Effect of Methyl Cellulose Coating and Pre-Treatment on Oil Uptake, Moisture Retention and Physical Properties of Deep-Fat Fried Starchy Dough System

American Journal of Agricultural and Biological Sciences 4 (2): 156-166, 2009
ISSN 1557-4989
© 2009 Science Publications

Effect of Methyl Cellulose Coating and Pre-Treatment on Oil Uptake, Moisture
Retention and Physical Properties of Deep-Fat Fried Starchy Dough System

1Jihad M. Quasem, 2Ayman Suliman Mazahreh, 3Khaled Abu-Alruz, 4Ibrahim A. Afaneh,
5Ala’a H. Al-Muhtaseb and 6T.R.A. Magee
1Department of Medical Allied Science, Zarka University Collage, Al-Balqa'
Applied University, Zarka, Jordan
2Department of Applied Science, Al-Balqa Applied University,
Princes Alia University College, P.O. Box 941941 Zarqa11194, Jordan
3Department of Food Science and Nutrition, Faculty of Allied Medical Sciences,
Applied Science University, Amman, Jordan
4Department of Food Processing, Faculty of Science and Technology,
Al-Quds University, P.O. Box 20002, Jerusalem, Via Israel
5Department of Chemical Engineering, Faculty of Mining and Environmental Engineering,
Al-Hussein Bin Talal University, Ma'an 20, Jordan
6School of Chemistry and Chemical Engineering, Queen’s University Belfast,
David Keir Building, Stranmillis Road, Belfast BT9 5AG, UK

Abstract: Problem statement: The influence of edible methyl cellulose coating and blanching pre-
treatment in reducing oil uptake and moisture loss during frying of starchy dough system was
investigated. Approach: Potato dough cylinder of 60 mm length and 22 mm diameter was used as a
model food system. Samples were coated with 0.5% methyl cellulose film-forming solution and
uncoated samples were used as control. Compared to the control samples, a reduction of 80% in oil
uptake was achieved, with an increase in frying temperature decreasing the oil uptake due to the gel
formation of methyl cellulose which was enhanced by higher temperatures. Results: No effect of
methyl cellulose coating on the final moisture content was observed. The effect of methyl cellulose
coating on structural properties (diameter, length, volume and bulk density) was also examined. The
results showed that methyl cellulose coating had a significant effect on dimensional and density
changes, as it enhanced the formation of crust, leading to a buildup in pressure within the cylinder,
which in turn caused considerable puffing of the sample. Conclusion: Blanching pre-treatment
(100°C, 5 min) was found to be effective in reducing the oil content of the potato dough samples.

Key words: Edible coating, methyl cellulose, blanching, deep-fat frying, starch system, physical
properties

INTRODUCTION
potatoes, the moisture content decreases from

approximately 80-2%. This high removal inevitably

One of the primary objectives of food processing is
leads to a considerable oil uptake which amounts to
to increase food palatability. Deep fat frying is a widely
about 35-45% of the chip mass; this high consumption
used method for preparing foods with an attractive and
of fat is a key dietary contributor to high blood
tasty surface. The soft and moist interior along with the
cholesterol, high blood pressure, coronary heart
porous crispy crust provides increased palatability to
diseases and conducive to obesity[1]. Hence, due to
foods[39]. Deep-fat fried food constitutes a major part of
consumer health concerns and the trend toward
daily food consumption[26]. French fried potatoes
healthier and low-fat food products, intensive research
represent one of the major items in the food market,
has been undertaken to reduce the amount of fat
either as a processed product or as a frozen par-fried[17].
absorbed in the fried foods. Various factors affect the
However, during deep fat frying of French fried
quality of the fried product, such as temperature of the
Corresponding Author: Jihad M. Quasem, Department of Medical Allied Science, Zarka University Collage,

Al-Balqa' Applied University, Zarka, Jordan
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heated oil, frying duration, frying oil volume, types of
Pedreschi et al.[27] also reported that penetrated surface
oil, product composition, shape, porosity, moisture
oil constituted the highest fraction of the total oil
content and pre-treatment of the food product[35].
content, confirming that oil absorption in potato chips is

