Neglected Food Bubbles: The Espresso Coffee

Food Biophysics (2011) 6:335–348
DOI 10.1007/s11483-011-9220-5
REVIEW ARTICLE
Neglected Food Bubbles: The Espresso Coffee Foam
Ernesto Illy & Luciano Navarini
Received: 13 December 2010 / Accepted: 23 March 2011 / Published online: 30 March 2011
# The Author(s) 2011. This article is published with open access at Springerlink.com
Abstract Coffee beverage known as espresso, must be
topped by a velvety thick, reddish-brown foam called
crema, to be considered properly prepared and to be
appreciated by connoisseurs. In spite of the relevant role
played by crema as a quality marker, espresso coffee foam
has not yet been the subject of detailed investigations. Only
recently, some aspects of the Physics and Chemistry behind
the espresso coffee foam have attracted the attention of
scientists. In addition to sharing several characteristics with
other food foams like beer foam, for instance, the espresso
coffee foam may contain solid particles (minute coffee cellwall fragments), it is subjected to a remarkable temperature
gradient and its continuous phase is an oil in water
emulsion rendering it a very complex system to be studied.
Moreover, in the typical regular espresso coffee cup volume
(serving) of 25–30 mL, crema represents at least 10% of the
total volume, and this is a limitation in obtaining experimental data by conventional instruments. The present work
is aimed at reviewing the literature on espresso coffee foam.
The traditional espresso brewing method will be briefly
described with emphasis on the steps particularly relevant
to foam formation and stabilization. In addition to present
up-dated experimental data on surface properties at solid/
beverage and air/beverage interface, recent advances on the
espresso foam formation mechanism, as well as on foam
stability, will be critically examined. The key role played
by carbon dioxide generated by roasting and the effects
of low and high-molecular-weight coffee compounds in
Ernesto Illy (Deceased)—July 18, 1925–February 3, 2008
E. Illy : L. Navarini (*)
Illycaffè S.p.A,
Via Flavia 110,
Trieste 34147, Italy
e-mail: luciano.navarini@illy.com
promoting/inhibiting the espresso coffee foam will be
discussed and emphasized.
Keyword Espresso coffee . Foam . Crema . Interfacial
properties
Introduction
Espresso coffee extraction is the most common brewing
method in Italy and it is increasingly becoming very
popular in many other countries around the world, where
more than 50 million cups of espresso are consumed every
day.1 Differently from other brewing techniques, conditions
normally used to brew espresso coffee enhance several
surface tension-related phenomena such as foam formation
and stabilization. In spite of the relevant role played by
foam in assessing the beverage quality, no systematic
chemical and physical studies have been devoted to better
understand this complex system. Espresso foam has often
been considered a sort of magic event with its fate linked to
the experience of a skilled barista (coffee bar technician)
instead of being the result of Chemistry and Physics.
Unfortunately, this situation has created various ingrained
beliefs and myths which, as in the case of other beverages,2
have precarious scientific foundations, if any.
One of the more indisputable popular statements on
crema claims that “any error in grinding or in percolation,
in temperature or extraction level, is immediately denounced by the color, the texture and the persistence of
the foam”. This statement is supported by almost a century
of “trial and error” rather than objective and reproducible
characterizations. It is worth mentioning that the espresso
brewing method and equipment were only technically
described almost 70 years after its invention and data on
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physicochemical and structural aspects have been reported
only in the last 20 years by Italian scientists.3,4
The aim of the present work is to organize and to
structure the knowledge so far achieved on espresso coffee
foam, from a scientific point of view. The reported
experimental data on the physical properties of the espresso
coffee foam will be discussed in view of brewing process/
chemistry interplay.
What is Espresso Coffee Foam
A foam is a coarse dispersion of gas bubbles in a liquid
continuous phase. In the case of espresso coffee, the gas
phase is mainly the carbon dioxide generated during coffee
roasting and entrapped within the cell structure, whereas the
continuous phase is an oil in water (O/W) emulsion of
microscopic oil droplets (90% <10 μm) in an aqueous
solution of several solutes (including sugars, acids, proteinlike material, and caffeine) containing solids coffee cellwall fragments of 2–5 μm.1 The typical pure Coffea
arabica regular (30 mL cup volume percolated in 30 s)
espresso liquid is an O/W emulsion of 0.2–0.3% volume
fraction, a suspension in which the dispersed phase is
represented by about 150 mg solid coffee particles
(corresponding to about 5 g/L) and a solution with total
soluble solids concentration of 52.5 g/L.1
According to Dickinson,5 the espresso coffee foam
(herein called crema for the sake of brevity) can be
classified as a metastable foam with a specific lifetime.
This is the time at which the foam disappears so as to
expose the dark surface of the beverage below, which can
be up to 40 min.3 During the short lifetime of crema, its
structure and properties change considerably. It starts as a
liquid bubbly foam in freshly prepared espresso and
becomes a dry polyhedral foam on aging. The latter,
however, is not of practical interest from a consumer point
of view, since espresso coffee is consumed within a few
minutes after preparation.
From a quantitative point of view, the crema should
represent at least the 10% of the volume of an espresso.1
Foam density, as a gross indication of the gas phase
content, in the range 0.40–0.60 g/mL has been reported.6
No detailed study has been published so far on crema
chemical composition as well as on bubble size distribution. For the latter, the technical literature describes pure
Coffea canephora (known as robusta) crema having larger
bubbles than that of coffee made from pure C. arabica.7 An
essentially monomodal distribution ranging from 10 to
150 μm has been reported for a regular pure C. arabica
espresso.8
It has been reported that taste-wise, crema is of little
sensory interest,4 although, if tasted in the absence of
Food Biophysics (2011) 6:335–348
beverage, some bitterness and astringency can be clearly
perceived.
Traditional Espresso Preparation Method
Although traditional espresso coffee preparation is a very
complex and challenging topic to which several detailed
reviews have been dedicated,1,7,9 it is possible to simplify
the process to the following three key events to describe the
whole set of steps involved:
–
–
–
Grinding of the roasted coffee
Coffee powder dosing and tamping
Brewing (more correctly Percolation)
Preliminary blending and roasting are taken for granted,
and will be discussed later. It has to be stressed out that the
brewing has to be performed at the moment, on demand.
