1. business and industrial
2. energy
3. oil

Journal of Food Engineering 55 (2002) 181–191
www.elsevier.com/locate/jfoodeng
Vacuum frying of potato chips
Jagoba Garayo, Rosana Moreira
*
Department of Biological and Agricultural Engineering, Texas A & M University, College Station, TX 77843-2117, USA
Received 31 October 2001; accepted 5 February 2002
Abstract
Vacuum frying was tested as an alternative technique to develop low oil content potato chips. The effect of oil temperature (118,
132, 144 °C) and vacuum pressure (16.661, 9.888, and 3.115 kPa) on the drying rate and oil absorption of potato chips and on the
product quality attributes such as shrinkage, color, and texture was investigated. Furthermore, the characteristics of the vacuumfried potato chips (3.115 kPa and 144 °C) were compared to potato chips fried under atmospheric conditions (165 °C).
During vacuum frying, oil temperature and vacuum pressure had a significant effect on the drying rate and oil absorption rate of
potato chips. Potato chips fried at lower vacuum pressure and higher temperature had less volume shrinkage. Color was not significantly affected by the oil temperature and vacuum pressure. Hardness values increased with increasing oil temperature and
decreasing vacuum levels.
Potato chips fried under vacuum (3.115 kPa and 144 °C) had more volume shrinkage, were slightly softer, and lighter in color
than the potato chips fried under atmospheric conditions (165 °C). It was concluded that vacuum frying is a process that could be a
feasible alternative to produce potato chips with lower oil content and desirable color and texture.
Ó 2002 Elsevier Science Ltd. All rights reserved.
1. Introduction
Potato chips represent 33% of total sales of snack in
the US Market. They have an oil content that ranges
from 35.3% to 44.5% w.b. that gives the product the
unique texture-flavor combination that makes them so
desirable.
In recent years, consumer’s preference for low-fat and
fat-free products has been the driving force of the snack
food industry to produce lower oil content products that
still retain the desirable texture and flavor.
Vacuum frying may be an option for production of
fruits and vegetables with low oil content and the desired texture and flavor characteristics. It is defined as
the frying process that is carried out under pressures
well below atmospheric levels, preferably below 50 Torr
(6.65 kPa). Due to the pressure lowering, the boiling
points both of the oil and the moisture in the foods are
lowered. Vacuum frying poses some advantages that
include: (1) can reduce oil content in the fried product,
*
Corresponding author. Tel.: +1-979-847-8794; fax: +1-979-8453932.
E-mail address: rmoreira@tamu.edu (R. Moreira).
(2) can preserve natural color and flavors of the product
due to the low temperature and oxygen content during
the process, and (3) has less adverse effects on oil quality
(Shyu, Hau, & Hwang, 1998).
Shyu and Hwang (2001) studied the effects of processing conditions on the quality of vacuum fried apple
chips. They used a single vacuum pressure condition,
3.115 kPa, and three levels of temperature, 90, 100, and
110 °C to fry the chips. After frying, the chips were
centrifuged for 30 min at 350 rpm to remove the surface
frying oil and then packed in polyethylene bags and
stored at 30 °C. Using texture (hardness) as an indicator of product quality, the optimum conditions were:
vacuum frying temperature of 100–110 °C, vacuum time
of 20–25 min, and a concentration of immersing fructose
solution of 30–40%.
Even though frying is an old process of manufacturing a food product worldwide, most of the research
found in the literature is related to atmospheric frying.
No studies are available on the effect of vacuum on the
oil content, crispness, color, and nutritional value of the
final product. The main objective of this research was to
develop a vacuum frying system to produce high quality
potato chips in terms of reduced oil content, good texture, and color.
0260-8774/02/$ - see front matter Ó 2002 Elsevier Science Ltd. All rights reserved.
PII: S 0 2 6 0 - 8 7 7 4 ( 0 2 ) 0 0 0 6 2 - 6
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J. Garayo, R. Moreira / Journal of Food Engineering 55 (2002) 181–191
The sealing mechanism consists of two Buna-N washers, the inner edges of which, under compression,
seal against the shaft outer diameter.
