generating this thirst
in advance of physiological needs for fluid are not completely understood but probably
involve a learned anticipation of future fluid deficit, for example during exercise (Noakes,
2007) or to assist in mastication during eating (Brunstrom, 2002). In addition, levels of
thirst and drinking decrease with age, making older people more vulnerable to
dehydration (Kenney and Chiu, 2001) possibly due to impairment of such anticipatory
thirst signals.
Using recent advances in neuroimaging techniques it is now possible to identify the brain
areas involved in the emergence of sensations of thirst and thirst satiation in humans.
The sensation of thirst was recently related to increased activity in phylogenetically
ancient areas of the brain including the insula, the cerebellum and some parts of the
hippocampal gyrus and anterior cingulate cortex which displays extensive connections to
the hypothalamus (Egan et al., 2003). In addition, activity of this latter area was found to
return to baseline level immediately after thirst satiation, whereas this occurres about 15
min later in the other areas. This time corresponds approximately to the time needed to
complete normalization of osmolality. From these findings, the authors concluded that the
anterior cingulate cortex is likely to be responsible for the emergence of thirst (Egan et al.,
2003). Another neuroimaging study compared brain activity induced by the presence of
water in the mouth as a function of thirst (de Araujo et al., 2003). In particular, thirst
results in a higher level of activity in the caudal orbitofrontal cortex, compared with brain
activity measured after thirst satiation. According to the authors, activity in the caudal
orbitofrontal cortex in response to water may reflect the thirst level or motivational state of
the subjects. With the emergence of these new techniques, brain responses to
consummatory behavior can now be analyzed in a non-invasive manner, allowing better
understanding of the mechanisms governing perceptions including refreshing and
motivated behaviors.
In summary, it appears that thirst is often anticipatory (in that a sensation of thirst is
perceived before significant dehydration has occurred) and that, even when dehydrated,
thirst satiation seems to be triggered by relief of oropharyngeal symptoms such as
mouth/throat dryness and unpleasant in-mouth perceptions and that this occurs well
before extracellular fluid osmolality is normalized.
Mouth/throat dryness
Brunstrom (Brunstrom, 2002) suggested that drinking produces a progressive increase in
parotid saliva flow and that this contributes to mouth wetting and hence to satiation of
thirst. During dehydration in humans, infusion of water intragastrically restored hydration
but thirst persisted until water was provided via the oral cavity (Figaro and Mack, 1997).
In agreement with this, the perception of refreshing is often linked to alleviation of
oropharyngeal symptoms (e.g. from mouth/throat dryness to mouth/throat wetting) rather
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than from completed rehydration (Brunstrom, 2002). The influence of oropharyngeal cues
on drinking is complex and depends on hydration state, initial mouth dryness and volume
consumed. In addition, for a given volume consumed, water at 5°C is perceived as more
thirst-quenching than warmer (22°C) water (Brunstrom and MacRae, 1997). Eccles
proposed that cold water is more effective than warm water in reducing thirst in humans
because of the higher impact of cold water on salivation (Eccles, 2000). This idea was
partly based on a previous study where parotid salivary flow was monitored and that
demonstrated that water at 0°C produces more salivation than does water at room
temperature (22°C) (Pangborn et al., 1970). It is also supported by other findings showing
that the presence of water in the mouth at low temperatures (3°C or 10°C) is more
efficient to increase salivation than warmer temperatures ranging from 22°C to 44°C
(Brunstrom et al., 1997; Lee et al., 2006).
In the same way, menthone and menthol from peppermint oil are recognized as salivary
flow enhancers, most likely in relation to the perception of coldness they induce (Haahr et
al., 2004). Both are volatile compounds which give plants of the Menthe species their
typical mint smell and flavour together with a cold or cooling perception. Coldness
perception is mediated by specific receptors found in trigeminal cold-sensing neurons.
These receptors are widely distributed (on the tongue, in the nasal cavity, and in the
peripheral nervous system) and are activated by cold temperatures and by cooling agents
such as menthone and menthol (for a review of mechanisms of temperature perception,
see (Patapoutian et al., 2003)).
Other unpleasant in-mouth feeling experienced when thirsty may influence drinking
behavior (Phillips et al., 1984; Brunstrom, 2002). Indeed, dry mouth has been identified
as one of the factors linked to release of the volatile sulfur compounds responsible for
bad breath (Feller and Blignaut, 2005). Water consumption may therefore contribute to
the removal of this dry unpleasant taste and bad breath by its rinsing and cleaning
actions.
In developed countries, extreme conditions of dehydration are rare. During everyday
drinking, liquids are perceived as refreshing mainly because of their ability to alleviate
oropharyngeal symptoms. Indeed, cold liquids are refreshing in part because they lead to
mouth and throat wetness and in part because they enhance salivary flow due to cold
stimulation. This may partly explain why cold or thin liquids are perceived as refreshing
while astringent and thick liquids seem to dry the mouth even though the wetting
effectiveness of astringent solutions is as efficient as that of water under dry mouth
conditions (Guest et al., 2008).
Mental fatigue/energy
As stated in the two dictionary definitions cited in the introduction, refreshing perception is
closely related to increased energy, arousal and stimulation. “Mental energy” has recently
been described as a mood state linked to the ability to engage in cognitive work
(O'Connor, 2004; Lieberman, 2006). It also refers to a functional state of the brain, such
as cortical activation as measured with electroencephalography (EEG) (Lieberman, 2007).
In this section, we examine how hydration status may affect mental energy in terms of
mood, cognitive performance and cortical activation.
Mood
The effect of mild dehydration (1-3% loss of body weight as water) on subjective reports
of physiological and mood states has been investigated during drinking restriction
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(Shirreffs et al., 2004; Szinnai et al., 2005). As expected, dehydration induces increasing
reported sensations of thirst and mouth dryness. Dehydrated subjects also reporte
decreased ability to concentrate, reduced alertness and higher levels of fatigue, as
compared to hydrated controls. These results confirm previous observations (Cian et al.,
2001). The physiological changes and-or physical discomfort induced by moderate
dehydration seem therefore to negatively influence subjective perceptions of mental
energy.
Cognitive performance
Deterioration of cognitive performance during mild water restriction has been observed
for a variety of cognitive tasks. Dehydrated subjects generally display lower performance
in tasks involving sustained concentration or short-term memory when compared to
hydrated controls (Gopinathan et al., 1988; Cian et al., 2001; Suhr et al., 2004; Bar-David
et al., 2005). The physiological factors responsible for cognitive impairment associated
with dehydration remain largely speculative and may be linked to general brain
metabolism perturbation as has been reported in mice (Thurston et al., 1983) or to
changes in plasma hormonal levels as observed humans (Maresh et al., 2006). For
instance, the increased levels of the stress hormone cortisol observed by Maresh et al.
(Maresh et al., 2006) after moderate dehydration (5% loss of body weight) should be
enough to impair learning and memory processes (Het et al., 2005; Oei et al., 2006).
On one hand, water restriction, or dehydration, seems to increase mental fatigue and
impair mental energy. But on the other, water consumption may also influence mental
energy, either in a positive or negative manner, depending on prior thirst level. In an
unpublished investigation on the psychostimulant effects of caffeine reported by Rogers
et al. (Rogers et al., 2001), it was found that water consumption alone (used as a placebo
treatment in the study) impaires visual attention processes compared to the “no drink”
condition. Rogers and colleagues then examined the effect of water consumption on
subjective alertness and performance of visual attention as a function of prior thirst status
(Rogers et al., 2001). Water consumption induces immediate subjective sensations of
being “alert” and “revitalized” in both the low and high thirst participants, while effects of
water consumption on visual attention task performance dependes on prior thirst level
(improved performance when thirst is high; decreased performance when low, as
compared to a “no drink” condition). These findings suggest that the positive influence of
refreshing on cognitive performance, but not subjective alertness, may be sensitive to
hydration status. These results should, however, be treated with caution until they have
been confirmed.
Besides water consumption, some other sensory stimulations driving refreshing
perception have been shown to reduce mental fatigue and to improve mental energy.
These effects have been consistently reported for coldness-cooling perception induced
by odorous cooling compounds such as peppermint oil that contains both menthol and
menthone. Peppermint is indeed widely reported to possess stimulating and invigorating
properties. As compared to air, ambient exposure to peppermint oil was found to reduce
sleepiness (Norrish and Dwyer, 2005), improve subjective alertness (Moss et al., 2008)
and enhance performance in a range of cognitive tasks assessing attention and memory
functions (Warm et al., 1991; Barker et al., 2003; McBride et al., 2004; Ho and Spence,
2005; Moss et al., 2008). Eccles suggested that the activation of nasal cold-receptors by
menthol is similar to “taking a breath of fresh air,” leading to an increased level of
alertness, or “cortical activation” (Eccles, 2000), as defined earlier (Oken and Salinsky,
1992). To illustrate his point, Eccles reported the example of the use of ‘smelling salts’,
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containing trigeminal stimulants such as menthol and ammonia, to arouse someone who
has temporarily lost consciousness. From these observations, one could argue that the
positive effects of smelling cooling compounds on mood and cognition might be mediated
by neurophysiological mechanisms and, in particular, to the stimulation of the trigeminal
system responsible for the coldness perception.
Cortical activation
Quantitative measurement of the functional state of the brain using EEG can provide
evidence of changes in alertness, in terms of amplitude fluctuations of cortical oscillations
(Oken and Salinsky, 1992). Consistent with subjective and performance data, recordings
of EEG during water consumption reveale a greater enhancement of cortical activation in
water-deprived subjects than in thirst-quenched subjects (Schmitt et al., 2000; Hallschmid
et al., 2001). Following water consumption (1-3 min after drinking), cortical activation
remaines enhanced in the low-alpha frequency range (8-10 Hz) only in water-deprived
subjects (Hallschmid et al., 2002). These findings are consistent with other neuroimaging
data on brain areas activated by water consumption (de Araujo et al., 2003). While
activation of the insula does not differ in response to water as a function of hydration
status, activation of the orbitofrontal cortex occurres only during the period before
subjects had drunk to satiation; i.e., while drinking is still refreshing and rewarding. Taken
together, these results suggest that increased cortical activation and activation of the
insula observed during drinking reflect non-specific sensory and motor activity. In contrast,
activation of the orbitofrontal cortex correlating with the process of thirst-quenching is
consistent with the role of the orbitofrontal cortex in human motivation and related mood
states. The persisting enhancement of cortical activation after thirst-quenching is
hydration-dependent and may reflect direct measurement of refreshing perception.
Other EEG studies have reported increases in cortical activation induced by nasal
stimulation with peppermint (Badia et al., 1990; Klemm et al., 1992) thus supporting
Eccles’ hypothesis (Eccles, 2000) of refreshing-induced alertness by cooling stimulation.
Similar effects are induced by the chewing of gums flavoured with aromatic compounds
including peppermint oil (Masumoto et al., 1998; Morinushi et al., 2000).
Consumption of mint or mint-flavoured candies, chewing gum or carbonated mineral
water after a meal is common in many countries. Although, to our knowledge, the
potential link has never been investigated scientifically, it may be in part driven by their
perceived influence on alertness and-or digestion. If a person feels a lack of mental
energy after eating (a phenomenon known as the post-prandial dip in alertness (Smith et
al., 1990; Monk, 2005)), he or she may seek to improve feelings of energy by consuming
a refreshing food or drink. A possible link with digestion is the observation that
peppermint oil seems to facilitate digestion, possibly via a spasmolytic effect on
gastrointestinal tract muscle (Grigoleit and Grigoleit, 2005).