This concept is clarified by the saying “the consumer, not
the espresso must wait!”.
Grinding The objective of grinding is to obtain a distribution of particles suitable to offer the proper fluiddynamics.10 An appropriate particle size distribution can
be modeled either by a power-law or log-normal distribution, occasionally with a typical bimodality or even
trimodality. Such a complex characteristic of particle size
is believed to produce a double effect: it forms a coarse
fixed structure, which allows the correct flow through the
cake and it forms large quantity of fines of high specific
surface, which permit the extraction of large amount of
soluble and emulsifiable material.1 The particle size range
is 0.2–650 μm.11
Grinding destroys the roasted coffee cell structure and
this results in a remarkable release of carbon dioxide, with
obvious consequences on crema, albeit not yet studied in
detail. Barbera12 demonstrated that over 70% of the carbon
dioxide was released following coffee grinding into 500μm particles. It has to be mentioned that even in the
absence of grinding, carbon dioxide is released from
roasted coffee beans and this degassing process has been
investigated since it represents a technological issue for
coffee packaging and shelf life.13,14 Since it is not possible
to carry out the coffee extraction without coffee grinding, to
limit the detrimental effect of the carbon dioxide loss on
crema it is necessary to be quick in preparing espresso
immediately after grinding. It has been reported that the
range 0–30 min is the interval between grinding and
espresso preparation, and should not be exceeded so as to
preserve crema quality.15
Dosing and Tamping The coffee portion, e.g., the weight of
roasted and ground coffee required for preparing one cup, is
Food Biophysics (2011) 6:335–348
normally confined in the range 6.0–8.0 g1 but the upper
limit can reach 9.0 g.16 The quantity of coffee powder and
the tamping are the two factors which affect coffee cake
porosity. The latter, is the coffee bed property which
governs the water/coffee contact time and then both the
extraction yield and the foam quality. The compacting force
may vary from a few kilograms, for a vertical upward thrust
(a tamping plate is usually built-in with the grinder), to
approximately 20 kgf for a downward compacting by a
hand tool.1
Brewing Traditional espresso requires specialized equipment that can heat water to a temperature of 92–94 °C and
then pressurize it to 9±2 bar.11 The portion is placed in a
perforated basket (also known as a filter) and compacted.
Once the proper porous medium is created it is possible to
start the percolation of hot water under pressure through the
coffee bed. It has to be stressed that the energy of the water
pressure is spent within the coffee cake.1 The beverage flow
into the cup should be ideally close to 1 mL/s11 but the
range 0.40–2.73 mL/s has been recently reported in a
survey of barista habits in Italian coffee shops.16 The
process is applied (percolation time) until the beverage
volume in the cup meets the personal preferences of the
consumer and/or the regional traditions, in Italy for instance
inside the range 15–50 mL, with an optimal outcome at 25–
30 mL (regular espresso). The whole set of factors
controlling the percolation is very crucial for the color of
the foam: if the latter is pale, it means that the espresso has
been “under extracted”, probably because of too coarse
grind, too low water temperature or too short time. If the
crema is very dark in hue and has a white spot in the
middle, it is likely that the consistency of the coffee
grounds was too fine or the quantity of grounds was too
large. An “over extracted” espresso exhibits either a white
foam with large bubbles if the water was too hot or just a
white spot in the center of the cup if brewing time was too
long.11
Foam color can also reveal the composition of the coffee
blend: a pure C. arabica blend leads to an espresso foam
characterized by a reddish-brown color with a “tiger skin”
or “tiger tail” pattern not presented by a pure C. canephora
crema which is characterized by a dark brown color with
grey tones.7,17 These two species are those commercially
used among the over 60 Coffea species (a genus belonging
to the Rubiaceae family). Differences between arabica and
robusta coffee are generally very pronounced and well
documented.1 The aroma profiles as well as the chemical
composition of arabica and robusta coffee brews are
different. Total lipids (about twice as high in arabica as
in robusta brews) and caffeine content (about twice as high
in robusta as in arabica brews) are the main differences
between the two types of brew. Because of its superior
337
quality and taste, arabica sells for a higher price than its
harsher, rougher “relative”.11
It has been claimed that the use of robusta is
indispensable to confer a proper crema to an Italian
espresso3 however this claim has been substantially contradicted by recent experimental data, as discussed in the
following. In Figure 1, the “tiger skin” pattern is shown.
Roasting and Carbon Dioxide The origin of the gas phase
in the espresso coffee foam resides in the roasting process.
Coffee beans are roasted using hot combustion gases or air
at temperatures above 200 °C to develop the characteristic
flavor, color, and aroma.18 Carbon dioxide is the major gas
produced during roasting (87% of gases released from
roasted coffee together with 7.3% of carbon monoxide),
formed as a result of the many reactions occurring, among
which Strecker degradation: a secondary step in the
Maillard reactions and pyrolysis of carbohydrates. Roasting
degree can be expressed in terms of total weight loss
(moisture+organic compounds) or organic loss (organic
compounds only, as% dry matter). For what concern the
latter: 1–5 corresponds to light; 5–8 to medium, 8–12 to
dark; >12 to very dark roasting degree. In a recent paper,
the initial carbon dioxide content, measured immediately
after grinding, has been found to range from 4.0 to 8.6 mg/
g of coffee, with an overall average of 5.7 mg/g (2.9 mL/g
at Standard Temperature and Pressure, STP).19 The pure
arabica sample analyzed by those authors was characterized
by a lower carbon dioxide content (average 4.6 mg/g coffee)
than that of the pure robusta one (6.9 mg/g coffee). Both
coffee samples contained more carbon dioxide in a dark
roast rather than at a medium roast.19
It has also been reported that two additional reactions
that occur in the espresso brewing method can be indicated
as a source of carbon dioxide. Both derive from bicarbonate
ions contained in the water: the second most important
ingredient to brew coffee. In particular, carbon dioxide
Fig. 1 Image of freshly prepared pure arabica regular espresso coffee
foam. The “tiger skin” effect is shown
338
from bicarbonate thermal decomposition and from bicarbonate neutralization by natural coffee acids.20–22
During percolation, the beverage should be conveyed
into a proper cup. As in the case of beer, both pouring style
and cup characteristics are very important factors which
may affect the crema. In particular, the cup shape and
material, the distance between cup and beverage conveyor
and the temperature of the empty cup are particularly
relevant. For the latter, it is well known that an excellent
espresso is always prepared in a warm cup.1,7
Surfactants and Foam Stability
It is well known that for food foams and emulsions the
preparation, properties, and long-term stability are determined by the adsorption and subsequent interactions
between various molecular species at air–water and oil–
water interfaces. In real food systems, the interfaces will be
populated by a large number of molecular species, which
may include various proteins, surfactants, lipids, and
polysaccharides.