2. Materials and methods
2.1. Vacuum frying setup
An electric pressure cooker (Wisconsin Aluminum
Foundry Co., Manitowoc, Wisconsin), which has a capacity of 24 l, was modified to serve as the vacuum
vessel. The inner diameter of the vessel is 28 cm and the
wall thickness is 0.8 cm. It is constructed of cast aluminum and can withstand an internal pressure of 2 atm.
The maximum temperature that the heater can generate
is 145 °C for an oil capacity of 6.5 l.
A dual seal vacuum pump (model 1402 Welch Scientific Co., Skokie, Illinois) that can generate a vacuum
up to 10 Torr (1.333 kPa) is used to provide vacuum to
the vessel. The vacuum frying system is illustrated in
Fig. 1.
The condenser consists of a dry-ice trap. The objective of the condenser is to prevent the water vapor
from the product to mix with the pump’s oil, thus prone
to damage the pump. The condenser has a removable
three-quart center well designed for dry-ice/alcohol
slurry. A 50–50 mix of dry ice and 95% ethanol was used
in this study.
The fryer’s basket rod is held by a motion feedthrough-type shaft-seal attached at an NTP-threaded
port (Kurt J. Lesker, Clairton, PA). This shaft-seal allows rotary motion of the shaft running from the atmosphere side to the vacuum side of the chamber wall.
2.1.1. Experimental conditions
Three levels of vacuum pressure (16.661, 9.888, and
3.115 kPa) and three levels of oil temperature (118, 132,
and 144 °C) were considered in this study. The vacuum
vessel was set to the target temperature and allowed to
operate for one hour before frying started. The oil volume in the vessel was 6.5 l. For this oil volume, there
was a temperature differential between the bottom layer
and the top layer of oil in the vessel of about 3 °C.
Fresh hydrogenated soybean oil was used in all experiments. The potato chips were fried until the equilibrium
moisture content was reached.
2.1.2. Atmospheric deep-fat frying experiments
For the atmospheric frying experiments, an electricfired type fryer (Hobart model HK3-2, Hobart Corp,
Troy, Ohio) was used. It is a bench top type fryer with a
frying oil capacity of 7.5 l. About 50–60 g of potato
slices was fried in each batch.
To compare the atmospheric frying to the vacuum
frying on potato chip quality and frying rate, an oil
temperature of 165 °C was considered. The atmospheric
fryer was set to the required frying temperature and left
for 30 min to ensure the oil temperature was equili-
Fig. 1. Schematic of the vacuum frying system.
J. Garayo, R. Moreira / Journal of Food Engineering 55 (2002) 181–191
Table 1
Solids content and color for the potato variety used in this study
Characteristics
Values
Solids content (%)
Color––lightness (L)
Color––green/red chromaticity (a)
21.67
67.04
1.495
brated. The samples were fried to the equilibrium
moisture content.
Potato samples: A food company provided the potatoes used in this study. The potato breed is special for
frying and was stored in a cooling chamber at a temperature range of 10–12 °C and a relative humidity of
55%. Table 1 shows the solid content and color specifications determined by the scientists of the food company for this potato variety.
Fig. 2 presents a block diagram of the vacuum frying
process for producing potato chips. The potatoes were
taken out of storage at least 12 h before frying to let
them reach room temperature and for the reducing
sugar contents to decrease. Once the potatoes were
peeled and sliced, they were soaked in water for a few
seconds, and then dried in paper towels prior to frying.
About 5–6 slices of potatoes (20–25 g) were fried each
time. Once the oil temperature reached the target value,
the potatoes were then placed into the basket, the lid
closed, the vessel evacuated. At this moment, the basket
was lowered to the oil and frying began for the desired
frying time. Once the potato slices were fried, the basket
was lifted from the oil and the vessel pressurized. Then,
the lid of the vessel was opened and the potato chips
were removed from the basket. The potato chips were
then allowed to cool to room temperature, dried with
paper towel, and later stored in polyethylene bags for
further analysis.
2.2. Analytical methods
Moisture content of raw potatoes: Potato tubers have
considerable variability in initial moisture content from
potato to potato. To account for this variability, the
183
initial moisture content of each potato was measured
before the slices of the potato were fried. The initial
moisture content was determined by drying samples of 5
g of the potato (slices) to a constant mass for 72 h at 105
°C (AACC, 1986). The tests were done in triplicate.