Appetite
Appetite status may be another modulator of how we perceive the refreshing attributes of
drinks (Rolls, 1993). To our knowledge, no systematic study has investigated the effects
of appetite on refreshing perception of foods and drinks. The nearest sensation to
appetite studied in relation to refreshing perception is the question on "filling" asked to
volunteers during an investigation of the perceived “drinkability” properties of beer
(defined as “the amount you think you can drink”) (Guinard et al., 1998). Drinkability is
increased by beers perceived as highly refreshing. In addition, refreshing and drinkability
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are both negatively correlated with strong flavour, viscosity, and aftertaste. Based on
these results, the authors concluded that weaker-flavoured beers are perceived as less
satiating and more refreshing than strongly-flavoured beers. This fits with previous
findings (McEwan and Colwill, 1996; Guinard et al., 1998; Labbe et al., 2007) showing
that high olfactory intensity, after-taste, sweetness and thickness are negative drivers of
refreshing since they represent sensory properties of high caloric content food and
beverage, generally not associated with water or other refreshing products.
Role of alliesthesia on refreshing perception
Alliesthesia, from the Greek word referring to altered sensation, is used to describe the
observation that a given stimulus can induce a pleasant or unpleasant sensation
depending on the subject's internal state (Cabanac, 1979). This phenomenon was
investigated for the pleasantness of water drunk at different temperatures as a function of
hydration status and body temperature (Boulze et al., 1983). Results showed that
dehydration increases the perceived pleasantness of cold water (0°C) and decreases the
pleasantness of warm water (50°C) compared to normal hydration status. In addition,
cold water is even more pleasant during hyperthermia than during normothermia. These
findings suggest that specific internal states, such as dehydration and hyperthermia, may
reinforce the effects of both positive and negative sensory drivers on refreshing intensity.
Alliesthesia is a phenomenon that should be taken into account when exploring
refreshing perception.
In summary, refreshing perception induced by drinking water seems to alleviate some
psychophysiological symptoms (including thirst, mouth dryness, and mental fatigue).
Coldness induced by water and other cooling compounds may actively contribute to this
process. As refreshing perception is associated with relief of psychophysiological
symptoms, a transfer of refreshing valence to coldness may therefore occur through
everyday associative learning. Further, internal incentives depending on physiological
states such as dehydration or high body temperature may further influence refreshing
perception.
Influence of experience on sensory drivers of refreshing
Role of learning during consumption
The degree to which a particular beverage or food is perceived as refreshing seems also
to depend on learning from everyday eating and drinking experiences. As presented in
the previous section, the positive experience of alleviation of unpleasant symptoms (thirst,
mouth-dryness, mental fatigue, feeling too hot) following consumption of water or of
another beverage leads to a learned association of the two and perception of the drink as
refreshing. This can explain why Zellner and Durlach found that, among a group of
American students asked to list foods, beverages and sensory characteristics they
considered to be refreshing, water is the most frequently mentioned drink (90% of
respondents) and cold temperature the most frequent sensory attribute (Zellner and
Durlach, 2002). Learning may explain other associations with refreshing, such as the
positive association with clear beverage colouring (Zellner and Durlach, 2003) and the
negative associations with sweetness, thickness (McEwan and Colwill, 1996; Guinard et
al., 1998; Labbe et al., 2007), intense flavour and after-taste (Guinard et al., 1998).
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Acid tastants elicit a salivary reflex. As with water drinking, this salivation leads to mouthwetting that is associated with thirst-quenching (French et al., 1995) and potentially with
refreshing. This may, in part, explain why acid beverages (Brunstrom et al., 2000), and
acid viscous fluids (Labbe et al., 2007) are often perceived as refreshing. Retronasal
olfactory characteristics of acidic fruits such as orange are also associated with
“refreshing” by consumers (Zellner and Durlach, 2002). A learned association between
sourness and an accompanying orange aroma may explain why this odour is often added
to products sold as “refreshing”.
Labbe et al. noted that, in peach and mint flavoured viscous products, coldness, acidity
and thickness are significantly correlated with perceived refreshing intensity (Labbe et al.,
2007). On cluster analysis, they found that differences in the importance of these sensory
characteristics for refreshing perception depends on the cluster considered. Coldness
and acidity are the main positive drivers for refreshing in Clusters 1 (36% of respondents)
and 2 (26%), respectively. For Cluster 3 (38%), coldness and acidity are less important
but thickness is the main negative driver of refreshing. The speculation that these
differences may be linked to past food experiences is supported by the further
observation showing that Cluster 1 (coldness driven) included a greater proportion of
heavy users of mint chewing gums compared to the other two clusters.
In summary, associative learning about sensory attributes often linked to positive
psychological or physiological consequences of water drinking may sometimes
generalize to the point where these attributes are perceived as refreshing in other
contexts. This type of learning about food experiences has been elegantly explored in
previous studies (Prescott, 1999; Koster et al., 2004; Mojet and Köster, 2005). However
further work is needed to demonstrate that “flavour-refreshing” learning is as robust as
other types of associative learning such as “sweet taste-olfaction” (Valentin et al., 2006;
Auvray and Spence, 2007).
Marketing communication
Advertisers use the term “refreshing” as a claim in the belief that it may influence
expectations about the product and later perception. These effects interact with personal
attitudes and/or cultural conventions about particular foods and drinks, acting via
memories and expectations. The influence of packaging features was investigated on
expected characteristics (including refreshing) of an unfamiliar fruit juice among English
consumers (Deliza et al., 2003). The authors manipulated background colour
(orange/white), information (none/medium: “1 liter, pure”/high: “1 Liter, 100% Pure,
Natural”), brand (none/minor UK brand/major UK brand), and image
(drawing/photograph). Results confirmed that packaging features can indeed influence
consumers’ expectations concerning refreshing intensity that is improved by the following
packaging conditions: white background, high information and major UK brand. The
authors speculated that the effects of brand on expected refreshment might come from
advertisements or from consumers’ own experiences.
Another example of how consumer learning may be used in marketing communication is
illustrated by a study exploring the expected refreshing properties of toothpastes as a
function of colour (Lee and O' Mahony, 2005). The “refreshing look” of 20 commercial
toothpastes of different colours was rated by 72 American consumers. Results showed
that transparent blue toothpaste is expected to be the most refreshing and off-white the
least refreshing. One can speculate that transparent blue may be perceptually the most
closely associated to water and therefore to refreshing. This idea is supported by the
results of a study showing that the same beverage evaluated in four differently coloured
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glasses (blue, red, green and yellow) is perceived as most thirst-quenching when
evaluated in the blue glass (Guéguen, 2003).
Zellner and Durlach reported that, for 50% of their sample of American students,
sweetness is an expected characteristic of a thirst-quenching beverage (Zellner and
Durlach, 2002). Using an open questionnaire study with 75 American students,
Clydesdale and co-workers demonstrated that the four colours most frequently
associated with thirst-quenching are clear, (36% of respondents), brown (24%), red (17%)
and orange (12%) (Clydesdale et al., 1992). These findings contrast with sensory studies
showing that brown (Zellner and Durlach, 2003) and sweetness (McEwan and Colwill,
1996; Guinard et al., 1998; Labbe et al., 2007) seem to be negative drivers of refreshing
perception. Part of this controversy may be explained by the fact that in some
circumstances, associating brown colour and sweetness with refreshing may result from
experiences of drinking a cola beverage to quench thirst (Clydesdale et al., 1992; Zellner
and Durlach, 2003). In addition, it is possible that frequently seeing advertising claims
that a major brand of cola is refreshing may further influence these associations.
Pleasantness as a consequence of symptom alleviation
Several studies have demonstrated that pleasantness and refreshing perception (or
related concepts) are positively correlated. In beverages: 1) for a wide range of different
drinks, rated by English assessors, thirst-quenching intensity and acceptability scores are
strongly correlated (McEwan and Colwill, 1996); 2) for drinks with different combinations
of flavour and colour, rated by American students, refreshing and hedonic ratings follow
similar patterns (Zellner and Durlach, 2003); 3) for bottled “nutritive drinks”, rated by
Japanese assessors, scores for refreshing intensity and overall palatability are positively
correlated (Kataoka et al., 2004); and 4) for the packaging rated as most liked is also
expected by UK consumers to contain the most refreshing beverage (Deliza et al., 2003).
Similar associations between pleasantness and expected refreshing intensity were
obtained among American students for toothpastes (Lee and O' Mahony, 2005). Lastly, a
study conducted with French consumers reported that refreshing intensity delivered by
viscous fluids is strongly correlated with perceived pleasantness (Labbe et al., 2007).
Since, as this review has noted, refreshing perception is often linked to physiological
needs, pleasure induced by refreshment may be a sign of satisfaction and fulfillment. This
interpretation is supported by the work of Brunstrom et al. (1997) who asked participants
to score preference for water at different temperatures (3, 13, 23 or 33 °C), under drymouth and non-dry mouth conditions. Pleasantness increases with lower temperatures
and drier mouth feel, and this is correlated with higher levels of saliva production.
Similarly, another study showed that respondents prefer and drink more cold water than
warm water during and after exercise (Sandick et al., 1984). In addition, they scores 16
°C water as more pleasant after exercise than on control days, possibly due to the more
noticeable relief of dry oral sensations induced by exercise. Guest et al. (2006) confirmed
that cold water is preferred under dry mouth conditions but, in contrast to the findings of
Sandick et al. (1984) and Brunstrom et al. (Brunstrom and MacRae, 1997), does not
observe enhanced saliva flow. This can be due to the small amount (0.75 and 1.5 ml) of
water ingested in the study of Guest and collaborators (Guest et al., 2006). Based on
these observations, it seems that refreshing substances are often perceived as pleasant
because they alleviate unpleasant feelings of dryness by either wetting the mouth of by
stimulating saliva production. This is in line with the concept of alliesthesia where
pleasantness is enhanced by the relief of physiological needs.
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De Araujo and collaborators (de Araujo et al., 2003), in a study involving volunteers
deprived of liquid for 6-8 hours, reporte a positive correlation between subjective
pleasantness of water (as it quenches thirst) and activation of several brain regions
including the orbitofrontal and anterior cingulate cortices. Other studies suggested that
the reward value, or pleasantness, of oral temperature, and of taste and flavour of food
are also represented in these brain regions (Rolls, 2006; Guest et al., 2007). Bringing
together these different oral representations in the brain regions linked to reward provide
evidence of neural integration of refreshing and pleasantness already shown at a
perceptual level.
In summary, pleasure experienced during the consumption of a refreshing food or drink is
likely to result from satisfaction of a psychophysiological need. In addition, the
phenomenon of alliesthesia, as shown for water temperature and preference, probably
plays a role in this association between refreshing perception and pleasure. Indeed, the
internal state of humans in a context of water deprivation or in a "need to be refreshed"
provides the motivation for consuming products with sensory properties that can alleviate
the associated psychophysiological symptoms. In such a context, this may explain why
sensory properties related to refreshing perception are also perceived as the most
pleasant ones.
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Conclusion
A schematic representation of how refreshing perception might be constructed is
proposed in Fig. 1, below. First, a range of physiological symptoms can be alleviated by
water drinking. In addition to body hydration mechanisms, water, through its specific
sensory properties, has the ability to alleviate psychophysiological symptoms, leading to
a refreshing perception and to pleasure. Everyday food and beverage experiences
reinforce these associations between sensory experiences induced by water
consumption and its related psychophysiological effects. Consequently, a refreshing
value (perceived or expected) may be attributed to foods or drinks: 1) sharing some
characteristics of water in terms of sensory properties (clear, cold, liquid); and 2)
alleviating psychological or physiological symptoms in a similar manner to water (e.g.,
acidic foods and drinks, cooling compounds). In addition, food experiences can be
expected to induce associative learning about perceptions conjointly present in refreshing
products (e.g. mint olfactory perception and coldness) and new beliefs in terms of
refreshing characteristics through marketing communication.
Figure 1: Schematic representation of the construction of refreshing perception
The alleviation of physiological symptoms by water drinking through its specific sensory
properties leads to refreshing perception and pleasure. The daily consummatory experiences of
water or any other food and beverage having sensory properties allowing to alleviate some of
described psychophysiological symptoms induce the transfer of a refreshing value to a non
exhaustive list of products attributes.
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3.2 Sensory foundations of refreshing perception
3.2.1 Sensory determinants of refreshing
Labbe,D., Gilbert,F., Antille,N. and Martin,N. (2009) Sensory determinants of refreshing.