The chemical nature of coffee surfactants has not yet
been fully clarified. In the first chemical characterization of
a pure arabica blend espresso coffee brew (both whole and
fractionated) some classes of surfactant have been quantitatively determined, albeit not recognized as surfactants at
that time.4 In particular, proteins and lipids have been
analyzed the latter in terms of triacylglycerides, diacylglycerides, and fatty acids, showing that the first five fractions
(about 5 mL each) corresponding cumulatively to a regular
espresso cup volume contained about 2.1 mg/mL triacylglycerides, 0.4 mg/mL diacylglycerides, and 0.09 mg/mL
fatty acids. The same investigation reported whole beverage
total lipids concentration of 1.80 mg/mL and of 0.55 mg/
mL in espresso for pure arabica (from a sample different
from that used for fractionation) and for pure robusta blend,
respectively.4
It has been also suggested that other classes of complex
molecules, like glycolipids or glycoproteins might be
involved in the formation and/or stabilization of the foam.1
Nunes et al.23 determined foamability and foam stability
of espresso coffee as a function of the roasting degree along
with other dependent variables like total solids, pH, fat,
protein, and carbohydrate contents. They observed a high
correlation between foamability and protein content, and
between foam stability and the fractions containing highmolecular-weight polysaccharides; these fractions are a
mixture of galactomannan and arabiinogalactan or better,
complexes between polysaccharides, protein, and phenolic
compounds caused by the roasting process, probably
products of Maillard reactions.24 The same investigation
Food Biophysics (2011) 6:335–348
put in evidence the strong negative correlation between fat
and foamability.23
The first foaming fraction isolated from ground and
roasted coffee has been obtained by Navarini et al. 25 and
characterized by the same group.26,27 Addition of ammonium sulphate to the coffee extract (defatted dark roast
arabica blend extracted by solid–liquid extraction with
water at 90 °C) produced a precipitate which was
redissolved, dialyzed and freeze-dried. This fraction, also
known as Total Foaming Fraction (TFF) when redissolved
in water, easily foams on shaking. By using the same
procedure on different arabica and robusta pure single
origin coffee samples, yield, chemical and physicochemical
properties resulted to be independent from coffee species
and origin.27 TFF was further fractionated by means of
isopropanol precipitation to get foaming fraction A (FFA)
and B (FFB). The former was found to be composed of
about 80% (on molar basis) of mannan, containing small
amounts of galactose and arabinose, and of about 20% of
arabinogalactan.25 FFA, characterized by high molecular
weight (34,000–47,000) and moderately low surface activity, was shown to be involved in foam stability whereas
FFB, melanoidin-proteinaceous material, with lower molecular weight (8,500–17,000) and high surface activity was
shown to be involved in foamability.27 TFF, FFA, and FFB
dissolved in water at 0.4% w/v gave an average surface
tension (De Nouy tensiometer at 25 °C) of 51.9, 59.5, and
46.5 mN/m, respectively.27
Very recently TFF, FFA, and FFB have been isolated
from an arabica (30%)/robusta (70%) blend and their
foaming performances and viscoelastic properties have
been determined.28 The whole set of experimental data
substantially confirmed previous findings. The interfacial
elasticity of 0.5% FFB in water plotted as a function of
time, described the typical behavior obtained for proteinlike biopolymers 28 characterized by values in the range
typical of food-stabilizing proteins.29
The role played by soluble proteins in promoting and by
coffee oil in inhibiting coffee foam volume and foam
stability has been also reported by Saliba and Ayoub.30
In order to avoid possible influence of the extraction
conditions on coffee foam positive compounds, D’Agostina
et al.31 isolated TFF, FFA, and FFB directly from a defatted
espresso obtained under strictly controlled brewing conditions using a pure arabica coffee blend. Chemical
composition and physicochemical properties were found
to be in reasonable agreement with findings of Navarini and
Petracco 26 and Petracco et al.27 The three foaming
fractions have been also organoleptically tested. FFA has
been found to be essentially tasteless whereas all panelists
agreed that FFB has an intense coffee aroma and persistent
bitter taste.31 Due to the higher foam promoting activity of
FFB, this fraction was further subfractionated by means of
Food Biophysics (2011) 6:335–348
solid phase extraction on C18 cartridges and subsequent
elution with different water/methanol mixtures. Four subfractions (FFB-1–FFB-4) were obtained but only the first
two, more hydrophilic, were characterized. FFB-1 showed
no foamability, whereas FFB-2, slightly more hydrophobic
(elution with 50% methanol), was very effective both in
foam formation and stabilization. As far as chemical
composition is concerned, FFB-2 was found to be essentially a melanoidin having cinnamic derivatives as well as
polysaccharides and modified proteins structural elements
characterized by a monomodal molecular weight distribution centered at 56,000.31
Caffeine shows appreciable surface activity in water
which is enhanced by the presence of sucrose,32 however,
in view of the entity of the adsorption phenomenon (for
1.0% pure caffeine at 20 °C monitored up to 20 h, it
showed a surface tension equal to 70.1 and 68.5 mN/m in
mixture with 6.0% sucrose) it seems difficult to attribute a
relevant role of caffeine content as a surfactant.33
Espresso Coffee Interfacial Properties
Air–Beverage Interface
In spite of the crucial role played by interfacial properties in
foaming, reported data on espresso coffee have been
confined to surface tension measurements carried out by
means of stalagmometry.17,34–37 Unfortunately, besides
indicating the presence of naturally occurring surfactants
in the beverage, this method cannot provide information
about their kinetics of adsorption. Reported surface tension
values at 20 °C for pure arabica espresso range from 46 to
49.7 mN/m whereas for pure robusta or robusta-rich blends
slightly higher values have been reported (48–50.5 mN/m).