Early results obtained by Pravisani and Calvelo[42]
mainly a surface phenomenon.
showed that oil content was independent on the frying

Moreira et al.[25] exhibit the effect of frying time on
temperature, range between 155 and 200°C. This
the moisture content of tortilla chip. They elucidated
result is in agreement with Gamble et al.[13,14] and
that although the result shows a moisture decrease
Moreira et al.[24] who studied the effect of different
along the frying period (60 sec); tortilla chips losses
frying temperatures on potato crisps, tortilla chips,
most of its moisture content within the first 15 sec. This
plantain slice and plantain slices, respectively. The
observation was in agreement with the work of
former found that although the early stages of frying
Diaz et al.[8] who reported a similar trend when
showed the lower oil temperature resulted in lower oil
examining the moisture loss in the frying of different
content, but the overall oil content is temperature
types of plantain for different frying time. They found
independent within the oil temperature examined.
that the drop in moisture content was higher during the
However, Lisinska and Leszczynski[41] found out that
first 90 sec frying than with the long frying time.
potato chips fried at higher oil temperature for a short

Many approaches to reduce oil absorption in fried
period of time contain less oil than those fried at lower
products have been reported in the literature, which are
temperatures. Moyano and Pedreschi[26] found out that
focused on cooking technology (time/temperature),
oil uptake increased approximately by 32% as the
ingredients addition (chemical compounds) and pre-
frying temperature decreased from 180 to 120°C at
treatment (pre-drying, osmotic pre-treatment and
moisture content ?1 g water/g dry solid. They also
blanching)[32]. Bunger et al.[4] reported that potato strips
reported that oil uptake was high even for short frying
soaked in 3% NaCl solution for 50 min prior to frying
times at the different temperatures tested (120-180°C)
significantly reduced oil uptake from 0.13-0.10 g oil/g
suggesting that oil wetting is an important mechanism
dry matter. However, soaking had no effect on moisture
oil uptake during frying. Ufheil and Escher[39]
loss during deep fat frying. Rubnov and Saguy[33] added
confirmed experimentally, that most of the oil is
fructose to restructured potato samples which resulted in
absorbed when the product is removed from the oil bath
a change of the surface properties, with a reduction of
and suggested that oil uptake is primarily a surface
absorbed oil. Krokida et al.[20] examined the use of
phenomenon. Moreira et al.[24] also found that most of
osmotic dehydration as a pre-treatment to produce low-
the oil penetrates within the structure of a tortilla chip
fat French fries, where four types of solutions (sugar,
during the post-frying cooling period and not during
NaCl, maltodextrine 12 and maltodextrine 21) were used.
immersion. They found that only 20% of the oil is
The results showed that osmotic pre-treatment decreases
absorbed during the immersion period, while 64% of
oil and moisture content. Rimac et al.[32] investigated the
the total oil content is absorbed during post-frying
influence of osmotic pre-treatment, using 0.5% NaCl
cooling, suggesting that oil absorption is related to
solution, on the oil absorption in fried potato. The
capillary pressure differences. Diaz et al.[8] found that
results obtained have shown the pre-frying treatment
during frying, oil uptake have two stages; the first
considerably decreased the oil absorption by 28%. Mai
stage remarked as considerable oil content between 0
Tran et al.[22] reported that crisp samples, soaked in
and 20 sec and the second stage was starting at 20 sec,
sugar solution (23 wt %) for 2 sec before frying, had
which noted as a decrease in the oil uptake rate.
about 30% less oil than untreated samples. However,
Pedreschi et al.[27] analyzed oil penetration into potato
treatment did not affect the final moisture content of
cylinders during deep-fat frying, distinguishing between
crisps.
the different oil fractions that are absorbed during the