Food Quality and Preference, 20, 100-109.
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3.2.2 Temporal Dominance of Sensation and Sensory Profiling: A Comparative Study
Labbe,D., Schlich,P., Pineau,N., Gilbert,F. and Martin,N. (2009) Temporal Dominance of
Sensations and Sensory Profiling: A Comparative Study. Food Qual Prefer, 20, 216-221.
Abstract
Temporal Dominance of Sensations (TDS) is a recent descriptive sensory method
consisting in assessing repeatedly, until the sensations end, which sensation is dominant
and in scoring its intensity. Compared to Time-Intensity, this method considers the
multidimensionality of the perceptual space over time. The objectives of this study were
first to compare description of viscous fluids containing different levels of odorants (peach
and mint), citric acid, cooling agent and xanthan gum obtained with TDS and with a
conventional descriptive method and then to explore the impact of mint and peach
odorant on long lasting perception. TDS provided reliable information close to standard
sensory profiling. In addition, TDS provided information on the dynamic of perception
after product consumption that was not available using a conventional profiling method
and that may be critical for the understanding of complex perceptions such as refreshing.
Keywords: Temporal Dominance of Sensations; Sensory profiling; Dynamic of perception
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3.3 Role of physiological factors in refreshing perception: mental energy and
saliva
3.3.1 The impact of refreshing perception on mental energy: changes in mood, cognitive
performance and brain oscillations
Labbe, D., Martin, N., le Coutre, J, &. J. Hudry to be submitted to Biological Psychology
Abstract
Previous sensory research has demonstrated that cold/cooling and sourness sensations
improve thirst-quenching and subsequent refreshing perception in food products. Other
research has suggested that thirst-quenching may improve mental energy in terms of
mood, cognitive performance and brain activation during rest. The aim of the present
research was to evaluate the effect of refreshing perception induced by an optimized
citrus-flavoured frozen snack on mental energy during mental workload. Comparison
treatments were a standard frozen snack differing in refreshing intensity but matched in
flavour and energy content, and a glass of water. Two experiments were conducted in six
and 18 healthy participants. Performances of sustained attention were assessed using a
rapid visual information processing task and related cortical activation with
electroencephalography (in Experiment I only). Results of Experiment I revealed that the
optimized frozen snack improved cortical activation in the alpha and beta powers known
to be involved in neural circuits of attention, working memory and sensory-motor
integration. The assumption that such enhancement of cortical activation is in favor of
optimal resources for task performance was demonstrated in Experiment II with a larger
sample of participants.
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Introduction
“Mental energy” is a term recently used by scientists to describe the psycho-physiological
resources necessary to engage, concentrate and perform well on cognitive tasks
(Lieberman, 2006; Lieberman, 2007; O'Connor, 2006). This term relates to a mood state,
mainly in terms of fatigue and alertness, to cognitive performance, as well as to a
functional state of the brain such as cortical activation measured with
electroencephalography (EEG). Mental energy may be influenced by a number of
variables including in particular the physiological status. Although the adverse effects on
mental energy of related factors, such as heat and dehydration, have been largely
examined, the scientific literature on refreshing and mental energy is quite limited.
"Refreshing" refers to a complex sensation similar to the one experienced during cool
water drinking, and corresponding to the relief of unpleasant physical symptoms such as
elevated body temperatures or mouth dryness (Brunstrom, 2002; Phillips, Rolls,
Ledingham, & Morton, 1984). Specific sensory properties of food and beverages in favor
of such a refreshing perception have been previously identified. Using a range of viscous
products varying in their compositions (levels of sugar, citric acid, cooling and flavour), it
has been shown that cooling was a key sensory driver of refreshing (Labbe, Gilbert,
Antille, & Martin, 2008). The importance of cold sensations in refreshing is corroborated
by other data showing that water at 5°C is perceived as more thirst-quenching than
warmer water at 22°C (Brunstrom & Macrae, 1997). As suggested, this superiority of inmouth cold stimulations may be due to both internal temperature and saliva flow changes
in favor of thirst-quenching perception. Sourness is another sensory characteristic
identified as a driver of refreshing in viscous products (Labbe et al., 2008) and of thirstquenching in beverages (McEwan & Colwill, 1996). Sour tastants add a refreshing value
to food products likely because they enhance the thirst-quenching experience by
stimulating saliva flow. Olfactory stimulations associated with sour fruits, such as lemon
and peach, seem as well to enhance refreshing intensity of yoghurts through a possible
perceptual associative learning mechanism (Martin et al., 2005). Taken together, these
sensory studies suggest that ingredients with cooling/cold and sour properties in food or
beverages may improve mouth-feel, thirst-quenching intensity and therefore refreshing
perception.
The idea that foods or beverages can reliably influence cognitive performance has
received great attention over the past decade and the concept of “functional food” has
been widely examined (for reviews see (Dye, Lluch, & Blundell, 2000; Messier, 2004).
Beside energy provided by macronutrients, other factors may influence cognition such as
refreshing induced by thirst-quenching. The impact of water consumption on some
aspects of cognition and mood has been previously investigated with regard to thirst
(Neave et al., 2001; Rogers, Kainth, & Smit, 2001). In the study of Rogers et al. (2001),
60 participants were randomly assigned to one of the following treatments: no-drink, 120
ml of water (10°C), 330 ml of water (10°C). They were a posteriori split into low- and highthirst groups according to pre-treatment thirst ratings. Compared to the no-drink condition,
the consumption of water induced immediate (2 min post-treatment), but not sustained,
subjective sensations of being “alert” and “revitalized” in both the low- and high-thirst
participants. In contrast, the effect of water consumption on performances of sustained
attention and working memory depended on prior thirst level, as measured with a 6-min
rapid visual information processing (RVIP) task. Hit rates were increased when thirst was
high and decreased when thirst was low. These changes were observed both 25 and 50
min post-treatment and occurred in a dose-dependent manner. Neave et al. (2001)
conducted a similar experiment in order to verify the benefits of drinking water upon
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mental energy in high-thirst participants. Thirst was induced by a ~11h (overnight) eating
and drinking restriction procedure. In addition to a 3-min RVIP task and a mood
questionnaire, the authors used a range of cognitive tasks measuring other aspects of
memory and attention. Twenty-four participants were involved in a randomized, balanced
crossover study employing a water (150 ml at 10°C) and a no-drink condition. Consistent
with previous findings by Rogers et al. (2001), the consumption of water improved
immediate subjective alertness as compared to the no-drink condition. However,
cognitive performances were neither positively nor negatively affected by the
consumption of water in participants in a high state of thirst. Substantial differences in
methodology (e.g., thirst and hunger levels of participants, experimental paradigm,
restriction procedure, treatments) between the two studies may explain the controversy in
findings. Yet it remains difficult to conclude on the potential effect of refreshing induced
by water consumption on mental energy, as measured with mood ratings and cognitive
tasks of attention and memory.
Non-invasive measurement of the functional state of the brain with EEG can provide
evidence of changes in mental energy, in terms of amplitude fluctuations of neuronal
oscillations reflecting various states of cortical arousal and activation (Barry, Clarke,
Johnstone, Magee, & Rushby, 2007). The effect of thirst-quenching on the brain state
during low cognitive load (mental count) was investigated with EEG in water-deprived and
thirst-quenched subjects after consumption of 400 ml of water (Hallschmid, Molle, Fischer,
& Born, 2002). Results revealed an increase of brain oscillatory activity in the lower alpha
band (8-10 Hz) in water-deprived subjects only, as compared to baseline measurements
obtained before drinking. Based on this, the authors proposed the enhancement of 8-10
Hz neuronal oscillations as an electrophysiological marker of thirst-quenching, viewed as
a reward response in relation to motivation. Similar changes in functional state of the
resting brain have been reported in the alpha (8-12 Hz) and beta (13-30 Hz) bands after
consumption of confectionery products with refreshing properties (Morinushi, Masumoto,
Kawasaki, & Takigawa, 2000). These changes were obtained after chewing a flavoured
gum the major constituents of which were aromatic oils with cooling (spearmint) and sour
(lemon balm) properties. The convergence of recent evidence suggests that oscillations
generated by the brain in different frequency ranges play a wide range of functions in
human cognition (for explicit reviews see (Klimesch, 1999; Tallon-Baudry, 2003; Ward,
2003). Theta (4-8 Hz) and alpha oscillations are two particular frequency bands with
strong cognitive correlates, largely assumed to reflect activity of multifunctional brain
networks involved in alertness, attention and memory processes. Faster oscillations, such
as those in the beta band, are thought to provide a means for the integration of
anatomically distributed processing and for the formation of transient neuronal
assemblies. Therefore, the enhancement of cortical oscillations obtained after water and
chewing-gum consumption may reflect an improvement of mental energy for optimal task
performance. However, in the absence of cognitive assessment, changes in brain
oscillatory activity should be interpreted with caution.
The literature on the relationship between refreshing perception and mental energy is
limited and inconsistent. Although it has been suggested that water drinking can enhance
mood and sustained attention performance in thirsty subjects (Rogers et al., 2001), the
improvement of cognitive performances was not replicated in subjects after an overnight
eating-drinking restriction procedure (Neave et al., 2001). The impact of refreshing
perception induced by water drinking or by food consumption on brain functional state
has not been thoroughly investigated and related changes in cognitive functioning have
not been assessed. There are a variety of reasons for the discrepancy in findings and for
the lack of studies on the effect of refreshing on human brain functions. Tangible
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refreshing is difficult to assess accurately using water as treatment since the effects will
mainly depend upon prior physiological status (Hallschmid et al., 2002; Rogers et al.,
2001). Inducing dehydration, or thirst, in a controlled and consistent manner is difficult
and may provoke non desired psychophysiological consequences, such as hunger,
stress or fatigue (Lieberman, 2007).
The aim of the present study was to explore further the relationship between refreshing
perception and mental energy. We partly replicated the experiment of Rogers et al. (2001)
and incorporated additional refreshing treatments and controls, as well as EEG
measurements. The benefits of a sweet frozen snack with sensory properties optimized
for refreshing perception was investigated on mood, cognitive performance (RVIP) and
related brain oscillations. Comparison treatments were a glass of fresh water and a
standard frozen snack matched in temperature and flavour with the optimized treatment.
We hypothesized that refreshing perception is not only a result of physiological benefits
consequent to eating or drinking, such as thirst-quenching, but also of benefits induced
on mental energy. In particular, we anticipated that sensory properties previously
identified as drivers of refreshing perception (Brunstrom & Macrae, 1997; Labbe et al.,
2008; McEwan & Colwill, 1996) would enhance brain oscillations implicated in cognitive
processes necessary to perform the task, (i.e. sustained attention and working memory)
leading to optimal task performance.
Materials and methods
Participants
Healthy right-handed volunteers between the ages of 18 and 35 were selected among
staff at Nestlé Research Center (Lausanne, Switzerland). Six participants (4 males) were
included in Experiment I and 18 other participants (7 males) in Experiment II. They were
healthy as determined by a medical questionnaire and had a BMI in the range of 18-30.
Exclusion criteria included eating disorders, diabetes, neurological disease, history of
head trauma, nose pathology or upper olfactory tract infection. All participants gave their
written consent and were paid for participation. The two experiments described here were
conducted in accordance with the Helsinki Declaration and were approved by the Ethical
Committee for Clinical Research of Medicine and Biology Faculty (Lausanne University,
Switzerland).
Treatment and design
A randomized, balanced, three-treatment crossover design was employed in order to
investigate the effect of treatment on mental energy. Participants received either 70g of
an Optimized frozen snack served at -17°C, 70g of a Standard frozen snack served at 17°C or a glass of 70 ml of Water served at 7°C. The Standard and Optimized frozen
snacks were based on a citrus flavoured sucrose water recipe. The cooling agent ‘L127039’ (Givaudan S.A., Dübendorf, Switzerland) were ingredients added to the frozen
snack optimized for refreshing at a ratio of 1:6. The two frozen snacks were matched for
energy (266 Kcal). The refreshing value of the Optimized frozen snack was previously
validated in another study held in 160 French consumers. The Optimized product was
judged significantly more refreshing than the standard frozen snack using 10-point rating
scales from ‘not at all refreshing’ to ‘extremely refreshing’.