Recently, dynamic tensiometry has been used to compare regular espresso coffee prepared by using pure arabica
(Brazil) and pure robusta (Congo). By using both maximum bubble pressure and drop shape tensiometry it has
been possible to follow adsorption kinetics in a wide range
of surface age ranging from 10−3 s to hours.38 A strong
reduction of surface tension was observed already at short
time, suggesting that in addition to high-molecular-weight
surfactants, low-molecular-weight amphipathic solutes are
involved. The comparison between the two different Coffea
species at 20 °C, revealed a similar behavior in the time
range from 10−3 to 10 s with surface tension values at about
4 s close to those obtained by stalagmometry (46.2 and
48.7 mN/m for arabica and robusta, respectively). However, for both species the surface tension tended to reach an
equilibrium value at time higher than 900 s this behavior
being particularly remarkable at 37 °C and especially for
arabica espresso. In other words, the robusta sample
339
tended to a steady state more rapidly that the arabica one.
The different behavior has been ascribed to the lipids
content in the brews. In particular lipids form insoluble
monolayers at the interface reaching equilibrium with a
slow adsorption process.39
This hypothesis has been indirectly confirmed by
studying the dynamic tensiometric behavior of several
coffee-based beverages and pure arabica espresso at
different cup (serving) volumes which differed in lipid
content.40
Surface tension (at 10 s) has been found to be influenced
by the cup (serving) volume (percolation time) and in
particular by increasing the cup volume from 10 to 60 mL.
The surface tension increased from 44 mN/m to about
48 mN/m following a power law with an exponent close to
0.042.41
Considering that the first sip of espresso coffee is
generally consumed at high temperature (ca. 60–65 °C
and accordingly surface tension values lower than those
measured at 37 °C) the dynamic tensiometric behavior of
the espresso coffee beverage has been suggested to be
consistent with that expected for systems with good wetting
properties of the oral cavity.41 As a matter of fact it has to
be mentioned that human whole saliva at 37 °C is
characterized by surface tension values remarkably higher
than those of regular espresso beverage at the same
temperature. Moreover, the critical surface tension of
wetting representative of a human saliva-coated tooth
surface and most restorative materials when exposed in
the oral cavity has been reported to be close to 35–38 mN/
m 42 and within the bio-adhesive range of 32–50 mN/m.43
In addition to surface pressure, the dilational modulus of
the interfacial adsorption layer plays an important role in
the surface properties. It has been stated that the dilational
modulus of a generic “coffee” is rather high and may
explain in part the stability of bubbles. However, no data
have been reported to support this claim.33
Solid–Beverage Interface
Up to now, surface properties of espresso coffee have been
investigated on Teflon surfaces only 40 with the purpose to
provide further evidence confirming the hypothesized good
wetting properties for the oral cavity. Wetting of espresso
coffee on a hydrophobic surface is higher than that of
popular drip filter coffee preparations. The reported data
show that the increase in temperature from 20 to 45 °C
promotes the spreading by reducing the contact angle and
the surface tension. In particular going from an initial
contact angle at 20 °C of about 60° to an initial contact
angle close to 40° at 45 °C.
In Figure 2, pure water (contact angle 97°±1°, upper
left), human whole stimulated saliva (upper right), drip
340
Food Biophysics (2011) 6:335–348
Solid–Foam Interface
Fig. 2 Pure water (contact angle 97°±1°, upper left), human whole
stimulated saliva (upper right), drip coffee (bottom left), and
espresso (bottom right) are compared on the Teflon surface used
by Ferrari et al.40
coffee (bottom left), and espresso (bottom right) are
compared on the Teflon surface used by Ferrari et al.40
The remarkable wettability of espresso coffee may be of
interest to interpret some peculiarities like the staining ability
of the beverage for the oral cavity surfaces and its long-lasting
after-taste. It is well known that taste and aroma of espresso
can be savored for as long as 20 min after it has been drunk.11
Human enamel and mucosal surfaces are very important
components of the oral cavity. Contact angles of water and
human whole saliva measured on enamel in vitro at 37 °C
show a better wettability of water (values close to 50°) than
whole saliva (values close to 65°). Very different is the
behavior of the two liquids on mucosal surface (in vivo
measurements at room temperature), being the contact
angle of water (75°–70°) significantly higher than that of
human whole saliva (close to 65°).44 It has been reported
that some coffee components adsorb onto saliva-coated
hydroxyapatite beads inhibiting the adsorption of Streptococcus mutans which is considered to be the major
causative agent of dental caries in humans.45 It is clear that
the ability to adhere to the tooth surface by binding water
insoluble polysaccharides and saliva constituents of the
acquired pellicle may reflect specific interactions occurring
in addition to surface properties (wetting and spreading).
The contact angles of a bitter taste system (1% w/v
caffeine aqueous solution) and a sweet taste system (6% w/
v sucrose aqueous solution) have been measured at 20 °C
on a hydrophobic surface (polyethylene) to mimic the
receptor membrane–tastant interaction. According to the
authors, the lower value found for caffeine of 85°±0.75°
versus 91°±0.95° for sucrose, suggests there is specific
adsorption of the bitter caffeine molecules on the lipidic
bilayer surface of receptor membrane.32
As in the case of surface tension, the above reported data
shows a very low contribution of caffeine in wetting of
espresso coffee.
