EFFECT OF PRE-DRYING METHOD ON THE QUALITY AND

polish journal of food and nutrition sciences
www.pan.olsztyn.pl/journal/
e-mail: joan@pan.olsztyn.pl
Pol. J. Food Nutr. Sci.
2007, Vol. 57, No. 4(C), pp. 555-562
EFFECT OF PRE-DRYING METHOD ON THE QUALITY AND MECHANICAL PROPERTIES
OF FRENCH FRIES
Agnieszka Tajner-Czopek1, Adam Figiel2, Grażyna Lisińska1
Department of Food Storage and Technology, 2Institute of Agricultural Engineering; Wrocław University of Environmental
and Life Sciences
1
Keywords: microwave and convective pre-drying, French fries, quality, mechanical properties
Application of microwave drying method in the process of pre-drying potato strips before frying can determine French fries quality to much higher
degree, than convective methods. The aim of the work was determination of the effect of the method and degree of pre-drying potato strips after blanching on fat content in French fries and their mechanical and organoleptic properties.
It was stated that the pre-drying applied to French fries resulted in decreased fat absorption, during I stage of frying, regardless drying method used
and more satisfactory results were obtained at dehydration by 1-3%. French fries pre-dried by microwave methods characterized better organoleptic
properties than the ones pre-dried by convective method. French fries became harder as the degree of pre-drying of potato strips increased and pre-drying according to convective method proved to be the least advantageous for French fries texture. Microwave drying at magnetrons power level 480W
occurred to be the most appropriate method of French fries pre-drying before frying.
INTRODUCTION
Consumers demands regarding French fries quality are
quite high and they seem to continuously grow. First of all,
there are assessed such basic quality determinants as: colour, taste and odour, then consumer’s attention is focused
on fat content in fried product and its texture [Lisińska &
Leszczyński, 1989; Lisińska, 2006].
Colour is an important factor deciding about the acceptance of fried product by consumers. French fries, regardless
the way they are produced, should be of light, golden colour,
without brown penetrations, black spots or streaks and they
should feature even length and regular shape [Talburt &
Smith, 1987; Agblor & Scalon, 2000].
French fries odour and taste beside the colour are quality
determinant which was first obtained. French fries odour and
taste, apart from colour, should be specific, potato-oily, without any flavour of bitterness, burning or rancid fat [Talburt &
Smith, 1987; Lisińska & Leszczyński, 1989].
Another considerable quality determinant of French fries
and their acceptance on the part of consumers is fat content.
French fries with too high fat content are of oily taste, while
too low fat content deprive French fries of taste and smell
typical for fried products. Fat content in French fries depends
on both raw material parameters i.e. chemical composition
of potato tubers, and technological factors like: strip thickness, kind and parameters of blanching, pre-drying and frying
[Lisińska & Leszczyński, 1989]. The amount of fat absorbed
by a product in the course of frying depends not only on the
temperature and time of the process, but also on the kind of
oil used [Kita et al., 2007; Mellema, 2003; Saguy & Dana,
2003].
One of more significant sensory properties of fried potato products is their crispy, delicate texture [Tajner-Czopek
2003]. Texture of French fries has two meanings: external texture – crispness, and internal texture-mealiness. The external
layer of French fries should be crisp and tender, without hard,
leathery or gummy texture, whereas the interior should be
mealy, not watery or mushy, and there should be no separation
between crust. When broken, French fries surface should be
smooth, without any cracks [Talburt & Smith, 1987; Lisińska
& Leszczyński, 1989].
Texture is determined by a structure, shape, chemical composition, viscosity and other physical properties of a product
[Lisińska & Leszczyński, 1989; Szczesniak-Surmacka, 2002;
Tajner-Czopek, 2003]. Consumers especially value the following texture properties: crispness, tenderness, delicateness,
juiciness and sappiness, while they do not accept, e.g.: leathery, soaked, watery, disintegrating, nobly or mucous French
fries [Bourne, 2002; Szczesniak-Surmacka, 2002]. In order
to assess food texture, including fried potato products, e.g.:
French fries, nowadays, there are different measuring techniques, which involve among the others, sensory methods (e.g.
quantitative descriptive analysis – QDA) and instrumental
methods using measuring devices of Stevens or Instron type).