Procedure
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Participants completed three separate test sessions starting at 11:00 am as well as a
prior practice session in order to familiarize themselves with the cognitive task and the
subjective ratings to be completed during the study. They were informed to refrain from
drinking alcohol for 12 h (overnight) prior to test sessions and to arrive at the EEG
laboratory at 7:30 am to get a standardized breakfast. Participants were then informed
that they should not drink or eat until the test session starts at 11:00 am. During one test
session, participants completed the mood questionnaire and cognitive task in this order
prior to treatment (Pre), immediately after treatment (Post1) and 15 min after treatment
(Post2). Following completion of pre-treatment assessments, 5 min were allocated to
consume the treatment. During the last period of the test session, 30 min after treatment
(Post3), participants had ad libitum access to water while completing a mood and an
additional distractive questionnaire of 10-15 min both. Without telling participants, the
amount of water drunk was taken as an indirect measure of their thirst levels.
For experiment I, six participants completed the practice and test sessions individually
while seated comfortably in front of a 17” computer screen, in a dimly lit, sound- and
electrically-shielded room. For experiment II, three groups of six participants completed
the sessions seating comfortably in front of a 17” computer screen, in dimly lit sensory
booths wearing an ear muff headset for noise reduction.
EEG data recording and reduction
In Experiment I only, continuous EEG activity was acquired through a Biosemi Active Two
system (Biosemi, Amsterdam, Netherlands) while participants performed the RVIP task.
EEG was recorded from 16 scalp active electrodes attached to a headcap and referenced
to the CMS-DRL ground (which functions as a feedback loop driving the average
potential across the montage as close as possible to the amplifier zero). Recordings were
sampled at 256 Hz with high- and low-pass filter settings of 0.33 and 128 Hz, respectively.
EEG-epochs
of
1000
ms
were
computed
using
CarTool
software
(http://brainmapping.unige.ch/Cartool.htm). The first EEG-epoch started randomly 5001500 ms following the presentation of the first stimulus onset; and the last EEG-epoch
started prior to the presentation of the last stimulus onset. In addition to the application of
an automated artifact criterion of ±100 µV, visual inspection of EEG data was performed
to exclude epochs containing artifacts due to facial movements or other sources of
transient noise. Pre-treatment epochs were used as baseline measurements in order to
isolate the task-specific activation changes induced by the treatments (Barry et al., 2007).
Artifact-free epochs were subjected to a fast Fourier transformation using a 100%
Hanning window. The mean power amplitude was computed for four separate frequency
bands (delta: 1–3 Hz, theta: 4–7 Hz, alpha: 8–12 Hz and beta: 13–30 Hz) for each
participant, scalp region (frontal: electrodes F3, Fz, F4, central: electrodes C3, Cz, C4,
parietal: electrodes P3, Pz, P4), test period (Pre, Post1, Post2) and treatment (Optimized,
Standard, Water).
Cognitive task
Performances of sustained attention were measured using a difficult version of RVIP task
requiring working memory for completion. Single yellow digits (1-9) were presented in
quick succession (100 digits/min) in a pseudo-random order on a black background
during 6-min (via E-Prime software package, E-prime 1.1, Psychology Software Tools,
Summit Software Company). Target sequences of three odd or even numbers had to be
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detected with a button press as fast and as accurately as possible. During one test, 48
possible hits (out of 600 trials) could be scored. Outcome measures for each test were
the number of hits and the mean reaction time for correct target detection.
Mood ratings
Mood was assessed using the standardized “Bond and Lader questionnaire” consisting in
16 bipolar 100 mm scales anchored at either end by antonyms (e.g., alert-drowsy, calmexcited) according to three dimensions of mood: 'alertness' (9 items), ‘contentedness’ (5
items) and 'calmness' (2 items). Scores for each dimension were the average number of
millimeters (max 100) from the individual scales contributing to the dimension.
Statistical analyses
EEG data were submitted to analyses of variance using a mixed model with repeated
measurements for each frequency band (delta, theta, alpha, beta). The factors ‘treatment’
(Optimized, Standard, Water), ‘topography’ (Frontal, Central, Parietal, Occipital) and
‘period’ (Post1, Post2) were set as fix, and the factor subject set as random. Cognitive
performances were submitted to analyses of variance using a mixed model with repeated
measurements for each outcome measure of the RVIP task (number of hits, reaction
times). The factors ‘treatment’ (Optimized, Standard, Water) and ‘period’ (Post1, Post2,
Post3) were set as fix, and the factor subject set as random. Mood ratings were
submitted to analyses of variance using a mixed model with repeated measurements for
each mood dimension (alertness, calmness, contentedness). The factors ‘treatment’
(Optimized, Standard, Water) and ‘period’ (Post1, Post2, Post3) were set as fix, and the
factor subject set as random. For EEG, performance and mood data, pre-treatment data
were considered as baseline measurements and used as covariate in the model. The
main advantage of this approach is elimination of between-subject variance in EEG
power, performance and ratings. These analyses focused on correspondence between
within-subject changes of subjective state and the changes in EEG spectral parameters,
not on the raw mood or performance and EEG data. The effect of the treatments on ad
libitum water intake was assessed by calculating analysis of variance with the factor
treatment set as fixed and subject as random. All calculations were performed with NCSS
software version 2007 (Number Cruncher Statistical Systems, Karysville, Utah, USA.).
Post-hoc pair comparisons were assessed by Student t-tests. Confidence level was set to
95% for all analyses
Results
EEG data, Experiment I
Results of statistical analyses performed on EEG data from Experiment I as a function of
treatment, topography and period for each frequency band are presented in Table 1.
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Table 1: F and p-values obtained for the three-way ANOVA performed on EEG data for each
frequency band
Fig. 1 illustrates the oscillatory activity in each frequency band as a function of treatment
and period. The factor treatment had a significant impact only on beta power. Pairwise
comparisons revealed that this effect was due to a higher beta power in the Optimized
condition during Post2, as compared to the Standard and Water conditions during Post2
and to all three conditions during Post1 (Fig. 1D). A significant period effect and treatment
x period interaction were obtained in the alpha power. These effects were caused by a
significant higher power in this frequency range in the Optimized condition during Post2,
as compared to the two other conditions during Post2 and to all conditions during Post1
according to pairwise comparisons (Fig. 1C). In addition, alpha power in the Water
condition was significantly higher than in the Optimized and Standard conditions during
Post1.
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Fig.1: Mean changes in EEG power as a function of treatment and period of time for A) delta, B)
theta, C) alpha and D) beta powers. Values are averaged across participants and scalp regions. a,
significantly higher than all treatment conditions during Post1 and than Standard and Water
conditions during Post2; b, significantly higher than Optimized and Standard conditions during
Post1; c, significantly higher than Optimized and Standard conditions during Post1 and than
Water condition during Post2; error bars, standard error of the mean; n.s., non significant effects
or interactions as revealed by analyses of variance).
Topography had a significant effect on oscillatory activity in the delta, theta and alpha
powers. The distribution of EEG power for each frequency band is given in Fig. 2 as a
function of topography and, for illustrative value, as a function of the three periods of time.
With time on test, EEG power tended to increase in the alpha and beta frequency bands,
to decrease in the delta frequency band and to remain unchanged in the theta frequency
band. Taking Pre values as covariate, these changes due to the factor period were
significant only in the EEG alpha power (Fig. 2C). A significant period x treatment
interaction was observed in this latter frequency band. Pairwise comparisons revealed
that this effect was due to higher alpha power during Post2 than Post1 in the frontal,
parietal and occipital regions. Other pairwise comparisons due to the factor topography
revealed that delta power was significantly higher in the occipital than in the other scalp
regions for all periods (Fig. 2A). Theta power was significantly higher in the frontal and
occipital than in the central and parietal scalp regions. Alpha power was significantly
higher in the occipital than in the other scalp regions (Fig. 2B).
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Fig.2: Mean changes in EEG power as a function of topography and period of time for A) delta, B)
theta, C) alpha and D) beta powers. Values are averaged across participants and scalp
treatments. Note that the Y axis scales were adapted for each frequency band.
Cognitive performances
Experiment I
For Experiment I, statistical analyses did not reveal any significant effect of the factors
treatment and period for the number of hits (F(2,24) = 0.35; p = 0.71; n.s.; and F(1,24) =
1.16; p = 0.29; n.s.; respectively) and reaction times (F(2,24) = 0.10; p = 0.9; n.s.; and
F(1,24) = 0.21; p = 0.65; n.s.; respectively). Interactions between the two factors were not
significant for both measures of hits and reaction times (F(2,24) = 1.56; p = 0.23; n.s.;
and F(2,24) = 1.04; p = 0.36; n.s.; respectively).
Experiment II
Performances of the RVIP task obtained in Experiment II are illustrated in Fig. 3. The
factors treatment and period displayed a significant effect on the number of hits (F(2,84) =
3.37; p = 0.04; and F(1,84) = 11.14; p < 0.01; respectively). Pairwise comparisons
revealed that the number of hits increased during Post2 as compared to Post1. In
addition, the number of hits scored during Post2 was significantly higher in the Optimized
condition as compared to the glass of water condition during Post2 and to all conditions
during Post1. Such effects of the treatment and period factors were not observed for the
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reaction times (F(2,84) = 2.09; p = 0.13; n.s.; and F(1,84) = 0.96; p = 0.33; n.s.;
respectively). No significant interaction was observed between the two factors for both
measures of hits and reaction times (F(2,84) = 0.63; p = 0.53; n.s.; and F(1,84) = 0.28; p
= 0.75; n.s.; respectively).
Fig.3. Mean changes in performances for the RVIP task obtained in Experiment II as a function of
treatment and period of time for the number of hits (left panel) and reaction times (right panel).
Error bars, standard error of the mean; n.s., non significant effects or interactions as revealed by
analyses of variance.
Mood ratings
Experiment I
In Experiment I, alertness was impacted by the factor period (F(2,53) = 7.96; p < 0.01)
due to a progressive decrease of ratings from Post1 (mean = 74.2 ± 3.4) to Post3 (mean
= 63.1 ± 3.4). No significant effect of treatment or interactions between the two factors
were observed on alertness ratings (F(2,53) = 0.56; p = 0.58; n.s.; F(4,53) = 0.60; p =
0.66; n.s.; respectively). Ratings of calmness and contentedness were not affected by the
factors treatment (F(2,53) = 0.92; p = 0.4; n.s.; and F(2,53) = 0.11; p = 0.74; n.s.;
respectively) and period (F(2,53) = 0.62; p = 0.54; n.s.; and F(2,53) = 0.98; p = 0.38; n.s.;
respectively) nor by any interaction between the two factors (F(4,53) = 0.30; p = 0.87;
n.s.; and F(4,53) = 0.22; p = 0.80; n.s.; respectively).
Experiment II
In Experiment II, alertness ratings were both impacted by the factors treatment (F(2,135)
= 17.3; p < 0.01) period (F(2,135) = 16.97; p < 0.01). As in experiment I, mean alertness
scores progressively decreased from Post1 (mean = 70.2 ± 2.1) to Post3 (mean = 60.4 ±
2.1). The effect of the factor treatment was due to significantly lower scores in the Water
condition during Post1, Post2 and Post3, as compared to the Optimized and Standard
conditions. Calmness was also both impacted by the factors treatment (F(2,135) = 6.55; p
< 0.01) and period (F(2,135) = 5.03; p < 0.01). Mean calmness scores progressively
increased from Post1 (mean = 63.4 ± 2.4) to Post3 (mean = 69.6 ± 2.4). These changes
were due to significantly higher calmness ratings in the Water condition during Post1,
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Post2 and Post3, as compared to the Optimized and Standard conditions. Contentedness
was impacted by the factor treatment (F(2,135) = 7.52; p < 0.01) but not by the factor
period (F(2,135) = 1.84; p = 0.16) nor by an interaction between these two factors
(F(4,53) = 0.53; p = 0.72). Contentedness ratings were significantly higher in the
Optimized and Standard conditions than in the water condition.