No investigations on solid–espresso foam interfacial properties have been reported so far, although recently,
Foschia46 observed a foam adhesion phenomenon similar
to that known as foam lacing or cling in beer.47 When beer
is poured, changes occur in the foam which convert it from
being liquid to being essentially solid, which leads to the
foam being left in contact with the glass surface when
liquid is drained. Those beers most prone to this change
give the most adhesive foam. It is interesting to underline
that beer foam is equally able to lace onto virtually any
type of solid surface indicating that there is no role for
specific types of foam–glass interactions in beer foam
lacing.47 Unhopped beer is not able to give rise to foam
adhesion; low-molecular-weight species (iso-alpha-acids)
in hopped beers and their interactions with foam positive
proteins have been suggested to reinforce and stiffen
bubble films and then to give cling.48 More recently, free
fatty acids and in particular di- and trihydroxyoctadecenoic acids have been shown to negatively affect beer foam
adhesion.49
Foschia,46 in a study on espresso coffee foam, observed
foam adhesion, independently of solid type (borosilicate
glass, polystyrene, polyethylene, and Teflon), for espresso
prepared with pure robusta (two single origins). Foam
adhesion has not been observed in the case of pure arabica
espresso (five single origins). The foam adhesion observed
in the case of pure robusta has been related to the observed
higher drainage rate and consequently, to the faster change
from a liquid foam to a dry one.46
A further difference between the foam obtained by pure
arabica espresso and that of pure robusta is the special
visual pattern known as “tiger skin” or “tiger tail” not
observed in the latter. This effect is attributed to the
presence in the foam of very fine coffee grounds along
with cell-wall fragments and it may reflect the different
cellular structure between the two coffee species and then
as well as the different behavior when subjected to
grinding.50 The presence of a high fraction of solid particles
in pure arabica crema may be the origin of the differences
evidenced in Figure 3. However in addition to the visual
aspect, the solid particles in the arabica crema may be
considered an additional class of surfactants which can play
a role in the foam stabilization.51
No detailed studies have been focussed at determining
the role played by solid particles in crema. Optical
microscopy was used in a preliminary observation.52 In
Figure 4 (pure arabica regular espresso freshly prepared), a
large bubble appears to be covered by solid particles (size
<50 μm). The particle surface seems to mostly remain on
the external side suggesting a contact angle (through the
aqueous phase) of less than 90°.51
Food Biophysics (2011) 6:335–348
341
a
b
Fig. 4 Optical microscopy image of a large bubble isolated from a
freshly prepared pure arabica regular espresso coffee foam. The scale
bar represents 50 μm. Silizio52
Foam Volume
Fig. 3 Optical microscopy image of a freshly prepared pure arabica
regular espresso coffee foam pattern b freshly prepared pure robusta
regular espresso coffee foam pattern. The arrow shows a solid particle
of 120 μm as a size reference, Silizio52
Earlier studies3 on espresso coffee prepared by using
arabica/robusta (40:60) blend reported the foam volume,
measured in a graduated beaker immediately after percolation, as a function of water content of the ground roasted
coffee, of compaction in the filter holder and of coffee
portion. Unfortunately, the roasting degree was not specified. The foam volume range is shown in Table 1.
According to reported data,3 a low water content (2.2%)
of the ground roasted coffee, high compaction force (10 kg)
and a 6.5-g portion led to the higher foam volume. The
detrimental effects of poor compaction as well as of
The particles in the dry espresso foam shown in Figure 5
(pure arabica), are clearly located in the Plateau border,
suggesting the tendency to be unattached. In fact, during
drainage, unattached particles predominantly follow the net
motion of the liquid.53 This observation strongly suggest
possible stabilizing role of the solid particles within the
crema.
Espresso Coffee Foam Properties
It has to be emphasized that the methods used to determine
the physical properties of espresso coffee foam are
empirical and have not yet been standardized, therefore,
for the sake of clarity, both methods and samples, when
mentioned, are briefly described in what follows.
Fig. 5 Optical microscopy image of a dry pure arabica regular
espresso coffee foam. The scale bar represents 50 μm. Silizio 52
342
Food Biophysics (2011) 6:335–348
Table 1 Espresso coffee foam volume reviewed data
Blend
A=arabica
R=robusta
Coffee portion(g)/cup volume
(mL)
Roasting
degreea
Water temperature (°C)
Water pressure
(bar)
Foam volume
(mL)
Reference
A/R
A
A
R
A
A/R
A
A
A/R
A
A/R
A
A
5.0–7.5/40
6.75/30
6.0/40
6.0/40
7.5/40
7.5/40
7.5/40
7.5/40
7.5/40
6.5–8.5/40
6.5–8.5/40
6.75/4–20
6.75/25
NR
Medium
Light–very dark
Light–very dark
NR
NR
NR
NR
NR
NR
NR
Medium
Medium
NR
93±1
NR
NR
96
96
92
88–98
88–98
92
92
93±1
93±1
NR
9±2
NR
NR
9
9
7–11
9
9
9
9
9
9
3.5–10.0
At least 2.7
1.2–3.5
1.4–3.9
4.4
6.8
5.1–6.9
4.3–4.5
5.9–6.9
4.4–6.0
5.5–9.42
1.37–4.5
2.5–12
3
40:60
20:80
20:80
20:80
34
23
23
17
17
35
36
36
37
37
46
6
NR not reported
a
See text for definition
prolonged time between grinding and extraction were
emphasized.
In particular, holding for 24 h at equilibrium with
ambient conditions led to a remarkable decrease of the
foam volume (from 10 to 6.5 mL).
Illy and Viani34 defined as “foam index” the ratio
between foam and liquid volumes. A “foam index” of at
least 10% is expected for a correct preparation.
Nunes et al.23 stimulated by the findings of Dalla Rosa
et al.3 reported a more detailed study. Pure arabica (Brazil)
and robusta (Uganda) were roasted to different roasting
degree (from light to very dark), degassed for 2 days at
room temperature and then espresso coffee was prepared
from these species and analyzed. The foam volume, was
measured immediately after extraction using a 50 mL
graduated cylinder and ranged from 1.2 to 3.9 mL, as
shown in Figure 6.
foam volume (mL)
5
4
3
2
1
0
0
5
10
15
20
Roasting degree (organic loss % dry basis)
Fig. 6 Foam volume (in milligrams) as a function of roasting degree
(organic loss% dry basis) after Nunes et al. 23 Triangle pure Arabica;
square pure Robusta
This range includes values lower than those measured by
Dalla Rosa et al.3 and lower than those expected for wellprepared cups (of identical volume);34 the roasting, however, had been performed at a temperature (200 °C) lower
than that normally used to roast coffee blend for espresso
brewing. The roasting temperature remarkably affects the
carbon dioxide coffee content, in particular roasting at
200 °C produces about 3 mL CO2/g coffee STP while
roasting at 230 °C produces about 9 mL CO2/g coffee
STP.12 As seen in Figure 6, the foam volume was found to
increase linearly with roasting degree for both coffee
species. No significant differences in the foam volume
were observed in comparing arabica and robusta (see
Figure 6). The water content of the roasted coffee was
not found to affect the foam volume, as previously
reported.3 A good correlation between beverage pH and
foam volume has been also observed.23 Maetzu et al.17
compared the foam volume (measured in a 100-mL
graduated cylinder) of espresso prepared by using pure
arabica (Colombia) and arabica/robusta (20:80) blend
without details on roasting degree. The reported values
(see Table 1) proved the higher foamability of the blend.