The latter ones enable conducting different tests (e.g.: shearing, bending or TPA tests) [Bourne, 2002]. Measurement of
French fries by an objective method enables the determina-
Author’s address for correspondence: dr inż. Agnieszka Tajner-Czopek, Department of Food Storage and Technology, Faculty of Food Science,
Wrocław University of Environmental and Life Sciences, Wrocław, ul. Norwida 25, 50-375, Wrocław, Poland; tel.: (48 71) 320 50 38; fax: (48 71) 320
52 21; e-mail: tajner@wnoz.ar.wroc.pl
© Copyright by Institute of Animal Reproduction and Food Research of the Polish Academy of Sciences
556
tion of selected properties: maximum shearing force (Fmax),
shear work (W) and shear stress (τ max). The maximum values
of shearing force, shear stress and shear work are mechanical properties which are important in assessing raw material
damaged when harvested or transported [Gieroba & Dreszer,
1993] and the texture of the ready product for evaluation.
TPA test (Texture Profile Analysis) makes it possible to
analyse French fries texture and, at the same time, to apply
sensory and objective analysis [Szczesniak-Surmacka, 2002].
The basic properties which characterized of TPA test are cohesiveness, adhesiveness and hardness (texture) [Bourne,
2002].
French fries texture can be modified thought appropriate
combination of technological parameters of particular selected stage of the process. The basic technological process includes: blanching, pre-drying, pre-frying and frying [Lisińska
& Leszczyński, 1989; Abglor & Scalon, 2000]. Blanching is a
one of the technological stages of French fries production, the
proper choice of parameters for that stage enables obtaining
good quality products as well as improvement of those properties of French fries which do not meet the norm [Lisińska &
Leszczyński, 1989]. The results of properly combined parameters of blanching is unified, even of colour of French fries,
decreased absorption of fat when fried and appropriate texture featuring ready product.
Another process which apart from blanching, can effect the
improvement of French fries sensory properties is pre-drying
of potato strips previously subjected to blanching. Pre-drying
before frying has the following advantages: it is very easy to
main uniform colour of the product during and after frying,
product stability is optima, weak and soggy product is not obtained after the frying process, the oven-ready product stays
crispy from the kitchen the table and the pre-dried product
absorbs much less fat during frying [Lisińska & Leszczyński,
1989; Abglor & Scalon, 2000].
The process of pre-drying and drying, applied in food processing factory, can be conducted with the use of convective,
microwave – at different magnetron power methods or the one
which combines the mentioned methods. Microwave drying
enables heating the material as a whole bulk, which considerably favors this technique in comparison to the conventional
ones [Jones, 1992]. Additional advantage of microwave method is shortening of drying time, which means shorter time of
exposing this material to oxygen contact at higher temperature and reduction of negative effects of biochemical alterations (i.e.: fat oxidation), as well as maintaining appropriate
colour and nutritive value of product [Drouzas and Schubert,
1996].
Probably, application of microwave drying method in the
process of pre-drying potato strips before frying can determine
French fries quality to much higher degree than convective
methods. Decreased fat content in French fries, maintaining
their proper colour and texture of a ready product can speak
for application of microwave method to pre-dry potato strips
in spite of its high exploitation costs [Wang et al., 2004].
The aim of the work was determination of the effect of the
method and degree of pre-drying potato strips after blanching on fat content in French fries and their mechanical and
organoleptic properties.
A. Tajner-Czopek et al.
MATERIALS AND METHODS
Materials. The material used for investigation were potatoes of Felsina cultivar, originating from French fries producing plant in the region of lower Silesia.