Moisture content of potato chips: Potato chips were
ground in a coffee grinder after frying. Moisture content
of potato chips was determined by weight loss after
drying 5 g samples of ground chips in a forced air oven
at 105 °C for 24 h (AACC, 1986). The test was performed in duplicate.
Oil Content: Total oil content of potato chips was
determined by using the Soxtec System HT (Pertorp,
Inc., Silver Spring, Maryland) extraction unit with petroleum ether (AACC, 1986). The test was performed in
duplicate.
2.3. Product characteristics in frying
Shrinkage: Degree of shrinkage in volume (Sv ) was
evaluated by:
Sv ¼
Vo V ðtÞ
100
Vo
ð1Þ
where Vo is the original volume of the sample (m3 ) and
V ðtÞ is the volume (m3 ) of the sample at time t. The
volume of the sample (elliptical shaped) at any given
time can be calculated by V ¼ ðpDd=4ÞL where D is the
larger diameter of the sample (m) and d is the smaller
diameter [m] of sample, and L the sample’s thickness
(m). Ten samples were taken to determine shrinkage
for each frying condition at the equilibrium time.
Color: The color of the potato chips was measured
using a Hunter Lab Colorimeter Labscan XE (Hunter
Associates Laboratory, Reston, VA). Color was measured in potato chips obtained at the equilibrium moisture content for each condition. Measurements were
taken for ten chips of each condition, and two readings
were taken for each potato chip by rotating the chip
with an 180° angle. The colorimeter was standardized
utilizing a white calibration plate, and a glass plate was
used as the standard. The samples were placed always
Fig. 2. Flow diagram of the vacuum frying process.
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against the same background to obtain the color measurements.
Texture: A rupture test was performed on the fried
potato chips obtained at different frying times using a
TA-XT2 Texture Analyzer (Texture Technologies Corporation, Scardale, New York).
The test involves applying a direct force to a sample,
which is placed over the end of a 0.018 m hollow cylinder. A 0.00635 m ball probe moving at a constant rate
was used to break the chips. The data for each sample
was saved for further manipulation. The data collected
included the force (N) at each time (s) and corresponding distance (mm).
Texture was evaluated on samples fried at several
times corresponding to different levels of moisture content for each condition. The tests were performed using
ten chips per condition. For good experimental practice,
all tests were run on the same day the chips were processed. The property used to describe the texture of the
samples was hardness, defined as the force at maximum
compression (Steffe, 1996). It was measured by recording the peak force value of each sample rupture test.
2.4. Data analysis
2.4.1. Vacuum frying experiment
The effect of temperature and vacuum pressure on the
drying curve, the oil content, shrinkage, color, and
texture of the chips was evaluated using a factorial de-
sign with three levels for temperature and three levels for
vacuum pressure. Analysis of variance, Duncan’s New
Multiple range test (a ¼ 0:05) and Tukey’s tests were
used to determine differences among treatments. The
statistical analysis of the data was conducted using SAS
statistical software package (version 6.10, 1996). Statistical significance was expressed at the p < 0:05 level. The
experiments were performed at least in duplicate. Calculations of non-linear regression parameters were done
using the Levenberg-Marquard iteration method with
the graphics software package PlotIt (version 3.2, 1999).
3. Results
3.1. Moisture loss
Figs. 3 and 4 represent the drying curves for vacuum
frying at various experimental conditions (temperature
and vacuum pressure). The loss of moisture during
frying exhibited a classical drying profile. The drying
process of foods is generally characterized by three
distinct periods. The first is an initial heat-up period
during which the wet solid material absorbs heat from
the surrounding media. The product is heated up from
its initial temperature to a temperature where the moisture begins to evaporate from the food. In vacuum frying, this initial heat-up period is very short and therefore
difficult to quantify.
Fig. 3. Effect of oil temperature on the moisture loss of potato chips versus frying time for different vacuum pressures: (a) Pvac ¼ 16:661 kPa; (b)
Pvac ¼ 9:888 kPa; and (c) Pvac ¼ 3:115 kPa.
J. Garayo, R. Moreira / Journal of Food Engineering 55 (2002) 181–191
185
Fig. 4. Effect of vacuum pressure on the moisture loss of potato chips versus frying time for different oil temperatures: (a) Toil ¼ 118 °C; (b) Toil ¼
132 °C; and (c) Toil ¼ 144 °C.