Recently, much attention has been given to the use
process: Structural oil (absorbed during frying),
of hydrocolloids (long chain polymers), especially
penetrated surface oil (surface oil suctioned during
cellulose derivatives, as potential solutions by forming
cooling) and surface oil (fraction of oil that remains on
a barrier to fat absorption during the frying process[34].
the surface). They found that only a small amount of oil
Hydrocolloids have been used as multifunctional
is absorbed as ‘structural oil’ during the immersion
additives in food processing. They are added to control
period, since the vigorous escape of water vapor
and to improve functional properties like viscosity,
precludes oil migration into the porous structure. Thus,
water binding capacity and emulsion stability[32].
they demonstrated that most of the oil was picked up at
Hydrocolloids are of a special interest because they
the end of the process, suggesting that oil uptake and
possess good barrier properties to oxygen, carbon
water removal are not synchronous phenomena.
dioxide and lipids[40]. Applicable hydrocolloids include
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proteins, cellulose derivatives, alginates, pectins,
Food Slicer, Kenwood, UK) to give smooth and
starches and other polysaccharides[1].
perpendicular edges. Samples were then rinsed in

Feeney et al.[12] reported the used corn zein as an
distilled water to eliminate some starch material
edible coating in French fried potatoes and reported a
adhering to the surface. Any surface moisture present
28% reduction in oil uptake. Mallikarjunan, Chinnan,
was adsorbed with filter paper and then weighed using
Balasubramaniam and Phillips[23] found that a reduction
a top-loading electronic balance (Denver Instrument®,
of 59, 61 and 83% in fat uptake was observed for
Model S-403) with an accuracy of ±0.001 g.
samples coated with Corn Zein (CZ), HPMC and Methyl

Cellulose (MC), respectively. Rimac et al.[32] mentioned
Osmotic pre-treatment: Potato samples were immersed
that 54% reduction in oil content was achieved by
in sodium chloride (AnalaR, BDH Laboratory Supplies,
blanching potato strips in 0.5% calcium chloride
UK) solutions of 5, 10 and 15% (w/w) concentration for
solution followed by immersion in 1% solution of
1, 4 and 8 h. Following the osmosis period, the samples
Carboxymethyl Cellulose (CMC). Salvador et al.[34]
were removed, rinsed and then blotted with filter paper to
reported that methyl cellulose performed effectively in
remove surface moisture.
reducing the amount of fat absorbed in a wide variety of

battered food items (marrow, meat and squid). Suarez,
Soaking in amylose solution: Potato samples were
Campanone, Garcia and Zaritzky[3] studied the effect of
soaked for 30 min in a 1.5% (w/w) solution of amylose
edible methyl cellulose coating in reducing oil uptake
(Corn Amylose, Practical Grade, Sigma, USA).
during frying of a dough system. The oil uptake
Following the soaking period, the samples were
reduction was 30% for coated dough samples, with no
removed, rinsed and then blotted with filter paper to
effect on the final moisture content and the quality,
remove surface moisture.
such as color and texture.


On reviewing the available literature, it is evident
Dipping in surfactant: Potato samples were immersed
that extensive research has been carried out to address
in polyoxyethylene-sorbitan-monolaurate (Tween 20)
various aspects of the frying phenomena. However, the
(BDH Laboratory Supplies, UK) solutions of 0.1, 0.5
use of edible ingredients as a surface treatment and
and 1% (w/w) concentration for 5 min. Following the
their effect on the oil uptake and water content during
soaking period, the samples were removed, rinsed and
frying are relatively limited in the literature. Therefore,
blotted with filter paper to remove surface moisture.
gathering more relevant data is necessary to promote

knowledge, simplify and delimit the subject to some
Dipping in Carboxyl Methyl Cellulose (CMC)
extent. With this in mind, the aim of this study was to
solution: Two investigations were carried out: (i)
(1) Examine the relationship between moisture loss and
samples were immersed in a 0.05% (w/w) solution of
oil adsorption with frying temperature and time; (2)
CMC (Aqualon Division, Hercules Ltd, UK.) for 5 min.
Investigate the influence of edible film coating
Following soaking, the samples were removed and then
(carboxymethyl cellulose) and osmotic pre-treatment
blotted with filter paper to remove surface moisture; (ii)
(NaCl, surface active agent, amylose) on the oil content
samples were dried to 15% (wet basis) in a convective
reduction and water loss phenomena during the frying
oven at 70±0.05°C, cooled and immersed in a 0.05%
of starchy food systems.
(w/w) CMC solution for 5 min. Following soaking, the

samples were removed and then blotted with filter
MATERIALS AND METHODS
paper to remove surface moisture.