Analysis. Washed and peeled potatoes were cut into
potato strips 10 x 10mm and then blanched in water at the
temperature of 75° C for 10 min. Blanched potato strips were
divided into five parts: first included blanched potato strips
without pre-drying application (control sample), four residue
of them were subjected to pre-drying, according to two methods: microwave and forced convective. Microwave pre-drying
was followed at lowered pressure, and three magnetron power
levels (240W, 480W and 720W) and changing pressure within
the range from 4 kPa to 6 kPa, by the use of Plazmatronika
SM 200 dryer. Another method involved forced convective
at the temperature of 55°C at drying air speed amounting 4
m/s. Convective pre-drying method was done using drying
equipment design and built up in the Institute of Agricultural
Engineering (Wrocław University of Environmental and Life
Sciences). The convective pre-drying enabled simultaneous
drying of 6 batches of material. Pre-drying of potato samples
according to each particular method was conducted within
the range from 23% to 30% of dry matter, i.e.: decreasing potato strips moisture by 1, 3, 5 and 7%.
Then there were made fries using two-stage frying method
in rape oil, from potato strips of all the samples. The first
stage of frying involved the temperature of 175°C, frying
time amounted 1 minute and the samples were subsequently
cooled and frozen to the temperature of –18°C. The second
stage of frying featured the temperature of 175°C and 5 minute frying time.
Dry matter was determined with the use of a dryer method,
in potato strips previously subjected to blanching and pre-drying (to 24%, 26%, 28% and 30% of dry matter) and in French
fries after first and second stage of frying [AOAC, 1995]. In
French fries after I and II stage of frying there was determined
fat content, according to Soxhlet method, throught sample
extraction with diethyl ether using the device of Büchi Distillation Unit B-316 type.
Five minutes after frying French fries were subjected to
mechanical properties measurement, i.e.: texture, shearing
work and TPA test. The measurement was done with the use
of measuring device of Instron 5544 type with Merlin software
and using different attachments. To measure of French fries
texture there was applied shearing attachment QTS-SB-25,
which was moving at the speed of 250 mm/min. To conduct
of TPA test a compressing plate was applied of 227 diameter.
Each texture measurement involved collection of 15 French
fries and shearing of each sample took place in the middle of
their length. There was determined maximum shearing force
Fmax (N) needed to shear each sample, as well as work done to
obtain this force value (shearing work Wtmax). TPA test measurement was repeated 6 times, each French fries underwent
shearing twice (second shift of a moveable plate amounts
70% of basic height). On the basis of the test there were determined the select, following parameters: hardness, tenacity,
springiness and gumminess.
557
Effect of pre-drying method on properties of French fries
Ten minutes after frying, French fries underwent organoleptic assessment according to a 5-point scale, as well as
regarding their colour – using objective method based on measuring device Minolta Chroma Meter CR-200 type. Differences of samples colour were provided as CIE system with L, a,
b coordinates, (value L – indicates lightness i.e.: the L value
represents light-dark spectrum with a range from 0 (black)
to 100 (white)., value a – measuring the colour of green-red
range, value b – measuring the colour of blue-yellow range)
[Minolta, 1994; Pomeranz & Meloan, 1994].
Statistical analysis. Results were analysed statistically
using Statistica v.7.1 programme with one-way analysis of
variance and multiple range test. There were determined homogenous groups and the least significant difference (LSD)
according to Duncan’s test (at significance level of p≤0.05).
Table 1. The content of dry matter in French fries after stage I and II
of frying as dependent on method and degree of pre-drying the potato
strips.
Microwave
(240W)
Microwave
(480W)
Microwave
(720W)
Convective
Pre-dying
degree (%)
Dry matter
(%)
0
1
3
5
7
0
1
3
5
7
0
1
3
5
7
0
1
3
5
7
23
24
26
28
30
23
24
26
28
30
23
24
26
28
30
23
24
26
28
30
Microwave
(240W)
Potato strips
Pre-drying
degree (%)
0
1
3
5
7
Dry matter
(%)
23
24
26
28
30
Mean of pre-drying degree
LSD
Microwave
(480W)
0
1
3
5
7
23
24
26
28
30
LSD
The process of pre-drying potato strips before frying results in the increase in French fries dry matter and therefore
it can effect on their quality. In Table 1 there was shown dry
matter content in French fries after I and II stage of frying,
depending on the method and degree of pre-drying potato
strips. It could be stated that depending on the degree of predrying potato strips, French fries dry matter increased after
I and II stage of frying. Pre-drying potato strips within the
range from 23% to 30% dry matter (i.e. decrease in moisture
by 7%) caused the incerase in French fries dry matter, after
their II stage of frying, predried using microwave method
(480W) – by about 20%, while in the case of those pre-dried
with other methods – average by 28%.