At a vacuum pressure of 3.115 kPa, for example, the
boiling point of water is around 25 °C. The temperature
in the potato slice is slightly higher than the boiling
point of water due to the presence of some solutes. Since
the temperature of the potato slices prior to frying was
at room temperature (23–24 °C), the slices only had to
warm up a few degrees for the water to start boiling. For
this reason, the heat-up period in vacuum frying is very
short.
The second drying period is known as the constant
rate period. Here, the rate of drying is limited by the rate
at which heat is transferred from the drying medium to
the product. The constant drying conditions continue as
long as the food surface remains wetted with water. In
the case of potato chips fried under vacuum frying, we
could not observe any constant rate period (see Fig. 3).
When the moisture level in the product is so low that
its surface is no longer wetted, the drying rate decreases
entering the falling rate period. During this period,
drying rate is controlled by moisture diffusion mechanisms. The water during this period is held in the
material by multi-molecular adsorption and capillary
condensation (Toledo, 1991).
Table 3 shows that potato chips fried at an oil temperature 144 °C and 3.115 kPa took the shortest time to
fry (360 s). Potato chips fried at the same pressure and at
132 and 118 °C, took 480 and 600 s to fry, respectively.
Decreasing the oil temperature increased the frying time
of potato chips fried under vacuum as expected. The
same behavior was observed for the other vacuum
pressures.
Gamble, Rice, and Selman (1987) reported that potato chips fried at atmospheric conditions at higher
temperatures resulted in an initial rapid fall followed by
a continuous drying period. Fig. 3a, b and c confirm this
point, where the drying rate in the initial 150 s of frying
was higher for 144 °C than for 132 °C. The difference
was even more pronounced when comparing the drying
rate at 132 and 118 °C at all pressures.
Fig. 4a, b and c show the effect of vacuum pressure
on moisture content history for potato chips fried at a
temperature of 118, 132 and 144 °C, respectively. The
rate of moisture loss was affected in a significant manner
(p < 0:05) by the vacuum pressure treatment. Fig. 4a–c
show that there is a negative correlation between vacuum pressure and moisture loss rate for the same oil
temperature. This is due to the fact that the more the
pressure is lowered; the further the boiling point of
water is reduced. As a consequence, the water in the
potato chips will begin to vaporize faster at a higher
vacuum level.
Table 2 shows that by decreasing the vacuum pressure, for a fixed value of temperature, it took less time
for the chips to reach the same final moisture content
(1:90 0:05% w.b.). For example, at 132 °C, the frying
time decreased from 480 s (Pvac ¼ 16:661 kPa) to 360 s
(Pvac ¼ 9:888 kPa) and then to 300 s (Pvac ¼ 3:115 kPa).
The equilibrium moisture content was significantly
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Table 2
Initial moisture content, moisture content after frying, and oil content for potato chips fried under different vacuum pressures and oil temperatures
Toil (°C)
Pvac (kPa)
FT (s)
IMC (% w.b.)
EMC (% w.b.)
FOC (% w.b.)
118
132
144
118
132
144
118
132
144
16.661
16.661
16.661
9.888
9.888
9.888
3.115
3.115
3.115
600
480
360
600
480
360
600
480
360
78:96 1:53a
77:92 0:52a
73:55 0:63a
77:98 1:01a
75:65 0:33a
73:45 0:35a
79:97 0:70a
80:11 0:68a
75:04 0:56a
1:84 0:19a;a (1:84 0:19)
1:90 0:21a;a (1:90 0:21)
1:93 0:02a;a (1:93 0:01)
1:31 0:10a;b (1:80 0:01)
1:37 0:04a;b (1:89 0:19)
0:94 0:02b;c (1:95 0:05)
1:06 0:19a;c (1:94 0:08)
1:12 0:01a;c (1:85 0:17)
0:75 0:00b;d (1:90 0:06)
25:74 0:36a
26:22 0:34a
26:66 0:26a
26:07 0:45a
26:16 0:20a
27:22 0:32a
26:66 0:31a
26:16 0:03a
26:63 0:02a
(600)
(480)
(360)
(480)
(360)
(200)
(360)
(300)
(190)
(35.54)
(36.48)
(37.33)
(35.90)
(36.09)
(37.88)
(36.89)
(35.89)
(36.67)
Tests were performed in duplicate. Means with the same letter are not significantly different (p < 0:05). Toil ¼ oil temperature, FT ¼ frying time
(values in parenthesis are the frying times to obtain chips with final moisture content around 1.90% w.b.), IMC ¼ initial moisture content,
FOC ¼ final oil content (values in parenthesis is oil content in dry basis); EMC ¼ equilibrium moisture content (number in parenthesis is the final
moisture content related to FT values in parenthesis––column 3).
affected (p < 0:05) by the vacuum pressure as indicated
in Table 2.