Potato: Potato samples (Pentland Dell variety), were
Experimental procedure: The frying experiments
stored in darkness at 4±0.5°C before processing. After
were performed in a commercial deep fat fryer
stabilizing at room temperature for at least 24 h prior to
(Kenwood) with temperature controller of ±1°C. The
use, the potato samples peeled, thoroughly cleaned,
oil temperature was also monitored using a type K
washed and then trimmed, samples of the appropriate
thermocouple (Omega Engineering). The fryer was
size were cut using a cylindrical stainless steel die. The
filled with 2 l of sunflower oil and set to one of the
samples were taken from the central medulla region of
required operating temperatures (135, 150, 165 and
the potato, due to the existence of a more uniform cell
180°C). The oil was preheated for approximately one 1
structure. The samples were all cored along the
h prior to frying to ensure steady-state conditions. A
longitudinal axis in order to provide one morphological
pre-weighed, single cylindrical sample was then
cell structure. The cored sample (16±0.5 mm length and
positioned on a metal mesh basket, immersed in the hot
19 mm diameter) was then trimmed on a slicer (SL400
oil and fried for a predetermined period of time (0.5,
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1.5, 3, 4.5, 6, 7.5, 9 and 10.5 min) to determine the
data were most accurately fitted to an exponential
amount of oil uptake and water loss in the samples as
function with respect to time. In contrast, a linear
function of time. Fried samples were removed from the
relationship was reported by Gamble et al.[13,14] and
unit and the excess surface oil absorbed with filter
Reddy and Dias[31].
paper. Samples were then allowed to cool to room

The effect of frying temperature on moisture
temperature for 5 min before moisture and oil content
content is in agreement with numerous researchers
analysis was done, using the methods detailed in the
including[6,7,18,19,22-24,30].
following sections. After each frying test, oil level was

checked and replenished; oil was changed after 1 h of
frying time. The frying characteristics of the cylindrical
potato samples were investigated with regard to the
operating parameters of oil temperature, frying time,
edible film coating and osmotic pre-treatments. Each
experimental run was performed in triplicate.

Oil content determination: The oil content of the
sample was determined using soxhlet extraction
according to the method Aa 4-38 of the AOCS (1993).
Samples were dried to constant weight in a vacuum


oven at 70°C prior to extraction. The dried samples
Fig. 1: Effect of emulsifier (MC) on the oil uptake
then grounded in a blender and extracted with 250 mL
during frying at 165°C (cylinder-60 mm length,
petroleum ether for 4 h. Petroleum ether was recovered
22 mm diameter)
under vacuum at 60°C by a rotary evaporator. The

recovered oil was left for 24 h in a vacuum oven at
70°C and weighted.

Moisture content determination: The moisture
content of the sample, raw or fried, was determined
gravimetrically by drying representative triplicate
samples in a conventional oven at 105°C for 8-10 h[2].

RESULTS

Influence of methyl cellulose emulsifying agent:

Preliminary experiments were conducted using potato

dough cylinders without emulsifying agent (MC).
Fig. 2: Effect of emulsifier (MC) on moisture content
However, the sample was found to disintegrate after
during frying at 165°C (cylinder-60 mm length,
frying for approximately 180 sec (at 150°C or higher).
22 mm diameter)

Therefore, subsequent experiments were carried out
using potato dough cylinders fortified with methyl
cellulose emulsifier (0.5% w/w); the emulsifier acted as a
binder. In a preliminary study the frying characteristics
of potato dough (without emulsifier), potato dough (with
emulsifier) and potato at 165°C were compared.
Comparison of the moisture and oil contents of the
samples, as shown in Fig. 1 and 2, reveal an added
advantage of emulsifier addition to the dough.