Fat content in French fries was one of determinants of
their quality. On the basis of the results tabled in Table 2 it was
Potato strips
Pre-drying
method
Mean of pre-drying degree
RESULTS AND DISCUSSION
Pre-drying
method
Table 2. The content of fat in French fries after stage I and II of frying
as dependent on method and degree of pre-drying the potato strips.
Microwave
(720W)
0
1
3
5
7
23
24
26
28
30
Mean of pre-drying degree
LSD
Fat content in French fries
(%)
(g/100 g s.m)
3.11c
2.10a
2.60b
3.41d
4.07e
11.51d
7.39a
8.49b
10.33c
11.44d
3.05AB
9.83B
0.19
1.05
3.11c
2.40a
2.83b
3.11c
3.22d
11.51c
7.82a
8.53b
8.99b
8.63b
2.93A
9.09A
0.09
0.65
3.11b
2.92a
3.05ab
3.40c
3.77d
11.51d
9.37ab
9.10a
9.91b
10.74c
3.25AB
10.13BC
0.14
0.46
3.11b
2.73a
3.40c
3.55d
3.97e
11.51c
8.86a
10.51b
10.66b
11.39c
3.35B
10.58C
LSD
0.11
0.71
Mean of pre-drying method
3.14
9.90
LSD
0.35
0.72
Convective
0
1
3
5
7
Mean of pre-drying degree
23
24
26
28
30
difference letters (a,b,c,d,e) indicate significant differences among results
of pre-drying degree (p ≤ 0.05); difference letters (A,B,C) indicate significant differences among results of pre-drying method (p ≤ 0.05); LSD
– least significant difference
Dry matter (%)
French fries
after stage I
of frying
27.04
28.40
30.59
33.02
35.55
27.04
30.70
33.20
34.60
37.31
27.04
30.80
32.35
33.30
34.85
27.04
31.15
33.50
34.30
35.10
French fries
after stage II
of frying
55.50
57.36
62.98
64.19
77.23
55.50
58.14
59.11
62.23
68.98
55.50
58.15
65.10
69.90
77.43
55.50
65.10
72.43
73.96
75.40
proved that average fat content in French fries after I stage of
frying, predried using microwave method (480W) was by about
4% lower in comparison to the samples pre-dried by microwave
method (240W), by about 10% lower in the case of microwave
method (720W) and by about 13% using convective metod.
The lowest fat content featured half-product (French fries after
I stage of frying) after pre-drying by microwave method (480W)
– average 2.93% (9.09g/100g dry matter), while the highest value
was obtained for pre-drying with the use of convective metod –
average 3.35% (10.58g/100g dry matter). In ready French fries
pre-dried acording to microwave method (480W) fat content
was the lowest and amounted average 11.34% (18.69g/100g dry
matter) and it was nearly 1.5 times lower than that of samples
pre-dried by convetive method (Table 3).
Analysing the results in Tables 2 and 3 it can be recorded that
fat content in French fries after I and II stage of frying showed
a tendency to decreasing in relation to not pre-dried sample
(control sample), regardless the method and degree of strips
pre-drying. I was observed that alteration in strips dry matter
558
A. Tajner-Czopek et al.
Table 3. The content of fat in French fries after stage II of frying as
dependent on method and degree of pre-drying the potato strips.
Pre-drying
method
Microwave
(240W)
Potato strips
Pre-drying
degree (%)
0
1
3
5
7
Dry matter
(%)
23
24
26
28
30
Mean of pre-drying degree
LSD
Microwave
(480W)
0
1
3
5
7
23
24
26
28
30
Mean of pre-drying degree
LSD
Microwave
(720W)
0
1
3
5
7
23
24
26
28
30
Mean of pre-drying degree
LSD
Convective
0
1
3
5
7
Mean of pre-drying degree
23
24
26
28
30
Fat content in French fries
Table 4. French fries colour (L, a, b – values) as dependent on method
and degree of pre-drying the potato strips.