3.2. Oil absorption
Oil absorption is a complex phenomenon that happens mostly when the product is removed from the fryer
during the cooling stage (Sun & Moreira, 1994). Potato
slices where fried for different times (and different operating conditions), until the equilibrium moisture content was reached, to study the effect of frying time on the
chip’s total (surface plus internal) oil content.
Figs. 5 and 6 present the oil absorption curves for
vacuum frying at various experimental conditions (temperature and vacuum pressure). It can be observed that
within the first 150 s, the chips’ oil content increased as
frying time increased for all temperatures and vacuum
pressures (Fig. 5a–c). This coincided with the period of
time at which water evaporated from the chips at the
fastest rate (Fig. 3a–c). The chips absorbed more oil
when fried at Pvac ¼ 3:115 kPa and Toil ¼ 144 °C when
the chips’ oil content reached a maximum value of about
56% d.b (33% w.b.) after 150 s of frying and then reduced to about 37% d.b. (27% w.b.) as frying reached
360 s.
Baumann and Escher (1995) found that the varying
frying oil temperature under atmospheric conditions
caused a slight increase in final oil content of the chips.
Higher oil temperatures lead to a faster development of a
solid crust and consequently surface properties that are
favorable for oil absorption. In the case of frying under
vacuum, the final oil content of the potato chips was not
significantly affected (p < 0:05) by the oil temperature
(Table 2). This suggests that the final oil content of the
potato chips is not a function of oil temperature, but it
depends on the frying time (thus remaining moisture),
which increases with decreasing oil temperature.
Fig. 5a shows that for a Pvac of 16.661 kPa the
product absorbed more oil, initially, when fried at 144 °C
than at the other two lower temperatures. This behavior
was similar for vacuum pressures of 9.888 and 3.115 kPa
as illustrated in Fig. 5b and c, respectively. So, the
higher the oil temperature, the higher the oil pickup by
the potato chips during the first 150 s of frying.
Note that the vacuum pressure level did not affect the
final oil content of potato chips fried at any temperature
(Table 2). Higher vacuum levels lead to faster development of a crust and thus to faster oil absorption rate
(due to loss of the characteristic hidrophilicity of raw
potatoes) during the process.
Therefore, the oil absorption rate was faster as the
vacuum level increased (Fig. 6a–c). Vacuum pressure
affects the speed at which moisture leaves the potato
chips (by lowering the Tsat of water). Fig. 6c shows that
the initial oil pickup (around 150 s) by the potato chips
was higher at 3.115 kPa than at 9.888 and 16.661 kPa,
respectively. However, the effect of vacuum pressure on
the chips’ oil content at different frying time was not as
large as the effect of temperature for the conditions
analyzed in this study. This can be observed by comparing Fig. 5a–c with Fig. 6a–c.
It is interesting to notice that the oil absorption versus frying time curves for the chips fried at Pvac ¼ 3:115
and 9.888 kPa at Toil ¼ 144 °C (Fig. 6c) behaved differently than those chips fried at the other Toil and Pvac
conditions, showing a maximum value for oil absorption
around 150 s of frying.
These results indicate that probably more oil adheres
to the chip’s surface as Pvac decreases (Tsat of water decreases, moisture loss rate increases, thus the crust develops fast). As the chip is removed from the oil bath,
the adhered oil intends to flow into the pore spaces
during the pressurization period. The amount of free
water in the product seems to be related to oil absorption, which is higher when more free spaces are available
(when the amount of free water is small). That is, an
increase in the surrounding pressure at constant temperature, during the pressurization period, causes the
vapor inside the pore to condense (at constant T and P).