A more in-depth series of frying experiments were
then undertaken using potato dough (with emulsifier) as
the raw material. As shown in Fig. 3, the experimental

results for the potato dough exhibited the expected

trend with respect to moisture content, i.e., the moisture
Fig. 3: Variation in moisture content with frying time
loss was found to increase with increasing frying time
and temperature (potato dough cylinder-60 mm
and temperature. The experimental moisture content
length, 22 mm diameter)
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Fig. 6: Effect of blanching treatment on sample oil
Fig. 4: Variation in moisture loss rate with frying time
content during frying at 165°C (potato dough
and temperature (potato dough cylinder-60 mm
cylinder-60 mm length, 22 mm diameter)
length, 22 mm diameter)






Fig. 5: Variation in oil content with frying time and
Fig. 7: Effect of blanching treatment on sample
temperature (potato dough cylinder-60 mm
moisture content during frying at 165°C (potato
length, 22 mm diameter)
dough cylinder-60 mm length, 22 mm diameter)



The moisture loss data was further represented in
Debnath et al.[7] found that pre-fry drying had a
terms of the moisture loss rate, as shown in Fig. 4.
significant effect on the moisture kinetic coefficient
Three stages of moisture loss could be identified in the
during deep fat frying of chickpea flour-based snack
moisture loss rate versus time graph.
food.

The corresponding oil content results for the potato

dough (with emulsifier) samples are shown in Fig. 5.
Potato dough with varying initial moisture content:
The maximum oil content was observed within the first
In order to simulate products with differing initial
60 sec, followed by a continual decrease as the frying
moisture content, but the same structural characteristics,
progressed.
potato dough samples with initial contents of 1.0, 1.5
?1

and 2.0 kg kg (dry basis) were prepared and fried at
Influence of blanching: Blanching is a pre-treatment
165°C. The oil uptake and the moisture loss rate were
method adopted during vegetable frying processes to
expected to be higher for the sample of higher initial
moisture content. The experimental oil content data are
enhance the frying characteristics and hence, product
shown in Fig. 8.
quality[15].


Figure 6 shows that the oil content of the pre-fry
Physical properties:
blanched sample has less oil compared to the standard
Dimensional changes: Potato dough samples were
sample (no pre-fry treatment).
fried at 135, 150, 165 and 180°C for different frying

As shown in Fig. 7, blanching also has a slight effect
times and the dimensional changes (diameter and
on the final moisture content of the samples. This is in
length) monitored. The diameter decreased as the water
agreement with the results of[18], who stated that
exited the potato dough. The cylinder was then pushed
blanching has an effect on the rates of moisture reduction
gently outwards by internal pressure. The results are
of potato chips samples, due to starch gelatinization.
shown in Fig. 11 and 12, respectively.
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During the early stage of frying, there is an initial
heating period when no water is lost. Heat is transferred
from the oil to the food via convection at the surface
and conduction through the uncooked solid and the
temperature of the food approaches the temperature of
vaporization of the liquid present, i.e., the food
temperature approaches 100°C[11]. This initial, short-
heating
phase
was
observed
to
proceed
for
approximately 10 sec. During this period all the heat
entering the potato was used for sensible heating, with
only a small amount of expanded air leaving the potato.

Such observations are in accordance with those of

Farkas[11,37] who reported that during the first stage of
Fig. 8: Effect of initial moisture content on sample oil
frying no evaporation of moisture occurred and the heat
content during frying at 165°C (potato dough
was transferred in convection mode between the oil and
cylinder-60 mm length, 22 mm diameter)
the sample. This period was followed by a sudden loss

of moisture, characterized by formation of a larger
Density: The variation in the density of potato dough
number of small bubbles, which form over the entire
samples during frying at different temperatures was
surface of the sample and leave rapidly. The maximum
also examined, as shown in Fig. 13, where it can be
water loss rate, in the period of ‘surface boiling’ occurs
observed that it falls significantly during frying. Frying
at between 30 and 100 sec. This surface boiling
temperature was also found to affect the density, which
increases the heat transfer coefficient, which in turn
decreases with increasing temperature, at a given time.

allows more heat to be transferred to the food for the
DISCUSSION
vaporization of water[24]. The rate of moisture loss was

then observed to fall as the sample crust thickened; the

As shown in Fig. 1, the presence of emulsifier
moisture driving force and moisture content decreases.
resulted in a significant decrease in the oil content of
The formation of water vapor bubbles becomes
the sample. This may be attributed to the fact that
limited to a relatively small number of formation sites.
following gelatinization, which takes place after
It is probable that these sites represent weak or
approximately 30 sec, the emulsifier acts as oil barrier.
damaged areas of the potato dough. It is worthy of
The insoluble, oil gel formed causes the oil to diffuse in
note that qualitative evaluation of the operating oil
a counter direction (from the inside to the outside of the
temperature could be carried out by observing the
food). Although the emulsifier accelerated the
volume of bubbles being formed during the early
formation of the crust, as revealed visually, this crust
stages of frying.
offers a barrier for oil penetration in comparison to the