The foam volume was found to be affected by water
pressure.35 As reported in Table 1, by increasing water
pressure, the foam volume steadily increases from 5.1 to
6.9 mL. The water temperature in the range 88–98 °C was
not statistically different as far as the foam volume is
concerned, whereas the coffee portion strongly influenced
foam volume which steadily increased.36,37,54
Unfortunately, in the above-mentioned investigations, no
details on water quality as well as on roasting degree have
Food Biophysics (2011) 6:335–348
Foam Persistence
Foam persistence is another property which has a special
importance in the consumer appreciation of espresso. Foam
should survive for at least a couple of minutes before
breaking and leaving a first uncovered black spot on the
surface of the beverage.9 Generally, the foam persistence is
defined as the time that the liquid phase below the foam
layer took to appear during cooling at room temperature in
the same container used to measure foam volume. The first
study on the subject3 reported foam persistence ranged
from 6 to 40 min. The factor positively affecting the foam
volume such as the reduced time between roasting and
extraction has been found to be negatively correlated to
foam persistence. Similar results have been reported when
coffee species have been claimed as having foam promoting factor, i.e., those that had higher foam volumes resulted
in a lower foam persistence.17,36
Differently from foam volume, the persistence has not
been found linearly correlated with the degree of roasting.23
As reported in Figure 7, for both arabica and robusta the
persistence reached a maximum (33.9 and 48.6 min,
respectively) at a medium roasting degree over the range
from light to very dark roast. The decline in foam
persistence at higher roasting degree has been suggested
to be related to thermally induced depolymerization of
polysaccharide-derived Maillard reaction products with
subsequent decrease in the viscosity of continuous phase.24
Process parameters such as water temperature and coffee
portion seem to have not a significant influence on foam
persistence whereas water pressure does. By increasing the
latter from 7 to 11 atm, foam persistence for pure arabica
espresso (7.5 g and 40 mL) lasted between 24.7 and
30.0 min.35
Foam Consistence
This physical property has been investigated less so few
papers deal with it. Two different methods have been
reported to determine foam consistence (also known as
foam solidity). In the first one, foam consistence is assessed
by measuring the time the foam bears a known weight (for
instance 1.5 g) of crystalline sucrose before penetrating the
crema layer and for this reason is known as “sugar test”.56
In the other method, similar in principle, the sugar is
replaced by a generally laboratory-made apparatus with
known weight and geometry (for instance 1.0 g and circular
with a 2 cm diameter and a metal net in the center).23
By using the “sugar test” Severini et al.56 reported a
foam consistence range from 0.5 to 14 s. As in the case of
foam volume, the prolonged time at ambient atmosphere
between grinding and extraction is deleterious for the foam
consistence: a storage of 36 h is sufficient to result in a
foam consistence of only 0.5 s. The range observed by
Severini et al.56 has been substantially confirmed by other
studies.15,16
3500
foam persistence (s)
been provided and carbon dioxide content has not been
determined.
By using back-scatter technique,55 Foschia46 investigated
foam volume as a function of percolation time (cup volume)
ranging from a few drops (e.g., 4 mL) to 20 mL espresso. In
this interval, the foam volume ranges from 1.37 to 4.5 mL
showing an almost linear trend. In the same study, the linear
correlation between foam volume and roasting degree has
been confirmed but not over the whole range of roasting
degree: a very dark roast coffee produced less foam than that
of a dark one.46
Two different methods to quantitatively determine the
espresso foam have been compared by Navarini et al..6
In addition to the graduated cylinder (100 mL) method,
the espresso has been percolated into a 100-mL separatory funnel and the crema determined gravimetrically
(foam weight) after the liquid was drained off. For the
first time, foam volume has been investigated as a
function of the carbon dioxide content of roast coffee.
Foam volume and foam weight was shown to be highly
correlated to carbon dioxide content of starting roasted
and ground coffee powder with a slightly better correlation for foam weight (R2 =0.907). The foam volume
ranged from about 2.5 mL for a 0.25 mg CO2/g coffee
to about 12 mL for a 4.5 mg CO2/g coffee. Carbon
dioxide content and foam volume correlation has been
also observed by Foschia.46
These latter findings constitute a possible rationale to
interpret previous data on espresso foam volume. As a
matter of fact, factors like compaction, prolonged time
between grinding and extraction, roasting degree, portion in
espresso machine, percolation conditions, coffee species,
etc. all remarkably affect the carbon dioxide content of the
roast and coffee powder and then the foam volume.
343
3000
2500
2000
1500
1000
500
0
0
5
10
15
20
Roasting dregree (organic loss % dry basis)
Fig. 7 Foam persistence (in seconds) as a function of Roasting degree
(organic loss% dry basis) after Nunes et al.23 Triangle pure Arabica;
square pure Robusta
344
It has also been found that there is high correlation
between foam consistence and foam persistence independently on coffee species at different roasting degree. A
maximum foam consistence has been observed in the
medium roasting degree between a light roast to a very
dark one.23
Foaming and Antifoaming Mechanisms
In spite of the fact that the carbon dioxide has been
frequently suggested as the gas phase responsible of the
espresso coffee foaming, the bubble formation mechanism
has not been investigated in detail.