As the water vapor condenses the pressure difference
J. Garayo, R. Moreira / Journal of Food Engineering 55 (2002) 181–191
187
Fig. 5. Effect of oil temperature on the oil absorption of potato chips versus frying time for different vacuum pressures: (a) Pvac ¼ 16:661 kPa;
(b) Pvac ¼ 9:888 kPa; and (c) Pvac ¼ 3:115 kPa.
Fig. 6. Effect of vacuum pressure on the oil absorption of potato chips versus frying time for different oil temperatures: (a) Toil ¼ 118 °C; (b) Toil ¼
132 °C; and (c) Toil ¼ 144 °C.
(Psurroundings Ppore ) causes the oil to be absorbed into the
pore space. Under these operating conditions, most of
the oil will be absorbed during the pressurization pro-
cess. For the chips fried at a higher Pvac (16.661 kPa), the
higher Tsat slowed down the crust formation so less oil
adhered to the chips’ surface. In addition, due to the low
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moisture loss rate, the higher free water content hindered the oil to enter the pore spaces during pressurization.
Once the free water is reduced to a critical level (at the
150 s in this case), oil absorption during the pressurization process decreases (since (Psurroundings Ppore ) is
negligible). Air then diffuses faster than the oil into the
pore spaces thus blocking the oil to flow into the
product. In this case, most of the oil is absorbed during
cooling.
These observations indicate that the pressurization
process plays an important role in the oil absorption
mechanism. It can increase or decrease oil absorption in
the product depending on the amount of surface oil and
free water presented in the product. The lower the vacuum pressure and the higher the temperature (i.e.
Pvac ¼ 3:115 kPa and Toil ¼ 144 °C), the highest is the oil
content of the chips during the initial period of frying
when free water is still available in the product. Therefore, for these conditions, the lowest oil content is obtained when the chips are fried as close as to the
equilibrium moisture content.
It seems that oil temperature has a major effect on the
oil absorption rate by increasing the moisture loss rate
(see Fig. 6a–c). Pressure affects the Tsat of water and thus
has a minor effect on the oil absorption phenomenon
during vacuum frying (Fig. 5a–c).
Since the best frying time and quality results were
obtained at 3.115 kPa and 144 °C with the same final oil
content and quality results, only the products fried
under this vacuum condition will be compared with
those products fried under the atmospheric condition.
Fig. 7a shows that the drying rate of potato chips was
similar for vacuum-fried potato chips and for potato
chips fried under atmospheric conditions (at 165 °C).
Table 3 shows that potato chips fried faster at atmospheric conditions (300 s) compared to the chips fried at
vacuum (360 s). Also, the equilibrium moisture content
for these two conditions shows significant difference
(p < 0:05) between the treatments, with the chips fried at
the atmospheric fryer showing lower value for EMC.
Table 3 also shows that the final oil content values
were significantly different (p < 0:05) for potato chips
fried under vacuum (37% d.b. (27% w.b.)) and atmospheric pressure (66% d.b. (40% w.b.)). Fig. 7b shows
the difference between oil absorption rate for potato
Fig. 7. Comparison between potato chips fried under vacuum pressure
and atmospheric pressure (a) moisture loss vs. frying time, (b) oil absorption vs. frying time.
chips fried at atmospheric conditions (at 165 °C) and
potato chips fried under vacuum at Pvac ¼ 3:115 kPa and
Toil ¼ 144 °C. The frying behavior of chips is clearly
different from each treatment as indicated by the oil
absorption curves in Fig. 7b.
One major difference between potato slices fried
under vacuum and atmospheric conditions is the surface
structure of the potato chips formed during the process.
Visual observations indicated that the surface of a vacuum fried potato chip had less expansion and numerous
small bubbles, as opposed to a potato chip fried under
atmospheric pressure, which showed more expansion
and lesser but larger bubbles.
The bubble formation at the product’s surface is the
results of gas expansion inside the pores. For the vacuum fried chips (Pvac ¼ 3:115 kPa, Tsat ¼ 24:6 °C), once
Table 3
Comparison of initial moisture content, moisture content after frying, and oil content for potato chips fried under vacuum and atmospheric conditions
Toil (°C), P (kPa)
144, 3.115
165, 101.35
FT (s)
360
300
IMC (% w.b.)
a
75:04 0:56
78:50 0:00a
EMC (% w.b.)
a
0:75 0:01
0:45 0:01a
FOC (% w.b.)