The water loss rate decreases gradually until the
crust formed on sample without emulsifier (this has
bubble end point is reached and the potato dough was at
properties favorable to oil uptake). The final oil content
its equilibrium moisture content (after approximately
of the dough (with emulsifier) was found to be
120 sec of frying time at 165°C). This is in agreement
comparable to that of the potato sample. This is in
with[7,20,22] who reported that the initial moisture loss
agreement with[3,8,13,17,23,28,32,34], who also reported the
was rapid and this was followed by a constant rate
barrier effect which a crust has on oil uptake.
drying period.

As shown in Fig. 2, the presence of emulsifier had

During the first 60 sec of frying the oil replaces the
a negative effect on the moisture loss from the dough
lost moisture, however, as the frying progresses the
sample. No effect on the final moisture content was
methylcellulose begins to gelatinize forming a barrier
observed. On comparison of potato and potato dough
(protective layer) on the cylinder surface, thus
moisture loss curves, it can be seen that the relative
restricting further oil uptake. The dependence of oil
degree of moisture loss is much lower for the potato
uptake on temperature is opposite to that observed
dough. This could be attributed to the sample structure,
during the frying of potato, with an increase in frying
with the dough samples having a decreased number of
temperature decreasing the oil uptake.
pores through which moisture can escape, compared

For low frying temperatures (below 135°C),
with potato and also due to the properties of the coating
although the oil temperature is sufficient to enhance the
film. Methyl cellulose coating film acted as protective
effect of methyl cellulose (gel formation), a certain
layer reducing material loss from the surface[23].
amount of oil is still allowed to diffuse into the
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cylinder. However, at high frying temperatures (150°C
Table 1: Moisture loss equation and R2 for data plotted in Fig. 9
and above) gel formation of methyl cellulose will be
Initial moisture content (kg kg?1, dry basis)
Equation*
R2
enhanced, resulting in less oil entering the sample.
1.0
y = 0.937e-0.002t
0.934
1.5
y = 1.42e-0.002t
0.934
From frying temperatures greater than 150°C, the
2.0
y = 1.92e-0.002t
0.968
barrier formed causes a buildup of internal pressure,
*: Equation: y: ±a × exp (-bt) where, y: Sample moisture content
leading to the sample ‘exploding’ after a period of
(kg kg?1, dry basis); t: Frying time (sec)
approximately 420 sec. Lower oil content during frying

at a higher temperatures is in accordance with the
results of[25-27,30].

This is due to the blanching effect on potato dough
structure. Gelatinization of the potato dough prior to
frying enhanced the resistance to oil uptake. This may be
due to gel formation decreasing the pores on the cylinder
surface. Selman and Hopkins[36], Lamberg et al.[21],
Debnath et al.[7], Rimac et al.[32] and Pedreschi and
Moyano[26] also found that pre-drying increases the
crispness dramatically and significantly reduces the oil
absorption of the blanched potato slices after frying.

The oil content at a given time was higher for

samples with lower initial moisture content. This is due

to crust formation and its favorable oil uptake
Fig. 9: Effect of initial moisture content on sample
?
characteristics. The 1.0 kg kg 1 sample has the lowest
moisture content during frying at 165°C (potato
moisture content, thus crust formation was faster and
dough cylinder-60 mm length, 22 mm diameter)
therefore, oil uptake was higher. Therefore, it would

appear that the effect of crust, which was enhanced by
the addition of methylcellulose, had the greater effect
than sample moisture content on the oil uptake
characteristics. The corresponding moisture loss data
and equations are shown in Fig. 9 and Table 1. Given
that the moisture content plots display similar trends as
confirmed by ‘b’ values of equal magnitude (Table 1),
this indicates that the moisture loss rate is not affected
significantly by initial moisture content.