Fond 22, in an attempt to interpret the dynamics of the
transient phase of the espresso extraction, reported the
possible role played by bicarbonate–carbonic acid equilibrium. The transient phase, well documented in literature57,58 is the initial wetting stage of espresso brewing in
which hot water fills into coffee particle voids while
simultaneously inter- and intraparticle gas is pushed out of
the coffee bed. In addition, mass transfer between coffee
particles and water occurs simultaneously. The transient
phase terminates when an equilibrium is reached and a
steady state phase takes place (pressure and flow rate
remain constant up to the end of percolation). In the
transient phase, two processes can be distinguished:
1. flow rate and pressure increase, in accordance with
Darcy’s law
2. according to the pump characteristics, pressure increases
while flow decreases in clear contradiction with classical
hydraulics
The anomalous behavior of process 2, has been
interpreted by Fond22 as the result of coffee bed compaction related to the bicarbonate ions present in the extraction
fluid (water) and the displacement of its equilibrium
according to the pH evolution in the course of brewing
(from 7.0–7.5 to 5.5–5.0). In other words, process 2 was
related to chemical reactions occurring at high temperature
in the coffee bed resulting in its compaction through CO2
degassing. The latter, according to Fond22, “contributes to
foam and emulsion generation”.
This view overlooked the importance of the carbon dioxide
already present in the roasted coffee, it doesn’t explain the
transient phase observed by using water at 4 °C 58 and it
partially explains the role played by bicarbonate ions in the
extraction fluid during espresso brewing.20,21 However, the
study by Fond22 is the first one which put in evidence the
relationship between carbon dioxide chemistry and foam
generation.
Bubble formation in espresso brewing has been discussed by Navarini et al.6
Food Biophysics (2011) 6:335–348
It has been hypothesized that during espresso coffee
brewing, the solubilization of carbon dioxide (present in the
coffee bed) in water at high pressure and temperature lead to a
supersaturation conditions in the beverage. In a usual coffee
bar environment, 50 mL of water and 13.5 g of coffee are used
for two cups of espresso. Thus, assuming about 4.5 mg CO2/g
coffee, 61 mg of carbon dioxide will be available to be
solubilized. The resulting carbon dioxide concentration is
approximately three times lower than CO2 solubility at a
pressure of 9.2 bar and at a temperature of 100 °C but
approximately two times higher than the solubility at a
pressure of 1 bar and at a temperature of 60 °C. This view is
compatible with a foaming occurring through bubble
formation by heterogeneous nucleation and bubble rise and
this has been related to the effervescence normally observed
immediately after espresso preparation. Micronic solid
particles and sub-micronic cell-wall fragments, present in
the beverage, may act as nucleation sites. Moreover, the
small volume of an espresso beverage offers a limited length
(about 1.5–2 cm in a standard espresso coffee cup) for
bubble rise. These two conditions have been related to the
size of the tiny bubbles characterizing espresso coffee foam.
From a quantitative point of view, the exercise proposed
by Navarini et al.6 leads to a volume of CO2 available for
foaming of about 7.6 mL (STP) per cup, a value within the
reported foam volume range. This hypothesis has not yet
been supported by further experimental evidences.
As far as the foam stability is concerned, it has been
suggested that the relatively high temperature of the
beverage may induce the reduction of the film thickness
between bubbles by evaporation of water and this mechanism rather than disproportionation can play a role in foam
collapse. 6 Moreover, due to water evaporation, the
surfactants concentration in the film increases and this
generally results in a lowering of the surface tension. This
effect is different between thin and thick part of the film
resulting in a stretching which causes film instability.59
The thermal gradient at which the crema is subjected
during its life may be relevant in interpreting the inverse
correlation between foam volume and foam persistence.
Too high of carbon dioxide availability, in view of its low
solubility in hot water, can destabilize the foam that was
just formed, whereas a lower carbon dioxide content in the
bubbles may be retained for a long time in view of the
viscosity increase of the continuous phase, on cooling: from
about 0.50 mPa s at 70 °C to 1.40 mPa s at 25 °C.41 Other
foam instability sources can be hypothesized looking at the
beverage chemistry.
The coexistence of high- and low-molecular-weight surfactants, for instance, in addition to the role played by CO2, can
represent a rationale to interpret the debate over the foampromoting role of robusta coffee in espresso blending or the
peculiar foam adhesion observed in pure robusta espresso.
Food Biophysics (2011) 6:335–348
Destabilization of beer foam by lipids is widely documented and easily demonstrated.60,61 Free fatty acids, in
particular, have been recognized as foam-negative
materials depending on chain length and degree of
saturation. The competition at the air/beverage interface
of proteins able to stabilize the foam by forming a
viscoelastic “skin” around the bubbles and free fatty
acids, destabilizes the foam by reducing the strength of
the adsorbed protein layer and increasing the probability
of bubble rupture and coalescence.60 However, fatty
acids with a chain length shorter than C12 are not surface
active enough in the concentrations present in beer to
compete with the proteins and destabilize the beer foam.
C12–C14 fatty acids and the unsaturated C18:1 and C18:2
fatty acids destabilize foams by disrupting and weakening
the adsorbed protein film whereas the saturated C16:0–
C18:0 fatty acids probably act through the formation of
hydrophobic aggregates that destroy the foam through a
film-bridging mechanism.60
Lipids represent well-known antifoaming agents also in
milk: the poor foaming behavior of whole milk as
compared with skim milk is in accordance with practical
experience.62 Skim milk tends to produce a lighter, airier
foam over whole milk leading to differences in the rheology
and in the visual texture of the corresponding foam.63 Other
important antifoaming agents in milk are the products of
lipolysis. Foaming of pasteurized milk is reduced when
unesterified fatty acids are present.64 According to
Buchanan,65 mono- and diacylglycerides, rather than free
fatty acids, are responsible for this effect.
In espresso coffee, no detailed studies on the above
subject have been produced. However the lipid content is
remarkably higher than that of beer but remarkably lower or
comparable than that of milk.