Ad libitum water intake
For both Experiment I and II, the factor treatment did not affected the amount of water
drunk (F(2,10) = 1.38; p = 0.29; n.s.; and F(2,25) = 0.31; p = 0.73; n.s.; respectively).
However, in both experiments, the amount of water drunk tended to be smaller in the
Optimized (Experiment I: 136.1 ± 26.9 ml; Experiment II: 109 ± 22.7 ml) than in the
Standard conditions (Experiment I: 171.1 ± 26.9 ml; Experiment II: 113 ± 22.7 ml) and
Water (Experiment I: 199.3 ± 26.9 ml; Experiment II: 135 ± 22.7 ml).
Discussion
The two experiments described here investigated the impact of refreshing perception
induced by a solid food on mental energy, as measured by subjective mood reports,
cognitive performances and related cortical activation. Refreshing perception was
optimized in a frozen snack by adding specific ingredients associated with sensory
properties previously identified as drivers of refreshing, namely cooling and sourness
(Labbe et al., 2008; McEwan & Colwill, 1996). In a first experiment (I) including 6
participants, superior benefits of refreshing were found upon alpha and beta frequency
ranges during Post2, suggesting an increase of brain activation for optimal task
performance 15 min after treatment. This suggestion was corroborated at the mood and
performance levels in a second experiment (II) including a larger sample of 18
participants.
Enhancement of brain oscillatory activity
In Experiment I, the beneficial effects of the frozen snack optimized for refreshing
perception was examined on brain oscillations during completion of a 6-min RVIP task
immediately (Post1) and 15 min after intake (Post2). Comparison treatments were a
Standard frozen snack matched in flavour and energy content, and a glass of fresh
Water. Power in two frequency bands (alpha and beta) was significantly affected by the
treatment condition. Oscillatory changes in the alpha power were the most prominent and
were affected by an interaction between the treatment and period factors (see Fig. 1C).
Three main changes in alpha oscillations are noteworthy. First, during Post1, EEG power
in the alpha power was increased in the Water condition, as compared to both frozen
snacks. This finding parallels those of a previous study of EEG and thirst-quenching
reporting a general increase of EEG power in the alpha range immediately after drinking
through a period of 7 min (Hallschmid et al., 2002). Second, whereas alpha power
remained unchanged in the Water condition with time on test, oscillatory activity in this
frequency range displayed a large increase in both frozen snack conditions from Post1 to
Post2. Changes in EEG power subsequent to food consumption have been previously
reported, mainly in terms of increased alpha and decreased delta activity during rest
(Hoffman & Polich, 1998; Wang, Szabo, & Dykman, 2004). Such changes were seen as a
general effect of arousal due to nutrient intake. Our finding that oscillatory activity in alpha
power was enhanced in Post2 in the two frozen snack conditions compared to the glass
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of water may also reflect an effect of nutrient intake, and in particular of carbohydrate.
However, to our knowledge, the effect of carbohydrate on brain electrical activity has not
been investigated over time. Third, the enhancement of alpha power during Post2 was
significantly higher in the Optimized than Standard condition, suggesting a superior
impact of the sensory properties optimized for refreshing perception. Similar oscillatory
changes obtained in the beta frequency range (see Fig. 1D) reinforce this assumption of
a superior impact of refreshing perception during Post2.
Possible mechanisms underlying changes in cortical activation induced by refreshing
perception
Cooling sensations are mediated by specific receptors found in trigeminal cold-sensing
neurons widely distributed over the tongue, throat and nasal cavity (Patapoutian, Peier,
Story, & Viswanath, 2003). These receptors are activated both by cold temperatures and
cooling agents. Their contributions to thirst quenching and consequent impact on arousal
have been highlighted in a review article (Eccles, 2000). The author suggested that
activation of the trigeminal system through a combination of cold and cooling stimulations
trigger specific brain mechanisms leading to an increased level of arousal and cortical
activation. A recent functional neuroimaging study in humans has identified a large
network of brain areas activated by oral temperature processing during water drinking
(Guest et al., 2007). This network includes the primary somatosensory cortices, parts of
the primary taste cortex, the premotor cortex, the fronto-parietal opercular cortex and the
orbitofrontal cortex. The assumption that trigeminal stimulations inducing cold/cooling
sensations can improve mental energy is corroborated by other studies using volatile
cooling compounds of menthol from peppermint oil that activates nasal cold-sensing
receptors. The smell of peppermint was found to reduce the feeling of sleepiness (Norrish
& Dwyer, 2005), to increase subjective alertness (Moss, Hewitt, Moss, & Wesnes, 2008)
and to increase performances on a variety of cognitive tasks (Ho & Spence, 2005;
McBride, Johnson, Merullo, & Bartow, Jr., 2004; Moss et al., 2008; Barker et al., 2003). It
is interesting to note that olfactory properties of other odorants may not induce any
benefit at all on mental energy, such as for example ylang-ylang (Moss et al., 2008), or
may trigger opposite relaxing effects, such as lavender (Moss, Cook, Wesnes, & Duckett,
2003).
Sour tasting compounds activate specific taste cells located on the tongue and
expressing proton sensing receptors responsible for sour perception. Of all taste
qualities, sour taste induces secretion of the largest volume of saliva by salivary glands
(Hodson & Linden, 2006). Increases of saliva flow rate and salivary cortisol concentration
have been observed in response to mental stress induced by cognitive activity (Bakke et
al., 2004). Acute exposure to stress, such as the one triggered by a cognitive demand or
challenge, may act as a mood enhancer and cognitive facilitator (Duncko, Cornwell, Cui,
Merikangas, & Grillon, 2007; Sandi & Pinelo-Nava, 2007). However, the link between
changes in saliva flow rate and the release of stress hormones such as cortisol or
adrenaline remains unexplored. By contrast, saliva has multiple well-known essential
functions in relation to mouthfeel (lubrication) and to the digestive process, including the
so-called ‘cephalic phase responses’ (Mattes, 2000). These physiological responses are
believed to prime the body to better absorb and use ingested nutrients. They coincide
with a rapid increase of insulin, promoting further glucose uptake into metabolic active
cells. This facilitation of glucose availability induced by digestive processes might be
improved by sour stimulation, possibly contributing to the superior impact of the optimized
recipe on cortical activation upon the standard recipe. The possible link between sour
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stimulation and brain activation remains purely theoretical as the impact of sourness on
mental energy has never been investigated. Further investigations are needed to answer
this question.
Taken together, our EEG findings suggest that, beyond carbohydrate or other nutrient,
the consumption of food with refreshing properties can enhance brain oscillations during
cognitive workload in frequency bands known to be involved in cognitive processes. The
activation of specific neural networks in response to trigeminal stimulation, together with a
hypothetic hormonal release or facilitation of glucose availability by sour stimulation, is
likely to improve mental energy.
Behavioral significance of enhanced brain oscillations during task performance
The success in detecting targets in the RVIP task is critically dependent on attentional
resources and on successful maintenance of the neuronal representation of previously
presented stimuli, up to three digits, in short-term memory. Numerous studies have
investigated the functional significance of alpha oscillations with regard to cognitive
processes. Whereas alpha oscillations typically increase during relaxation or when
subjects are in a passive state, recent research describes these oscillations as key
components of alertness, selective attentional processes and goal-directed behavior
(Jensen, Gelfand, Kounios, & Lisman, 2002; Klimesch, Doppelmayr, Russegger,
Pachinger, & Schwaiger, 1998; Palva, & Palva, 2007; Dockree, Kelly, Foxe, Reilly, &
Robertson, 2007). Using a visual continuous performance task of attention and memory,
Klimesch et al. (1998) found that alpha oscillations, and in particular in the posterior and
bilateral central regions, play a crucial role in neural circuits responsible for working
memory. In the present study, alpha oscillations were most prominent over the occipital
region while participant performed the RVIP task, but the treatment condition was not
affected by the topography. In addition to a clear involvement in cognitive processes, the
power of brain oscillations in alpha and beta bands have been identified as valuable
predictive measures of performance during tasks of sustained visual attention (Besserve
et al., 2008; Dockree et al., 2007). Oscillatory activity in the beta or higher frequency
ranges seems to play an important role in the functional organization of neuronal activity
that underlies visual perception, attention and working memory (Gross et al., 2004;
Tallon-Baudry, 2003). The possibility to predict performance based on EEG oscillatory
activity is further supported by other data showing that oscillations below 20 Hz,
especially those peaking between 7-12 and 16-20 Hz, are positively correlated with
driving performances (Liang et al., 2005). From these EEG data, alpha and beta
oscillations may account for an increase of antero-posterior synchrony with better
performance, possibly reflecting the activation of a neural network involved visualattention control.
In Experiment I, although clear effects of treatment condition on cortical activation were
found in the 6 participants involved, these effects were not found at the task performance
and mood levels. Nevertheless, using a larger sample of participants, we found effects of
treatment at the behavioral levels in Experiment II (see Fig. 3) that corroborate EEG
changes obtained during Post2 in Experiment I. The monitoring of changes in brain
oscillations have been validated as a highly sensitive method to assess the effects of
psychostimulants on the human brain (Siepmann & Kirch, 2002), even in the absence of
related changes in cognitive performance (Deslandes et al., 2005). In Experiment II, a
pattern of results comparable to the EEG changes observed in alpha and beta powers as
a function of treatment and period was obtained for the number of hits and for the
subjective ratings of alertness and calmness. Accuracy in detecting targets increased
during Post2, and this improvement was significantly higher in the Optimized than in the
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Water condition. Similarly, participants reported higher alertness and lower calmness
levels in the Optimized condition during Post2. Interestingly, participants felt happier after
consumption of any of the frozen snacks than the glass of water, contentedness ratings
being lowest in the latter condition.
In conclusion, data presented here indicate that key sensory properties of refreshing
perception can produce objective effects on cognitive performance, subjective effects on
mood, as well as physiological effects on brain activation. In agreement with previous
studies, our findings highlight the involvement of alpha oscillations in relation to refreshing
perception, subjective alertness and processes of sustained attention. Whereas the
benefits of trigeminal stimulations inducing cooling sensations are corroborated by other
data, the involvement of sour stimulations remains largely unexplored. Further
experiments will be conducted to elucidate the impact of individual sensory drivers of
refreshing on mental energy and their potential additive or synergistic effects. In addition
to providing insights into the impact of specific sensory stimulations into mental energy,
the present data open the discussion on the origin of the association between refreshing
and mental energy. In other words, is mental energy improved by certain sensory
properties because their combinations induce a refreshing perception? Or, inversely, are
combinations of certain sensory properties described as refreshing such as coldness and
sourness because they improve mental energy?
Acknowledgements
We thank Anny Blondel-Lubrano, Sylviane Oguey-Araymon and Maurice Beaumont from
the Metabolic Unit of the Nestlé Research Center for medical support, assistance with the
recruitment
and
scheduling
of
participants.
CarTool
software
(http://brainmapping.unige.ch/Cartool.htm) has been programmed by Denis Brunet from
the Functional Brain Mapping Laboratory (Geneva, Switzerland) and is supported by the
Center for Biomedical Imaging (www.cibm.ch) of Geneva and Lausanne. We thank JeanFrançois Knebel and Micah Murray (CIBM) for their support in the study, and Sabrina
Rami for her support in the statistical analyses.