At a given frying time, the final moisture content is
dependent on the initial moisture content and frying
conditions. Therefore, to eliminate the effect of the initial

moisture content, it is more appropriate to compare the

experimental data by adopting the parameter, oil content
Fig. 10: Oil content criterion against frying time during
criterion (U
frying at 165°C (potato dough cylinder-60 mm
R). This parameter is defined as the amount of
oil uptake (g) divided by the amount of water lost (g).
length, 22 mm diameter)
The experimental data were recalculated on this basis

and the results are shown in Fig. 10.

Visual observations were carried out on each

The initial moisture content of the potato dough
sample after frying, by cutting through sample cross-
significantly affected the oil content criterion. The
section to observe the internal structure. The crust area
lower initial moisture content resulted in a higher oil
(very rigid texture) was clearly distinguished from the
uptake to water loss ratio. The higher initial moisture
inner core of the sample which was completely
content results in pores of smaller radius being formed
gelatinized and rubbery in texture. This observation was
during frying due to the higher water diffusion rate[24].
used to explain the similarities in the moisture loss
Materials with a lower initial moisture content usually
trend during frying at different moisture content and
have higher porosity and have a greater tendency to
also different frying temperatures, due to the effect of
build up pressure within the pores during frying, thus
methyl cellulose. The moisture loss was evenly
causing an enlargement of the pore size which results in
distributed over the cylinder (visual evidenced for the
a lower pressure drop for oil absorption, particularly
bubbles) and the oil was forced to locate just within the
during cooling[24].
crust, since the gelatinized internal structure as a barrier
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for oil penetration, along with the resistance caused by
surface cracking occurred which led to no internal
the space between the crust and the gelatinized layers.
buildup and hence no visible puffing effect (e.g., 150°C
However, the higher initial moisture content delayed
diameter result).
the formation of the crust layer. Mallikarjunan et al.[23]

stated that moisture removal and consequent fat uptake
occur mainly in the crust and, therefore, the role of the
edible film coatings in retaining moisture and reducing
fat uptake was limited only to the surface.

This is in agreement with[10,21,23,27] who reported
that crust is the domain of interest regarding oil
penetration since oil deposition during the frying
process is mostly limited to the crust region.

As shown, the diameter of the potato dough sample
was found to decrease at the onset of frying,
irrespective of the frying temperature, due to the
moisture evaporation. It was expected that the diameter


would continue to decrease as the frying progressed,
Fig. 11: Variation in sample diameter during frying
however, the diameter was found, in fact, to increase.
(potato dough cylinder-60 mm length, 22 mm
This may be attributed to the presence of methyl
diameter)
cellulose, which gelatinizes during frying to form a

solid structure. Although the potato dough losses a
significant portion of its water content, the solid
matrix remains fixed and it is this structure that
ensures
that
dimensional
changes
are
small.
Methylcellulose has the further function of enhancing
the formation of crust which leads to a buildup in
pressure within the cylinder which in turn causes
considerable puffing of the sample. This is revealed
through the excluded samples when they had irregular
puffing in certain places over the cylinder surfaces or
in certain cases of sample explosion. Figure 11 also
shows that the effect of time on the diameter changes

was more significant than the effect of temperature;

however, the effect of temperature was significant on
Fig. 12: Variation in sample length during frying
sample length (Fig. 12).
(potato dough cylinder-60 mm length, 22 mm

The increase in length is attributed to the
diameter)
proportionately large degree of water movement within

the sample in the axial direction toward the cylinder
ends (visual evidence for moisture evaporation) which
caused them to swell. Therefore, the increase in
cylinder length is due to swelling at each end. As
shown in Fig. 12, sample length decreased during the
first 30 sec of frying, irrespective of frying temperature
(135°C continued to decrease during the first 120 sec).
As previously mentioned, the rate of moisture loss per
unit area is greater in axial ends than the radial
direction. The water movement within the sample
towards the two ends created a certain pressure on the

internal side of the two plat ends which changed the flat

end to a significant concave shape. Therefore, the
Fig. 13: Variation in bulk density during frying (potato
sample length increased. The increase in length may be
dough cylinder-60 mm length, 22 mm
partially attributed to crust development. In some cases,
diameter)
163