Pre-drying
method
Potato strips
Pre-drying Dry-matter
degree (%)
(%)
0
23
1
24
3
26
5
28
7
30
(%)
(g/100 g s.m)
11.81c
10.58a
10.88b
12.98d
15.56e
21.28d
18.44b
17.27a
20.22c
20.14c
Microwave
(240W)
12.36AB
19.47A
Mean of pre-drying degree
0.14
1.10
11.81c
9.98a
10.23b
12.23d
12.45e
21.28d
17.16a
17.31a
19.65c
18.05b
LSD
Microwave
(480W)
11.34A
18.69A
Mean of pre-drying degree
0
1
3
5
7
0.15
0.27
11.81a
12.35b
13.45c
13.69d
18.19e
21.28b
21.23b
20.66a
19.58a
23.49c
Microwave
(720W)
13.90BC
21.24B
Mean of pre-drying degree
0.15
1.25
11.81b
10.82a
15.12c
17.30d
20.13e
21.28b
16.62a
20.87b
23.39c
26.69d
Convective
15.03C
21.77B
Mean of pre-drying degree
23
24
26
28
30
LSD
0
1
3
5
7
23
24
26
28
30
LSD
0
1
3
5
7
23
24
26
28
30
French fries colour
L
a
b
56.07a
53.00d
51.25c
50.75b
48.25a
2.71a
4.10bc
3.30b
3.40b
3.70bc
30.82c
30.35c
27.05a
30.50c
28.50b
51.86A
3.44C
29.44A
0.22
0.45
1.25
56.07b
57.00c
61.75e
57.60d
49.95a
2.71d
1.09b
0.40a
1.33c
1.01b
30.82c
31.10c
29.40b
32.10c
28.20a
56.47C
1.31A
30.32A
0.10
0.22
1.1
56.07d
56.75e
55.00c
51.55b
50.75a
2.71c
0.55a
1.80b
3.78d
2.85c
30.82b
28.16a
31.85b
33.18c
27.70a
54.12B
2.33B
30.34B
0.17
0.90
1.11
56.07c
53.90a
56.85d
55.00b
56.70d
2.71c
5.85d
0.40a
1.10b
1.95b
30.82cd
32.40d
25.60a
27.60b
30.50c
55.70BC
2.40B
29.38A
LSD
0.15
1.15
LSD
0.18
0.68
1.85
Mean of pre-drying method
13.15
20.29
Mean of pre-drying method
54.53
3.12
29.87
LSD
1.74
1.65
LSD
2.01
1.01
0.95
difference letters (a,b,c,d,e) indicate significant differences among results
of pre-drying degree (p ≤ 0.05); difference letters (A,B,C) indicate significant differences among results of pre-drying method (p ≤ 0.05); LSD
– least significant difference
difference letters (a,b,c,d,e) indicate significant differences among results
of pre-drying degree (p ≤ 0.05); difference letters (A,B,C) indicate significant differences among results of pre-drying method (p ≤ 0.05); LSD
– least significant difference
amount ranging from 23% to 24% (i.e. dehydration by 1%) did
considerably effect on fat content lowering in French fries predried according to the examined methods (fat content showed
the lowest, the most advantageous values). However, as the degree of strips pre-drying increased within the range from 26% to
30% (potato strips dehydration from 3% to 7%) fat content in
semi-product and in French fries also did increase. Krokida et
al. [2001b] and Mayano et al. [2002], report that drying of potatoes before frying using microwave and hot-air treatment has
resulted in reduction in oil content of different products.
The application of strips pre-drying by different methods,
before frying significantly effected on French fries colour. It
was noted that depending on pre-drying metod determinant
“L” value ranged from 56.47 to 51.87, as far as “a” determinat
was concerned – the values amounted from 3.44 to 1.31, while
“b” determinant showed the values from 29.44 to 30.34 (Table
4). On the basis of determinants L, a and b obtained it was possible to state that the lighnest (desirable) colour characterised
French fries made of strips pre-dried by microwave method at
power applied ranging (480W), while the darkest colour (the
least desirable) featured French fries pre-dried by microwave
method (240W). It was recorded that longer potato strips predrying, prior to frying, within the range from 26% to 30% of dry
matter (increased strips dehydration from 3% to 7%), regardless pre-drying method, did considerably worsen the colour of
ready product. The advantage of microwave pre-drying is the
fact that this treatment alters dryied material colour only to a
small degree, e.g. potato slices [Bouraoui et al., 1994], apples
[Feng & Tang, 1998] or banana [Maskan, 2000] in comparison
to pre-drying by convective method (with hot air blow) which
results in significant worsening of product colour. Krokida et
al. [2001a] investigated the effects of different drying methods
on the colour of the obtained products. They found that colour
characteristic are significantly affected by the drying methods
and that the changes in “a” and “b” values. In paticular, air-,
vacuum and microwave drying caused extensive browning in
fruit and vegetables with a decrease in “L” value and an increase in “a” and “b” values improved colour characteristic.