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article 10
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Modulation of saliva flow, saliva lubricating properties and related lingering
perceptions by refreshing frozen snacks
Labbe,D., & Martin,N. (to be submitted to Physiology & Behavior)
Refreshing in food and drinks is a perception strongly related to mouth state after product
consumption. Oral dryness and roughness are perceptions negatively related to refreshing
whereas mouth wetting perception is a positive driver of refreshing perception. Since saliva
seems to be related to mouth wetting, we explored if salivary flow and saliva lubricating properties
could be potential markers of refreshing perception. To reach our objective, we explored saliva
flow and saliva lubricating properties after the consumption of a frozen snack optimized to be
perceived more refreshing than the standard snack. As key results, the optimized frozen snack
induced the highest salivary rate and saliva production with the lowest friction coefficient. These
results were validated perceptually. Indeed according to sensory results obtained with a group of
41 assessors, the optimized product delivered after consumption the most intense salivating
perception. Our finding seems to validate the positive association between refreshing and mouth
wetting perceptions.
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Introduction
Refreshing perception is a complex perception induced by: 1) the perceptual combination
of several key sensory stimuli such as high coldness, high sourness, low thickness and
low sweetness (Labbe et al., 2009a); and 2) the alleviation by these sensory stimuli of
psychophysiological symptoms such as mental fatigue, thirst, mouth dryness and hot
feeling (Labbe et al., 2008 submitted). The authors demonstrated that sensory drivers of
refreshing and psychophysiological impact of related sensory stimuli were close to those
experienced by water drinking.
Regarding psychophysiological parameters associated with refreshing perception, it was
assumed that consumption of a refreshing product enhances mental energy in terms of
cortical activation and cognitive performances thanks to these sour and cold sensory
properties (Labbe et al., 2008 submitted). Consumption of a glass of water, a product
generally associated with refreshing by consumers (Zellner and Durlach, 2002), has
already been shown as enhancing subjective alertness and performance during a
cognitive task (Rogers et al., 2001c) and increasing cortical activity (Hallschmid et al.,
2001). These results were obtained only for subjects who were submitted to a drink
restriction prior to the study ("thirst" condition). These findings suggest that the alleviation
of thirst and related mental fatigue contribute to refreshing perception induced by water.
Refreshing perception may also be associated with the alleviation of oral dryness and
consequently to mouth wetting perception (Brunstrom, 2002). This assumption was
corroborated by perceptual findings showing that refreshing was negatively correlated to
astringency (Guinard et al., 1998; Labbe et al., 2007), astringency being related to oral
dryness and roughness (Lesschaeve and Noble, 2005). The two objectives of the present
study were to compare the impact of two frozen snacks varying in refreshing intensity on
oral wetting in terms of: 1) saliva flow rate and saliva lubricating effect; and 2) salivating
perception. We hypothesized that the most refreshing product would induce the highest
saliva flow rate and production of saliva with the highest lubricating properties and that
physiological results could be validated by perceptual evidences. A glass of cold water
(+8°C) was used as reference for the physiological experiment since cold water was
reported as enhancing saliva flow rate (Pangborn et al., 1970; Brunstrom et al., 1997; Lee
et al., 2006)
At the end of the session subjects were free to drink ad libitum water. Amount of drank
water was considered as an additional indicator of the product abilities to perceptually
increase mouth wetting perception. Indeed thirst-quenching perception, which is strongly
associated with refreshing perception (Guinard et al., 1998), was suggested as positively
related to mouth wetting perception (Figaro and Mack, 1997; Brunstrom, 2002). Moreover
thirst sensation has also been described as "the consequence of the need to moisten the
mouth” (Brunstrom et al., 2000).
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Experiment 1: physiological measures
Material and methods
Products
Two flavoured citrus frozen snacks were used (standard frozen snack and optimized
frozen snack). They were similar to those described in the study from Labbe et al. (2008).
They mainly contained water (70g) and sucrose (25g). Difference in refreshing intensity
between both products was obtained by adding to the optimized frozen snack 0.04% of
cooling agent L-127039 provided by Givaudan (Dübendorf, Switzerland) and 0.20% of
citric acid as coldness and sourness are drivers of refreshing perception (Labbe et al.,
2007). Preliminary studies validated that compared to the standard frozen snack, the
optimized frozen snack was significantly perceived as being: 1) more intense in sourness
and coldness by sensory profiling carried out with trained subjects; and 2) more
refreshing in a consumer test. Frozen snacks were served at -17°C. Water was a glass of
70 ml of Vittel served at 8°C.
Subjects
Saliva was collected from six male volunteers between the ages of 18 and 35 selected
among staff at Nestlé Research Center (Lausanne, Switzerland). They were healthy as
determined by a medical questionnaire and had a BMI in the range of 18-30. Exclusion
criteria were eating disorders, diabetes, neurological disease, history of head trauma,
nose pathology or upper olfactory tract infection. All participants gave their written
consent and were paid for participation. The study was conducted in accordance with the
Helsinki Declaration and was approved by the Ethical Committee for Clinical Research of
Medicine and Biology Faculty (Lausanne University, Switzerland). Subjects were required
not to consume any food or drink after a standardized breakfast from 07:50 to 12:00 in
order to ensure comparable conditions among subjects.
Saliva collection and product administration
Subjects arrived at 12.00 at the Metabolic Unit. First saliva was immediately collected
before product consumption during 5 min (PRE). Subjects were seated and at rest, i.e.
they did not stimulate salivation, but waited till they had too much saliva in their mouth to
spit out into a beaker. Then, one of the three products (glass of water, standard frozen
snack or optimized frozen snack) was administrated. The complete consumption of a
frozen food lasted 5 min. One product was evaluated per day; the study was therefore
completed in three days. Product order was randomized between sessions and subjects
according to a Latin Square experimental design. During the sixty seconds following
product consumption, they were asked to swallow normally in order to eliminate in-mouth
product residues and finally during 5 min saliva was collected. All collected saliva
samples were weighed and kept in their individual beakers at 37ºC in a water bath till
instrumental analyses were conducted at 13.00 pm
Assessment of saliva lubricating effect by measurement of friction coefficient between
tongue and palate using tribometry
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Saliva lubricating properties directly affect in-mouth friction between tongue and palate.
Indeed the higher the saliva lubricating effect, the lower the friction coefficient. In vivo
measurement of friction coefficient between tongue and palate is technically challenging,
this is why friction coefficient measurements were conducted using a tribometer. The
device was developed to reproduce the friction movement between the tongue and the
palate (Fig.1) (Ranc et al., 2005; 2006). The tribometer enforces a reciprocating sliding
motion between a PCTFE (polychlorotrifluoroethylene) hemisphere which stands for the
palate (it was chosen due to its Young’s modulus that was close to the hard palate bone
elastic modulus) and a rough silicone rubber which represents the tongue. It was selected
due to its viscoelastic properties that were close to those of the tongue. Moreover, the
Young’s modulus of the silicone samples (1.5mPa) was in the same order of magnitude
as the tongue elastic modulus.
Contact is ensured through application of a normal force. The tangential force is
measured, and from this the friction coefficient is determined automatically. During the
test, the temperature was maintained at 37°C with a temperature-controlled vessel to
which the silicone surface can be fixed. The pin in PCTFE is first screwed at the end of
the loading arm and comes into contact with the silicone surface with a contact load not
exceeding 0.04 N. Then, a load of 0.5 N was applied thanks to dead weights that were
suspended on the tribometer arm, to obtain a Hertzian pressure of 310 kPa that
approximately corresponds to the pressure recorded in vivo with pressure transducers.
To reproduce the velocity of tongue movements found in the literature, a triangular
waveform with a frequency of 1 Hz (that corresponds to a velocity of 10 mms-1) was
applied. The displacement of the pin in PCTFE is measured thanks to the relative motion
of a laser placed on the arm and recorded by a photodetector (Hamamatsu, France).
The systems allows measuring the tangential force magnitude (Ft) corresponding to the
friction force, the normal force and the temperature. Thus, the friction coefficient (µ) is
obtained by dividing the friction force by the applied normal force (Fn) magnitude
according to the following equation:
µ=
Ft
Fn
µ: Coefficient of friction
Ft: Friction force (N)
Fn: Normal force (N)
The friction coefficient is first measured under dry conditions until the plateau region is
reached. Then, the motion is stopped and the loading arm is lifted up with a wedge, to
add 0.5 mL of saliva with a micropipette. The pin in PCTFE comes again into contact with
the silicone surface and the motion is started again after stabilizing the temperature.
To calculate the average friction coefficient during dry and lubricated contact, at least 100
data points of the plateau region of the friction coefficient-versus-time curve are taken into
account. All tests were performed in duplicate and new silicone surfaces and PCTFE pins
are used for each test.
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Fig.1 (a-d): Representation of a tribometer
a: The tribometer
b: The tribo-pair material does a reciprocating sliding motion between a hard hemisphere of
PCTFE and a flat soft silicone surface to simulate the contact tongue/palate
c: A hard hemisphere in PCTFE
d: The silicone surface.
Amount of ad libitum drunk water
At the end of the session subjects were asked to fill in a questionnaire to occupy them for
10 min during during which they were free to drink ad libitum Vittel mineral water. Replies
were not used as an outcome. An individual 1500 ml bottle of water was available and the
spontaneous amount of water drunk was measured.
Statistical analysis
The amount of produced saliva was expressed in g collected during five min. Product
impact on saliva production collected after product consumption (POST) was measured
according to a product (Optimized frozen snack, standard frozen snack, glass of water) x
Subject covariance analysis with interactions. Saliva production measurement before
product consumption (PRE) was considered as baseline and used as a covariate variable.
Product impact on friction coefficient in POST was measured according to a product
(Optimized frozen snack, standard frozen snack, glass of water) x Repetition x Subjects
covariance analysis with interactions. Friction coefficient measurement in PRE was
considered as baseline and used as a covariate variable.
The effect of product consumption on the amount of drunk water at the end of the session
was analysed by a product (Optimized frozen snack, standard frozen snack, glass of
water) x Subject analysis of variance.
Analyses of Variance (ANOVA) were calculated, using NCSS software version 2007
(Number Cruncher Statistical Systems, Karysville, Utah, USA.). Post-hoc pair
comparisons were conducted by a Student t-test. Confidence level was set to 95% for all
analyses
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Results
There was no significant impact of products on the salivary production after consumption
[F(2,26)=0.73, P=0.49] (Fig.2). But comparing saliva product before and after
consumption of the products, Student paired T-test showed a significant increase of the
saliva production after consumption of the optimized frozen snack (p-value<0.05).
Saliva produced (g) during five min (panel mean)
4
*
3
2
1
0
PRE
Glass of water
POST
Standard frozen snack
Optimized frozen snack
Fig.2: Impact of the three products on mean saliva production (+/- SEM). A star means a
significant effect (p<0.05) according to Student t-test pair comparisons
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There was a significant impact of products on friction coefficient [(F(2,61)=15.6, P<0.01).
Saliva produced after consumption of the glass of water led to the highest friction
coefficient whereas saliva produced after consumption of the optimized frozen food
consumption had the lowest friction coefficient (Fig.3). According Student t-test paired,
friction coefficient remained constant before and after consumption of the glass of water
whereas it decreased after consumption of both frozen snacks (p-value<0.05%).
Friction coefficient (panel mean with 2 repetitions)
0.6
a
a
a
a
0.5
b
0.4
c
0.3
0.2
0.1
0
PRE
Glass of water
POST
Standard frozen snack
Optimized frozen snack
Fig.3: Impact of the three products on mean saliva friction coefficient (+/- SEM). Products with the
same letter are not significantly different according to Student t-test pair comparisons (p<0.05)
The amount of drunk water at the end of the session did not significantly change
according to the product consumed during the session [F(2,10)=0.87, P=0.44]. But the
optimized refreshing snack consumption showed a trend in inducing the lowest volume of
water consumption (Fig.4)
Amount (g) of drunk water (panel mean)
250
200
150
100
50
0
Glass of water
Standard frozen snack
Optimized frozen snack
Fig.4: Product impact on the mean amount of drunk water (+/- SEM).
* means significant effects or interactions (p<0.05) as revealed by analyses of covariance
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Experiment 2: Sensory evaluation
Material and methods
Products
The two flavoured citrus frozen snacks of experiment 1 was used, but not the glass of
water since sensory properties of water and frozen snacks are not comparable.