In particular, in a regular espresso (25 mL) the total
lipids range from 45 mg 4 to 146.5 mg 17 for arabica and
from 13.65 mg 4 to 119.25 mg 17 for robusta. On average,
pure arabica espresso contains a higher content of total
lipids than robusta espresso, and therefore the probability
of lipid induced foam destabilization is higher for arabica
than for robusta. Since espresso coffee is well known to
contains emulsified lipids, the lipid induced foam destabilization may also occur through oil spreading at air/
beverage interface66 and this is consistent with the lower
surface tension generally observed in pure arabica espresso
in comparison with pure robusta one.41
The only paper in which the lipid amount and composition of regular espresso brews prepared with the two
different Coffea species has been compared reported a
similar lipid content (57 mg and 58 mg for arabica and
robusta, respectively) but a polar lipid fraction (including
phospholipids and free diterpene alcohols) three times
higher in robusta than in arabica.67
345
Solid particles may destabilize the foam by film bridging
but only in the case of hydrophobic particles through
surface dewetting otherwise the film remains stable. The
wetting nature of the solid particles present in espresso
coffee has not yet been investigated. However, from one
hand, the appreciated “tiger skin” effect observed in
arabica espresso may suggest a foam stabilizing role of
the solid particles, otherwise the antifoaming effect should
be macroscopically rapid,68 from the other it is well known
that roasting and some types of packaging induce the
spreading of the oil on the coffee cell walls.1 In view of the
previous discussion on the subject, the former hypothesis
seems to be more credible.
The different foam adhesion observed between the two
coffee species seems to reflect a different rate in the
transition from liquid to dry foam. The arabica espresso
foam rheology seems to be of the liquid-viscous type for a
longer time; this system eventually drains and collapses
leaving no residue whereas the solid-like nature of robusta
espresso foam deriving from a higher drainage rate seems
to be the prerequisite for adhesion.
In a study on foamed lipid emulsions, the physical state
of the lipid phase from liquid to mostly solid has been
investigated.69 It has been shown that solid lipid was
destructive to the formation of the foam even though it
subsequently proved to enhance the overall stability. The
latter has been suggested to derive from colloidal interactions between dispersed lipid particles within the continuous phase resulting in slower foam drainage. It may be
speculated that the different content, composition, and
physical states of the lipid phase (including solid particles)
in espresso prepared with different pure coffee species may
affect foam drainage rate through colloidal interactions as
observed in different systems.69 In this regard, it is
interesting to underline that the viscosity of coffee oil at
92 °C is 7.8 mPa s whereas at 20 °C is 70.6 mPa s. 1
From a chemical point of view, the hydroxy fatty acids
suggested to negatively affect beer foam adhesion, have been
reported to be originated by enzymatic and autooxidation of
linoleic acid resulting in the formation of its hydroperoxides
which are transformed into mono-, di-, and trihydoxyoctadecenoic acids.49 It is well documented that linoleic acid is the
most abundant fatty acid in the green coffee triglycerides
lipid fraction (arabica 52.2–54.3%; robusta 43.9–49.3%)
and, upon roasting, it is the only one whose content
decreases slightly as the roasting temperature increase.70
Concluding Remarks
The espresso coffee foam is a challenging topic for both the
intrinsic complexity and the lack of scientific literature. It is
clear that by adopting the “proper approach” a number of
346
Food Biophysics (2011) 6:335–348
apparently unrelated phenomena may be interpreted and
considered simply the results of Chemistry and Physics.
The key to interpret the several factors affecting the crema,
seems to be the carbon dioxide content of roasted coffee in
addition to CO2 possibly present as bicarbonate ions in the
water ingredient. Most of the data reported in the present
review may suggest that espresso brewing can be described
as “a quick way to transfer carbon dioxide from roasted and
ground coffee to a small cup by means of hot water under
pressure”. This then leads to the facts that for espresso
coffee, carbon dioxide has to be:
–
–
–
–
–
generated by roasting
maintained in the bean by proper packaging
maintained in the ground coffee
solubilized in water
released into the beverage.
In this framework statements such as “any error in
grinding or in percolation, in temperature or extraction
level, has an immediate effect on denounced by the color,
the texture and the persistence of the foam” or “the foam is
the signature of a well-prepared espresso” can be well
justified. In fact, foam volume, persistence, and consistence
are the consequences of the carbon dioxide content
originally present in the coffee. In addition to the
importance of carbon dioxide in espresso coffee foam, we
believe that carbon dioxide can play a role even from a taste
point of view. This aspect has not yet been the subject of
investigation.
From a chemical point of view, some foam-positive
compounds have been isolated and characterized whereas
foam-negative ones have been suggested but not yet
sufficiently investigated, although the negative influence
of lipids on foamability seems to be more than an isolated
observation. Lipids and solid particles may affect the foam
stability but this topic has to be investigated in more detail.
The role played by coffee species in espresso foam is not
established and it cannot be considered a consolidated belief.
As a matter of fact, the contrasting experimental results in the
comparison (when performed) between arabica and robusta
seem to suggest the statement like: “the high foamability of
robusta” or “it is necessary to use robusta to increase the
espresso foam” cannot be generalized. The interplay between
carbon dioxide content and lipid content seems to be more
relevant rather than coffee species as far as foamability and
foam stability is concerned. However, the scarce literature on
subject puts in evidence differences in color and foam
texture between the foam obtained by the two species.
It is indisputable that up to now the espresso bubbles
have been neglected from a scientific point of view, but the
authors’ opinion is that in addition to beer foam or other
famous food foams the espresso crema may represent a
good model system to merit attention in foam Science.
Of course, under the foam, the espresso has to be
organoleptically excellent, without off-tastes and off-flavors
and it has to ensure pleasure during and a smile after drinking.
Acknowledgment It is a pleasure to thanks all our friends and
colleagues for the useful discussions, suggestions, and constructive
criticisms given with warm enthusiasm. In particular Roberto
Cappuccio, Michele Ferrari, Oriana Savonitti, Theodore P. Labuza,
Massimo Barnabà, Marino Petracco, and Furio Suggi Liverani. We are
indebted to Fabio Silizio for optical microscopy images and to
illycaffè S.p.A (Trieste, Italy) for the photographic materials kindly
provided.
Open Access This article is distributed under the terms of the
Creative Commons Attribution Noncommercial License which permits any noncommercial use, distribution, and reproduction in any
medium, provided the original author(s) and source are credited.
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