559
Effect of pre-drying method on properties of French fries
Table 5. Organoleptic estimation (points 1-5) of French fries frying as dependent on method and degree of pre-drying the potato strips.
Potato strips
Pre-drying degree
Pre-drying method
Dry matter
Microwave
(240W)
Microwave
(480W)
Microwave
(720W)
Convective
Mean results
of organoleptic
estimation
(points 1-5)
(%)
(%)
0
23
4.3
4.3
4.3
4.3
4.3 b
1
24
4.5
4.5
4.6
3.9
4.3 b
3
26
4.0
4.7
4.5
4.0
4.3 b
5
28
4.3
4.3
4.6
4.1
4.3 b
7
30
4.0
4.5
4.3
3.6
4.1 a
4.2 B
4.5 C
4.4 BC
3.9 A
Mean
organoleptic estimation (points 1-5)
LSD pre-drying degree
0.19
LSD pre-drying method
0.25
difference letters (a,b) indicate significant differences among results of pre-drying degree (p ≤ 0.05); difference letters (A,B,C) indicate significant differences among results of pre-drying method (p ≤ 0.05); LSD – least significant difference
Basing on the results of organoleptic assessment (acording to a 5-points scale) of French fries, depending on potato
strips pre-drying metod and its degree, it was stated that the
method of pre-drying did considerably effect on French fries
assessment (Table 5). French fries pre-dried by mirowave
methods scored higher (from 4.2 to 4.5 points) in comparison to those pre-fried using convective method (3.9 points).
Among French fries dried by microwave methods, higest were
assessed the samples pre-dried at the power of 480W (4.5
points), while lowest assessment values were given to those
pre-fried at the power of 240W (4.2 points). French fries predried within the range of 23% to 28% of dry matter (i.e. decreased moisture by 5%), gained 4.3 points and only strips
dehydration to 7% caused lower assessment – 4.1 points.
Frech fries texture measurement by the objective method
enables determination of its characterisic mechanical properties, i.e. maximum shearing force (cutting) and cutting force
[Figiel & Frontczak, 2001; Tajner-Czopek & Figiel, 2003].
On the basis of French fries texture assessment it was proved
that as the degree of potato strips pre-drying before frying
increases within the range from 24% to 30% of dry matter,
French fries ecame harder and harder (Table 6) and the values of French fries maximum shearing force were also higher
(Figure 1). It was reported significant effect of French fries
pre-drying method prior to frying on the increase in ready
product hardness appropriate texture featured french fries
obtained from potato strips pre-dried by icrowave metod at
480W power applied – average 32.4 N. Those fries were of
the best texture as compared to French fries dried by convective method – average 35.5 N (too hard). Texture is a meaningful factor deciding about French fries quality [Krokida
et al., 2001b] and therefore, potato strips pre-drying before
frying should be conducted mainly by microwave method,
with appropriately combined parameters and the possibility of eliminating pre-drying by forced convective method.
Bushway et al. [1984] compared the texture of microwave
pre-treated fries with convectional French fries and they
noted an improvement of their texture caused by microwave
pre-treatment.
Table 6. The texture of French fries as dependent on method and degree
of pre-drying the potato strips.