Subjects and attributes
An external panel of 41 subjects experienced in TDS measurement was recruited. They
had previously been screened by tests selected in ISO 8586-1 (1995). They did not follow
specific trainings for this study since they knew the TDS method and the five attributes
used in this study: flavour intensity, sweetness, bitterness, coldness and salivating. The
list of attributes was set up following a 90-min session where a list of terms was
generated and then reduced according to ISO 11035 (1995). The reduction phase
consisted in deleting: 1) attributes what did not characterise all products; and 2)
synonymous and antonymous attributes (the most meaningful attribute to keep was
defined by the panel). For instance sourness and astringency were not selected since
they were synonymous and antonymous with salivating. All the attributes were presented
simultaneously on the computer screen with their corresponding 10 cm unstructured
linear scale, anchored at the extremities with “not at all intense” and “very intense”.
Method principle
TDS is an adapted time-intensity method which allows to consider the multidimensionality
of the perceptual space over time (Pineau et al., 2003; Labbe et al., 2009b). This
descriptive sensory method consists in assessing iteratively at each specific time until the
perceptions end, which perception is dominant and in scoring its intensity. In this study
dominant was the most intense sensation. The evaluation was conducted over 4 min
during eating and then continued for 6 min after eating.
When the subject started to suck the product, he clicked on a start button on the screen
to begin the evaluation. During an evaluation, the subject had to select the attribute
considered as dominant and score this attribute on a linear scale. When the dominant
perception changed, the subject had to score the new dominant sensation. The subject
was free to choose several times the same attribute or conversely to never select an
attribute as dominant.
Tasting conditions
Data acquisition was carried out on a computer with FIZZ software Version 2.20E
(Biosystemes, Couternon, France). Products were evaluated by all subjects during one
session and presented according to a design balancing position and order effects, based
on Williams Latin Squares.
Products were coded with three-digit random numbers and the 70 ml portion was served
at -17°C on a plastic plate. Rinsing was done between products with water and unsalted
crackers during a 5-min break. Tests were conducted in an air-conditioned room (22°C),
under white light in individual booths.
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The products were consumed according to a standardized procedure, i.e. subjects were
asked to suck every 5 s the product and to swallow normally during the evaluation.
According to this procedure, duration required for complete product consumption was
around 4 min.
Statistical analyses
TDS data were represented by curves showing for each product the percentage of
subjects who selected the attribute as dominant at a specific time, i.e. the dominance rate.
The frequency of selection of each attribute as dominant by all subjects is represented in
percentage (Axis Y) each 1.5 s, which corresponds to the data collection interval, during
the 5-min period (Axis X). The 200 points of the evolution curve were smoothed using a
non-weighted moving average with a 10 points window. The curve showed over time the
dominant attributes at a panel level. For each 1.5-s step, the proportion of scoring for an
attribute was considered as significantly dominant when above Ps (the smallest
dominance rate to reach to be significantly above the chance limit) as defined according
to the equation (2), and as dominant when between P0 (dominance rate obtained if all
subjects scored by chance) and Ps, in this case we considered the results as a trend.
Ps = P0 + 1.64
P0 (1 − P0 )
n
With chance level P0 = 1/(k+1), k being the number of attributes, n the number of
subjects and 1.64 the one-tailed normal law z value for α=5%.
Results
TDS curves showed that after consumption of the standard frozen snack, no clear
dominant sensation could be highlighted (Fig.5a) since salivating and coldness curves
were above the significant level only during a short time period. Regarding results
obtained after optimized frozen food consumption (Fig.5b), lingering perception was
clearly dominating in terms of salivating perception since the salivating attribute curve
was above the significant level during 100 s and then slowly decreased to reach the
chance level 500s after the end of consumption. In addition, for salivating attribute, panel
agreement was higher for the optimized frozen snack than for standard frozen food with a
maximal dominance rate of 40% and 33%, respectively.
Interestingly, during consumption, the flavour intensity was significantly different between
products. Flavour intensity of the optimized frozen snack was perceived dominant during
all 4 min consumption with a maximum dominant rate of 45% whereas flavour intensity
delivered by the standard frozen snack was dominating during very short periods and
with a lower panel agreement since the maximal dominance was 33 %.
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a)
b)
Fig.5a-b: Dominance rate in terms of subject percentage over a 10 min time period for a)
standard frozen food and b) optimized frozen food. Attributes are representing as
following:
—— flavour intensity — — salivating —— coldness ‧‧‧‧‧‧bitterness
— — sweetness
and
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Discussion
The saliva flow rate after consumption did not significantly differ between the three
products. But only the optimized product perceived as the most refreshing significantly
enhanced the saliva flow rate after consumption. The mouth wetting ability of the product
was shown as a physiological property of products perceived as refreshing and thirst
quenching (Figaro and Mack, 1997; Brunstrom, 2002). This effect is probably due to the
addition of citric acid that elicited a salivatory reflex (French et al., 1995). Cooling agent
might also enhance saliva production since a previous study showed that cold water (at
0°C and 3°C caused more salivation than water at 22°C or 33°C (Pangborn et al., 1970;
Brunstrom et al., 1997). In addition a study showed that a mouth pre-treatment with
menthol, a cooling agent, increases in-mouth coldness of water (Green, 1984). The
cooling agent might have potentialised the enhancing impact of the frozen snack on
saliva flow.
Comparing both frozen snacks, optimized snack consumption led to saliva production
with the highest lubricating properties in terms of friction coefficient. A saliva production
with a high lubricating effect likely reduces perception of oral roughness, dryness or
astringency that are negative drivers of refreshment (Guinard et al., 1998; Brunstrom,
2002; Labbe et al., 2007). This latter finding can physiologically explain the higher
refreshing perception caused by the optimized frozen snack compared to the standard
frozen snack. The physico-chemical mechanisms underlying the enhancement of saliva
lubricating properties by citric acid and or cooling agent remains unclear. In addition our
results are not consistent with a recent finding showing that citric acid aggregates and
precipitates salivary protein leading to disruption of the salivary film lubricating the oral
mucosa
At a perceptual level the optimized frozen snack was perceived significantly more
salivating than the standard frozen snack in terms of dominance and duration. In addition,
the higher dominance of fruity flavour intensity induced by the optimal frozen snack might
be explained by: 1) an increase in volatile release by citric acid; or 2) perceptual
interactions between olfactory and sour taste caused by citric acid. We support the
perceptual interaction origin since such an effect was previously shown (Bonnans and
Noble, 1993) and another study did not show effect of citric acid on volatile compounds
release from orange and strawberry flavoured sucrose beverages (King et al., 2006).
Finally the amount of drunk water was the lowest after consumption of the optimized
frozen snack consumption. Even if this result is only a trend, it might be related to the
increase in saliva flow rate and salivating perception since Brunstrom et al. (2000)
showed that an increase of saliva production during drinking reduces the need to
continue drinking for mouth wetting purposes.
Conclusion
The physiological and perceptual outcomes validated our two working hypotheses, i.e.
saliva flow rate was the highest and saliva production was the most lubricating after
consumption of the optimized frozen snack. Consequently this product delivered a more
intense oral wetting perception. This finding may partly explain that the optimized frozen
snack was perceived more refreshing than the standard frozen snack by consumers
since oral wetting is a positive driver of refreshing perception. Additional studies are
required to understand: 1) the impact of citric acid and cooling agent on saliva flow rate
and lubricating properties; and 2) mechanisms underlying the increase in saliva
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lubricating effect by citric acid and cooling agent comparing saliva chemical composition
when collected after consumption of each frozen snack.
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4
PART 4: General discussion and perspectives
4.1 Familiarity and exposure strategy modulate perceptual interactions
The role of product familiarity and exposure strategy, i.e. analytical task (sensory profiling)
vs. synthetical task (sorting task with free verbalisation), in the modulation of perceptual
interactions during food experience has been clearly established in the set of experiments
performed for the PhD work. Differential effects of product familiarity on perceptual
interactions were demonstrated using sensory profiling. As expected, vanilla odorant
increased sweetness in coffee and coca drinks whereas it surprisingly increased
bitterness in caffeinated milk. We proposed that an interaction between unpleasantness
and bitterness can explain this finding. The planned fMRI study will aim at investigating
the neural correlates of these psychophysical findings. Two main assumptions are
considered to explain the mechanisms underlying changes in taste perception induced by
vanilla odour in both the familiar and unfamiliar drinks. First, modulation of taste may be
driven by bottom-up processes that are involved in sweet and bitter perceptions through a
differential activation of taste receptors and/or primary taste cortex. Second, other topdown processes may result in differential activation of the integrative orbitofrontal cortex
responsible for flavour integration and reward/pleasure evaluation. We favor the second
mechanism since we assumed that unpleasantness could be at the origin of bitterness
enhancement during sensory/reward integration.
Concerning the impact of exposure strategy on coffee odour perception, we showed
differences between product sensory maps built up from sensory profiling and sorting
data. We assumed that perceptual interactions modulate perception and therefore
product grouping done by consumers. This is highlighted by the use of taste-related
words, such as ‘bitterness’, by consumers to describe the coffee smell. This term was
relevantly used to describe Robusta coffees which are known to be bitter due to their
chemical composition. In addition specific coffee olfactory notes (e.g. roasty, earthy) are
typical of Robusta coffees and can therefore predict coffee taste (Lindinger et al., 2008).
Olfactory taste association constructed during prior repeated exposure to coffee may
impact coffee aroma perception. The role of attentional strategy on perceptual
interactions was also confirmed during co-exposure to unfamiliar odorant and sweet taste.
Indeed synthetical attentional strategy exposure (triangle test) to the combined stimuli
promoted the construction of perceptual interactions whereas the analytical strategy
(sensory profiling) did not. This is probably because assessors from the analytical
attentional strategy group learnt during training to dissociate perceptual dimensions on
the studied products before conducting the sensory profiling. This approach limited the
construction of perceptual interactions and their impact on sensory profiling results.
4.2 Multiplicity of perceptual interactions between olfactory, trigeminal and tactile
perceptions
As expected, and already shown in the literature, we highlighted that olfaction modulates
taste perception (sourness, bitterness, sweetness) through perceptual interactions. As
new findings, we also demonstrated that olfaction impacts cold trigeminal perception and
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modulates taste-taste and trigeminal-taste interactions. The multiplicity and overlapping
of
olfactory-trigeminal-taste perceptual interactions in complex food systems should be
taken into account in product formulation since the use of odorants may have unexpected
effects on overall perception. More investigations are required to better understand if
interactions between cold trigeminal and bitter perceptions have only a perceptual origin
or if physiological factors are involved, for instance via common peripheral transduction
mechanisms consecutive to receptor binding by trigeminal compounds.
4.3 Food experience contributes to construction of refreshing perception
We demonstrated that refreshing perception induced by our range of viscous liquid
products is driven by four sensory dimensions: sweetness, coldness, acidity and
thickness. A large majority of consumers agreed that the sweeter the product the less
refreshing. Considering the three other sensory characteristics, a high coldness, a high
acidity and a low thickness enhance refreshing intensity, but the importance of these
drivers differ among consumers. Among the group of consumers who mainly associated
refreshing perception with coldness, we identified a significantly higher proportion of
chewing gum heavy users for breath refreshing purposes. Since these chewing gums
generally contain cooling agents, this might explain why consumers strongly associated
coldness and refreshing perception. The limited number of questions asked in the
consumer questionnaire did not allow to explain the association between refreshing
perception and either high acidity or low thickness for each of the other consumer groups.
But we might assume that consumers who frequently drink sour fruit juice such as orange
for refreshing purpose are more prone to associate acidity and refreshing. As for
perceptual interactions, food experience may strongly impact the construction of complex
perceptions.
The other novelty of this work about refreshing complex perception compared to studies
previously done on creaminess (Richardson Harman et al., 2000; Tournier et al., 2007)
and freshness (Peneau et al., 2006; Peneau et al., 2007) was the identification of
psychophysiological determinant strongly related to refreshing perception, i.e. saliva
properties and mental energy. However, the following questions have still no answer: 1)
what is the respective contribution of the citric acid and cooling agent in modification of
saliva properties and in mental energy enhancement; and 2) is the impact on
psychophysiological determinant required to perceive a food as refreshing?