26:63 0:03a (36.67)
39:65 0:16b (66.20)
Tests were performed in duplicate. Means with the same letter are not significantly different (p < 0:05). Toil ¼ oil temperature, Pac ¼ vacuum pressure,
FT ¼ frying time, IMC ¼ initial moisture content, EMC ¼ equilibrium moisture content, FOC ¼ final oil content (values in parenthesis is for oil
content in dry basis).
J. Garayo, R. Moreira / Journal of Food Engineering 55 (2002) 181–191
the fryer is evacuated, the water vapor in the pores expands with little resistance (since the crust has not been
formed and/or starch gelatinized) even before the product is fried. During frying, little expansion may be
produced by the superheated vapor trying to escape the
pore space. For the chips fried under atmospheric conditions, the expansion happens when the product is
immersed in the oil. Water is heated first and then the
vapor expanded. As it is heated starch will gelatinize
producing a barrier for the saturated vapor to escape
(Kawas & Moreira, 2001). As a result, large, but few,
bubbles will be formed at the chips surface.
Mass transfer mechanisms in atmospheric frying can
be divided into two periods: the frying period and the
cooling period. The frying period is characterized by a
heating-up stage in which the product’s temperature is
raised from room temperature (23–24 °C) to the boiling
point of water (100 °C). This process occurs at an extremely fast pace, since liquid water has a high specific
heat. Once the water inside the potato chip reaches the
boiling point, it vaporizes. Once the pores are filled out
with gas (water vapor and air), the temperature, and
then the pressure, increase at a fast rate (due to high
temperature differential with the oil). During this period,
the capillary pressure is negligible (Moreira, CastellPerez, & Barrufet, 1999), so there is no driving force for
the oil to flow into the chip’s pores.
During the cooling period the surface oil that adhered
to the surface of the potato chips during frying penetrates into the pores of the chips. As the potato chip
cools down, the pressure inside the pores changes as a
consequence of the so-called capillary pressure rise
(Moreira & Barrufet, 1995). This pressure difference
between the surface and the pores of the potato chips
creates a driving force for the oil and air to penetrate the
pores.
The mass transfer mechanisms in vacuum frying can
be understood by dividing the process into three periods:
the frying, the pressurization, and the cooling period. At
the beginning of the frying period, the boiling temperature of the water has been lowered to a level lower
than 100 °C (56.0 °C for Pvac ¼ 16:661 kPa, 45.4 °C for
Pvac ¼ 9:888 kPa, and 24.6 °C for Pvac ¼ 3:115 kPa). So,
the water in the potato chips evaporates quicker under
vacuum conditions and it will be less vigorous. Capillary
pressure (between oil and gas) during this period is
negligible. Thus, no oil is absorbed at this stage. The
crust in vacuum frying begins to form as soon as the
potato slices are immersed in the oil.
The second period in vacuum frying is the pressurization from vacuum to atmospheric conditions. This
stage begins when the potato chips are removed from
the frying oil and are kept under vacuum and temperature conditions (the fryer vessel is still closed). At this
stage, as the vessel is vented, the pressure in the pores of
the potato chips rapidly increases to atmospheric levels,
189
as air and surface oil are carried into the empty pores
spaces until the pressure reaches atmospheric levels.
However, because of the low pressure, gas diffuses much
faster into the pore space thus obstructing the oil’s
passage to enter the product’s pores.
The third period starts when the potato chips are
removed from the fryer and it is known as the cooling
period, when part of the adhered oil continues to penetrate the pore spaces. Since less oil adheres to the chips
surface during vacuum frying, less oil is absorbed during
cooling.
3.3. Product characteristics
3.3.1. Shrinkage
Fig. 8a presents the effect of oil temperature on the
degree of volume shrinkage of potato chips fried under
vacuum frying as a function of temperature and vacuum
pressure.
Volume shrinkage during early stages of frying very
nearly equals the volume of water loss; however, in the
Fig. 8. Shrinkage in volume of potato chips as function of frying time.
(a) Effect of oil temperature and vacuum pressure; (b) comparison
between vacuum-fried chips and atmospheric-fried chips.