Pre-drying
method
Microwave
(240W)
Potato strips
Pre-drying degree
(%)
0
1
3
5
7
Texture
Dry matter
(%)
23
24
26
28
30
Mean of pre-drying degree
1.33
0
1
3
5
7
23
24
26
28
30
Mean of pre-drying degree
1.59
0
1
3
5
7
23
24
26
28
30
Mean of pre-drying degree
30.1a
31.0a
33.2b
35.6c
37.9d
33.5B
1.41
LSD
Convective
30.1a
31.2ab
32.0bc
33.5c
35.3d
32.4A
LSD
Microwave
(720W)
30.1a
32.1b
33.2bc
34.1c
36.9d
33.3AB
LSD
Microwave
(480W)
(N)
0
1
3
5
7
Mean of pre-drying degree
23
24
26
28
30
30.1a
32.5b
35.4c
38.4d
41.3e
35.5C
LSD
0.89
Mean of pre-drying method
33.67
LSD
1.05
difference letters (a,b,c,d,e) indicate significant differences among results
of pre-drying degree (p ≤ 0.05); difference letters (A,B,C) indicate significant differences among results of pre-drying method (p ≤ 0.05); LSD
– least significant difference
560
A. Tajner-Czopek et al.
Figure 1. The shearing work of French fries as dependent on the degree
and method of pre-drying the potato strips.
Figure 2. The maximum shearing force of French fries as dependent on
the degree and method of pre-drying the potato strips.
Table 7. Chosen parameters of determination of the TPA test as dependent on the method and degree of pre-drying the potato strips.
Pre-drying
method
Microwave (240W)
Potato strips
Pre-drying degree
(%)
0
1
3
5
7
Dry matter
(%)
23
24
26
28
30
Mean of pre-drying degree
LSD
Microwave (480W)
0
1
3
5
7
23
24
26
28
30
Mean of pre-drying degree
LSD
Microwave (720W)
0
1
3
5
7
23
24
26
28
30
Mean of pre-drying degree
LSD
Convective
Mean of pre-drying degree
0
1
3
5
7
23
24
26
28
30
TPA parameters
Hardness
(N)
142a
159a
189b
199b
216c
Cohesiveness
(mJ)
0.17a
0.19a
0.24b
0.31c
0.34c
Springiness
(mm)
2.34a
2.96a
3.66b
3.99bc
4.30c
Gumminess
(N)
24.0a
35.6b
49.2c
54.7cd
64.0d
181AB
0.25AB
3.45B
45.5B
16
0.03
0.63
10.7
142a
145a
152b
159b
162b
0.17a
0.20a
0.29b
0.35c
0.39d
2.34a
2.97b
3.88c
4.02c
4.39c
24.0a
34.8b
41.9bc
49.6c
51.2c
152A
0.28B
3.52B
40.3A
11
0.03
0.62
11.0
142a
159a
195b
210b
279c
0.17a
0.18a
0.21b
0.23b
0.31c
2.34a
2.89a
3.67b
4.12bc
4.53c
24.0a
35.6b
48.4c
56.2cd
60.3d
197B
0.22A
3.51B
44.9B
15
0.02
0.54
11.2
142a
198b
259c
296cd
335d
0.17a
0.18b
0.22bc
0.26c
0.32d
2.34a
2.79b
3.09b
3.54c
4.39d
24.0a
39.8b
59.4c
71.3cd
83.0d
246C
0.23A
3.23A
55.5C
LSD
45
0.04
0.43
14.5
Mean of pre-drying method
194
0.24
3.42
46.55
LSD
44
0.03
0.20
4.5
difference letters (a,b,c,d,e) indicate significant differences among results of pre-drying degree (p ≤ 0.05); difference letters (A,B,C) indicate significant
differences among results of pre-drying method (p ≤ 0.05);
LSD – least significant difference
561
Effect of pre-drying method on properties of French fries
On the basis of the investigation conducted it was stated
that the value of French fries shearing work increased as the
degree of potato strips pre-drying before frying increased,
within the ranged from 24% to 30% of dry matter (Figure 2).
Shearing (cutting) work was strictly dependent on texture results (Figures 1 and 2). On the basis the results was stated
that the lowest values of shearing work (from 0.07 J to 0.09 J)
were obtained for shearing French fries made of potato strips
pre-dried by microwave method at the power of 480W (proper
texture 32.4 N), while the highest work values (the range from
0.08 J to 0.15 J) at congvective pre-drying (to hard texture –
35.5 N). Tajner-Czopek & Figiel [2003] stated that values of
shearing work depended on the maximum shearing force (texture) of French fries, alterations of related data were directly
proportional.