As next step, it would be interesting to recruit consumers having filled a food habit
questionnaire related to refreshing food and to test the impact of three liquid viscous
product containing either only citric acid or only cooling agent or neither citric acid nor
cooling agent (and therefore very sweet) on refreshing intensity rating and mental energy.
It would then be possible to explore the relationship between consumer food habits and
potential consumer groups differing in terms of refreshing sensory drivers (acidity or
coldness) and to highlight if mental energy is more impacted by cold or acid product
according to the consumer group.
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4.4 Alternatives to attribute rating (sensory profiling) for further exploring
perceptual interactions and understanding consumer preference
We successfully used the attribute rating approach in my PhD work to explore: 1) the role
of familiarity on perceptual interaction; 2) the impact of attentional strategy during
exposure on the construction of perceptual interaction via sweetness rating procedure; 3)
multi-sensory interactions in complex food systems; and 4) the sensory drivers of
refreshing perception. Indeed by limiting the exposure to the studied products during
training, assessors do not learn to dissociate specifically all product sensory dimensions.
Attribute rating therefore remains suitable for exploring perceptual interactions and
understanding consumer perception. For further exploring mechanisms underlying
perceptual interactions and understanding consumer preference, it would be interesting
to combine the attribute rating method with different paradigms with a more synthetical
approach as we did with the sorting task. However the sorting task procedure did not
allow the exploration of perceptual interactions between specific senses.
Nguyen et al. (2000) used a method based on Garner (1974) filtering paradigm
developed for investigating the notion of separability and integration between two sensory
dimensions in the field of visual and auditory perceptions. The notion of separable
dimension was defined by Garner (1974) as the possibility to give a perceptual judgment
on one dimension independently from perceptual information provided by another
dimension. Nguyen and co-workers (2000) applied this psychophysics method to
chemical senses. They asked assessors to make categorization judgments according to
the level of the sweetness intensity of a range of four samples combining factorially two
concentrations of sucrose and two concentrations of vanillin. First assessors are trained
to taste and memorize the four different samples and then they had to categorize
samples as a function of sweetness (high vs. low). Categorization performance on sweet
intensities was impacted by vanilla. This finding demonstrated that assessors were
unable to ignore the olfactory signal when categorizing the solutions according to taste
probably because of perceptual interactions. Same findings were obtained on sourness
categorization using four samples combining factorially two levels of lemon odorant and
acid tastant.
Signal detection theory is based on the Thurstone's model stating that a same repeated
stimulus induces a slightly different perception distributed according to a normal law (see
Fig.1). According to the signal detection theory (Green and Swets, 1966), when one has
to distinguish a signal, she/he has to distinguish in reality a signal plus a noise from a
noise (Fig.2).
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Fig.1: Distribution of perceived intensity
for a same stimuli (according to Thurstone's model)
170
Fig.2: Representation of the
Signal Detection Theory
A noise is a group of spontaneous and disorganized stimulations having an external or
internal origin (Reuchlin, 1978). Subject response is related to: 1) his sensitivity to the
signal, i.e. his capacity to distinguish "noise" from "noise+signal", this can be translated
for example by the discrimination between "control" and "test" samples during a different
from control task; and 2) his strategy, i.e. some subjects have a tendency to reply more
often "control" and other subjects "test". Subjects are therefore called "conservator" and
"liberal", respectively. This means that the perceptual difference separating the test from
the control product is smaller for the liberal subject than for the conservator subject.
During discriminative tasks the sensitivity and strategy indices can be calculated (O'
Mahony, 1988). Signal Detection Theory approach can also be applied to categorization
task on a specific product sensory dimension. In this situation, subjects reply is generally
"low intensity" or "strong intensity" instead of "control" or "test".
To summarize, the discriminative and categorization task based methods have the
following advantages compared to attribute rating: 1) they engage a synthetical approach
since the subjects do not dissociate their overall percept into sub-unit perceptions using a
list of attribute; and 2) they allow to measure the respective contribution to the subject
response of his sensitivity to the signal and of his behavior (liberal vs. conservator). To
our knowledge, no other study has been published about the exploration of perceptual
interactions using this approach. This could mainly be explained by technical issues
related to the complexity of the training which required that the panelists memorise all the
samples as explained by Dacremont (2007) who reviewed the potential applications of
such approach in exploring perceptual interactions between chemical senses. The
applied statistical treatments when the number of sensory dimensions and/or levels within
each dimension is higher than two are very complex. Another potential application of this
approach is to measure strategy index of assessors and to use this information for panel
formation according to the objective of the test. For instance if the objective of a
discriminative task is to validate that a new coffee recipe (Test) is similar to the current
coffee product (Control) commercialized from a long time and targeting hard core
consumers, the use of conservator assessors is more adequate since product consumers
are probably highly sensitive to slight differences between test and control products.
As another alternative to attribute rating for exploring perceptual interaction, White and
Prescott (2007) used a rapid detection task like paradigm for exploring perceptual
interaction between olfactory (strawberry and grapefruit) and taste (sweet and sour)
interactions. Assessors had to identify as fast as possible a tastant by pressing one of
two buttons related to sweet and sour response. By presenting randomly an olfactory
stimuli (strawberry, pineapple or a blank) simultaneously to a tastant (sweet or sour)
according to a 3x2 factorial design, the authors highlighted: 1) grapefruit olfactory
perception led to faster identification of citric acid (lower reaction time for pressing the
button "sour") than sucrose (sweet), whereas the strawberry odorant led to a reverse
pattern. A previous study demonstrated perceptual interactions between olfactory and
taste perceptions using a same type of detection task paradigm (Djordjevic et al., 2004a).
To conclude about alternatives to sensory profiling for investigating perceptual
interactions, several different paradigms successfully demonstrated perceptual
interactions in model solutions comprising olfactory and taste stimuli. But using such
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methods for highlighting perceptual interaction between more than two sensory
dimensions remains challenging. In addition more methodological development is
required for considering such methods as potential synthetical alternatives to
conventional sensory profiling allowing to better understand consumer preference.
4.5 Role of memory and neural integration processes in the construction of
perceptual association
Regarding the integration of olfactory and taste perceptions, we observed in our study
Subthreshold olfactory stimulation can enhance sweetness (pp. 52-62) that: 1) common
odorants related to taste (e.g. strawberry-sweet) can enhance taste intensity with
olfactory stimulus at both suprathreshold and subthreshold level); and 2) unfamiliar
odorants (e.g. cactus) co-exposed with sucrose can enhance sweetness during post
exposure tests when presented at suprathreshold level but not when then presented at
subthreshold level. It is worthwhile to indicate that the post exposure tests with cactus
odorant were conducted with a suprathreshold concentration the day after co-exposure
and with a subthreshold concentration one week after exposure which may partly explain
the results.
Olfactory and taste associations occurring during life experience involve first
implicit/incidental learning and then consolidation into long term (from days to months)
and or long-lasting (from months to life) memories during repeated co-exposure over life
time. At the brain level, simultaneous olfactory and taste stimulations are integrated in the
orbitofrontal cortex to generate flavour perception as shown in monkey (Rolls and Baylis,
1994) and humans (Rolls, 2006). Other key structures, referred as to key nodes of the
flavour network, have been recently implicated in olfactory and taste integration
processes (Small and Prescott, 2005). These include the anterior insula, frontal
operculum and anterior cingular cortex. The amygala has also been identified as an
essential structure of this flavour network in humans (de Araujo et al., 2003).
Electrophysiological research in monkies has identified: 1) unimodal neurons in the
orbitofrontal cortex that respond specifically to input from one sensory modality amongst
taste, smell or touch; and 2) multimodal neurons responding to input from several of
these sensory modalities (e.g. Kadohisa et al. (2005a). Synaptic connections between
unimodal olfactory and taste neurons may develop in the orbitofrontal cortex as a result of
repeated and simultaneous exposure to a given olfactory and taste combination during
life time. This assumption is reinforced by the presence of olfactory-taste bimodal
neurons in this brain area identified in monkeys (Rolls and Baylis, 1994; Critchley and
Rolls, 1996; Kadohisa et al., 2005a; Kadohisa et al., 2005b; Kadohisa et al., 2005a;
Kadohisa et al., 2005b); which may result from evolution processes. Such a complex
neural integration system allows sensory inputs from separate sensory modalities to
converge into a unitary perception. Moreover, in the flavour network, the amygdala has
been proposed to potentiate cortical information processing of stimuli that become
associated through life experience (Dalton et al., 2000). The impact of food experience in
terms of congruency between olfactory and taste stimuli has also been evidenced at the
brain level in humans (de Araujo et al., 2003; Small et al., 2004).
In the light of psychophysical findings obtained in our study and previous research (see
Valentin and co-workers (2006) for a review) perceptual interactions between sucrose
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and a congruent suprathreshold odorant appears as the consequence of long lasting
memorization processes within the flavour network. In these types of sensory interactions
generated during life time, a congruent odorant presented at subthreshold level can also
impact sweetness. A possible mechanism explaining this phenomenon is the activation of
olfactory receptors occurring below the detection threshold (unconscious perception) but
still leading to projections on bimodal neurons responding to these specific olfactory-taste
combinations in the flavour network. In our study, co-exposure to unfamiliar odorant and
sweet taste combinations during a week implied learning processes and consolidation
into short term memory (from seconds to days). Short term learning under experimental
conditions seems sufficient to induce sweetness enhancement with suprathreshold
odorant previously co-exposed with sucrose. By contrast, when tested one week after coexposure, this effect was not obtained for odorant presented below threshold level. The
perceptual association learnt at short-term level may not have been consolidated into
longer term memory. Other studies dealing with the impact of co-exposure between
unfamiliar odorant and taste carried out the post-exposure test for validating the
construction of perceptual interactions only immediately after co-exposure (Dalton et al.,
2000; Prescott et al., 2004; Pfeiffer et al., 2005; Yeomans et al., 2006; Miyazawa et al.,
2008). To our knowledge, the robustness of perceptual associations experimentally
constructed has never been investigated over time.
Additional psychophysics work is therefore needed. Indeed many aspects of memory and
neural integration processes involved in the construction of perceptual associations
during unfamiliar odorant and taste co-exposure remain unknown: 1) the robustness over
months or years of newly memorized associations (long-lasting memory); and 2) the
optimal period of co-exposure (i.e. learning phase duration, frequency and duration of
sessions during the learning phase) needed to robustly memorized new associations
leading at short term (immediately after co-exposure) and longer term (more than a week
after co-exposure) to taste modulation with a subthreshold odorant concentration.
The measurement of brain activity with fMRI in response to taste- new aroma coexposure before, during, and repeatedly after learning (from 1 week to several months)
may bring relevant insight into the plasticity of the flavour network and how unitary
perceptions are generated.
4.6 Anthropological approach of perceptual interactions
To enlarge our knowledge about mechanisms driving perceptual association, our
psychophysical investigations were completed by an anthropological approach. This was
conducted in the frame of the study Impact of concomitant exposure to new aroma and
sweet taste stimuli on perceptual interaction between tastant and odorant at supra and
subthreshold concentrations (pp. Error! Bookmark not defined. p.77). Indeed,
additionally to the two groups exposed according to either an analytical or a synthetical
attentional strategy, a third group was exposed according to the analytical strategy with
individual interviews after each training and profiling sessions. A total of five interviews
were therefore conducted by M-N. Ottavi (PhD student in Anthropology at the University
of Nice Sophia Antipolis) for each of the twelve assessors. The interview method is based
on a validated technique based on the self-confrontation method. The analyses of the
interviews is currently on-going and should allow to highlight: 1) naive theories elaborated
by each subject for describing the flavour perception (aroma and taste); and 2) the
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PART 4: General discussion and perspectives
173
respective contributions during theory elaborations of the sensory training and of the
dynamic construction (i.e. through the five daily interviews done consecutively over one
week) of flavour representations (individual and collective).
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