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Am. J. Agri. & Biol. Sci., 4 (2): 156-166, 2009





Fig. 14: Variation in bulk density during frying of potato
Fig. 15: Variation in sample volume during frying
dough with different initial moisture content
(potato dough cylinder-60mm length, 22 mm
(cylinder-60 mm length, 22 mm diameter)
diameter)


Table 2: Equation and R2 for data plotted in Fig. 14
irrespective of the frying temperature and the absence
Frying temperature (°C)
Equation*
R2
of a crust. After 90 sec frying (for a temperature of
135
y = 0.320e-0.576x
0.890
165°C) the crusts start forming (the equivalent at 135°C
150
y = 0.343e-0.518x
0.974
is 180 sec) and thus an internal gradient pressure was
165
y = 0.238e-0.745x
0.976
formed, hence, the volume exhibits an increase.
180
y = 0.303e-0.613x
0.958

*Equation: y: ±a × exp (-bx), where, y: Bulk density (g cm?3), x:
Sample moisture content (kg kg?1, dry basis)
CONCLUSION



The following conclusions can be stated from the

The results are in contrast to those of[9,29] who
present research in which frying of coated starchy food
reported no changes in restructured potato sample
systems has been studied:
volume during frying. However, Krokida et al.[20]
reported that as the drying time increases, the specific

•
volume of French fries samples decreases, which means

Methyl cellulose emulsifier has been shown to be
that the shrinkage phenomenon gets more intense.
capable of significantly reducing the amount of oil
Kawas and Moreira[16] and Taiwo and Baik[38] reported
uptake during frying of potato dough with a negative
an increase in the diameter, thickness and volume with
effect on the moisture loss from the dough sample
time during the frying of tortilla chips and sweet
• The dependence of oil uptake on temperature is
potatoes, respectively.
opposite to expectations, with an increase in frying

The relationship between moisture content and
temperature decreasing the oil uptake. Increasing
density was also determined; as moisture content
temperature increased the gel formation of methyl
increased the bulk density increased exponentially, as
cellulose, resulting in less oil entering the sample
revealed in Fig. 14 and Table 2. However, it has been
• Blanching
pre-treatment
process
affected
mentioned that, while there were slight changes in the
significantly oil uptake process. Gelatinization of
dimensions of the potato dough cylinder during frying,
the potato dough sample, prior to frying, decreased
the mass of the cylinder decreases considerably.
the pores on the surface and hence, enhanced the
Therefore, the changes in density can be considered to
resistance to oil pick up
be controlled by the amount of moisture loss rather
• The oil content at a given time was higher for
than dimensional variations. This finding is in
samples with lower initial moisture content. This is
agreement with Krokida et al.[20], Kawas and
due, to crust formation and its favorable oil uptake
Moreira[16] and Taiwo and Baik[38] who reported a
characteristics
decrease in bulk density as moisture content decreased
• The effect of crust, which was enhanced by the
during frying of French fries, potato, tortilla chips and
addition of methylcellulose, had the greater effect
sweet potatoes, respectively.
than sample moisture content on the oil uptake

Figure 15 shows the variation in volume during the
characteristics
frying process. During the first 30 sec the volume
• The structural properties were also affected by
decreased slightly due to the evaporation of water,
methyl cellulose coating, which increased the
164

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Am. J. Agri. & Biol. Sci., 4 (2): 156-166, 2009

diameter and the length of the sample, while the
10. Farkas, B.E., R.P. Singh and M.J. McCarthy, 1991.
density decreased with time and temperature
Movement of oil/water interface in foods during

frying. Proceeding of the Annual Meeting of
ACKNOWLEDGMENT
Institute of Food Technologists, (AMIFT’91),

Dallas, TX., pp: 285-295.

Researchers would like to thank Mr., Atalah Al-
11. Farkas, B.E. Singh R. P. and T. R. Rumsey, 1996
Khalayfeh, at the Department of Chemistry, for his
Modeling heat and mass transfer in immersion
technical support.
frying. II, model solution and verification. J. Food

Eng.,
29:
227-248.
DOI:
10.1016/0260-
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