TPA test parameters enable accurate and thorough analysis of French fries texture. In Table 7 there were shown the
results of determination of selected TPA test parameters (hardness, cohesiveness, springiness and gumminess) in French
fries, depending on the method and degree of potato strips
pre-drying. There was recorded considerable effect of potato
strips pre-drying degree (within the range from 24% to 30% of
dry matter) before frying on the increase in hardneness values
cohesiveness, springiness and gumminess of ready product.
French fries pre-dried by microwave method (480W} characterized the lowest hardness (average 152 N) and gumminess (average 40.3 N), and those values were 1.5 times lower
as compared to those of French fries pre-dried by convective
method. Microwave method at the power of 480W applied to
French ries pre-drying did improve such parameters as cohesiveness (average 0.28 mJ) and their springiness (average
3.52 mm).
The results of both tests conducted – shearing resistance
and TPA proved that appropriate texture belonged to French
fries pre-dried by microwave method at 480W power, while
too hard ready product was obtained after potato strips
pre-drying by convective method. The pre-drying processes
depend on the physical state of the material subjected to drying [Lewicki, 2006], and selecion of the method aims al the
improvement of product quality. Microwave pre-drying has
significant advantage than convective method, because microwave method is rapid, more uniform, convinient and more
efficient compared with convectional hot air pre-drying [Wang
et al., 2004] The products pre-dried using microwaves maintain appropriate colour, texture and taste. Should be remember that microwave pre-drying is know to results in a poor
quality if not properly applied of parameters the microwave
method [Drouzas & Schubert, 1996].
CONCLUSIONS
1. Pre-drying applied to French fries resulted in decreased
fat absorption, during I stage of frying, regardless drying
method used and more satisfactory results were obtained at
dehydration by 1-3%.
2. French fries pre-dried by microwave methods characterized better organoleptic properties than the ones pre-dried
by convective method.
3. French fries became harder as the degree of pre-dry-
ing of potato strips increased and pre-drying according to
convective method proved to be the least advantageous for
French fries texture.
4. Microwave drying at magnetrons power level 480W occurred to be the most appropriate method of French fries predrying before frying.
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WPŁYW METODY PODSUSZANIA FRYTEK ZIEMNIACZANYCH NA ICH JAKOŚĆ
I WŁAŚCIWOŚCI MECHANICZNE
Agnieszka Tajner-Czopek1, Adam Figiel2, Grażyna Lisińska1
Katedra Technologii Rolnej i Przechowalnictwa, 2Instytut Inżynierii Rolniczej, Uniwersytet Przyrodniczy we Wrocławiu, Polska
1
Zastosowanie metody suszenia mikrofalowego w procesie podsuszania krajanki przed smażeniem, w zdecydowanie większym stopniu może
kształtować jakość frytek, w porównaniu z metodami konwekcyjnymi. Celem pracy było określenie wpływu metody i stopnia podsuszania krajanki
ziemniaczanej po blanszowaniu, na zawartość tłuszczu we frytkach oraz ich właściwości mechaniczne i organoleptyczne.
Na postawie badań stwierdzono, że podsuszanie frytek przed smażeniem wpłynęło na mniejszą chłonność tłuszczu, podczas pierwszego
stopnia smażenia niezależnie od stosowanej metody podsuszania, przy czym lepsze rezultaty uzyskano przy odwodnieniu o 1-3%. Frytki podsuszane metodami mikrofalowymi charakteryzowały się lepszymi cechami organoleptycznymi, niż podsuszane metodą konwekcyjną. Wraz ze
wzrostem stopnia podsuszania krajanki przed smażeniem otrzymane frytki były bardziej twarde, przy czym najmniej korzystnie na teksturę frytek
wpłynęło podsuszanie metodą konwekcyjną. Suszenie mikrofalowe, przy poziomie mocy magnetronów 480W, jest najbardziej odpowiednią metodą do zastosowania przy podsuszaniu frytek przed smażeniem.