190
J. Garayo, R. Moreira / Journal of Food Engineering 55 (2002) 181–191
final stages of drying the volume shrinkage is smaller
(Johnson, 1999). Therefore, volume shrinkage depends
on water transfer within the product. For the same frying time, higher temperatures result in a higher mass
diffusivity, higher water loss, and consequently lower
volume shrinkage. The final shrinkage in volume of potato slices fried under different vacuum pressures was
shown to decrease with oil temperature (Fig. 8a). In
addition, higher oil temperature caused the potato chip
surface to become rigid more rapidly, thus producing
increased resistance to volume change. Lamberg, Hallstrom, and Olsson (1990) suggested that the formation
of a rigid surface in atmospheric frying at higher drying
rates.
Fig. 8a also shows that by decreasing the vacuum
pressure, the chips shrunk more. Probably, the structured formed in the chips during low vacuum pressure
was less rigid than that formed under high pressures
thus resulting in less resistance to volume change.
Fig. 8b represents a comparison of the degree of
shrinkage in volume for potato chips fried under vacuum and atmospheric conditions. Potato chips showed a
higher degree of volume change when fried under vacuum at Pvac ¼ 3:115 kPa and Toil ¼ 144 °C than when
fried at atmospheric conditions at Toil ¼ 165 °C. This is
probably because the product becomes more rigid when
fried under atmospheric frying than when fried under
vacuum frying.
3.3.2. Color
The statistical analysis showed that there were no
significant differences (p < 0:05) for lightness (L ), green–
red chromaticity (a ), and blue-yellow chromaticity (b )
of potato chips as a function of vacuum temperature
and pressure (not shown).
A comparison of the color attributes of potato chips
fried under vacuum and atmospheric conditions is shown
in Fig. 9. The potato chips fried at Pvac ¼ 3:115 kPa and
Toil ¼ 144 °C had L values that were significantly higher
(p < 0:05) than the values corresponding to the potato
chips fried under the atmospheric condition (Toil ¼ 165
°C). A higher L value indicates a lighter color, which is
desirable in potato chips.
The a values were significantly higher (p < 0:05) for
potato chips fried at atmospheric pressure than for potato chips fried at the vacuum conditions, indicating
more Maillard reaction occurred at the atmospheric
frying conditions.
The blue–yellow chromatically (b ) values were also
significantly higher (p < 0:05) for the potato chips fried
at atmospheric pressure and Toil ¼ 165 °C than for potato chips fried at vacuum conditions.
Visual observation confirmed the results obtained
with the colorimeter, since the potato chips fried under
atmospheric conditions were darker, more red, and yellowish than potato chips fried under vacuum. The change
Fig. 9. Comparison of color of potato chips fried under vacuum
pressure and atmospheric pressure.
in color in fried potato chips is due to the interaction of
an amine group with a reducing sugar, which is a nonoxidative browning also known as Maillard reaction.
3.3.3. Texture
During frying, most of the water is removed from the
potato slices resulting in textural changes. Table 4 shows
the effect of pressure and temperature on the texture
characteristic of potato chips at the end of frying. The
force required to break the chips (hardness) increased
for oil temperature range used in this study. However,
the oil temperature did not affect significantly (p < 0:05)
the chip’s texture. Higher vacuum levels (lower vacuum
pressure) produced chips with lower hardness values.
However, the texture was not significantly (p < 0:05)
affected by vacuum pressure.
For the potato chips fried under atmospheric conditions (Table 5), during the first 50 of frying, their
structure was very weak and collapsed easily when applying the rupture test. For the potato chips fried under
vacuum conditions, the texture in the initial stages of
Table 4
Effect of frying time on the texture (hardness) of potato chips fried
under different operating conditons
Toil (°C)
Pvac (kPa)
FT (s)
Hardness (N)
118
118
118
16.661
9.888
3.115
600
600
600
2:73 0:81a
2:47 0:44a
2:23 0:51a
132
132
132
16.661
9.888
3.115
480
480
480
3:13 0:64a
2:58 0:42a
2:05 0:40a
144
144
144
16.661
9.888
3.115
360
360
360
3:65 0:42a
2:86 0:86a
2:71 0:79a
Means with the same letter are not significantly different (p < 0:05).
Toil ¼ oil temperature, Pvac ¼ vacuum pressure, FT ¼ frying time.
J. Garayo, R. Moreira / Journal of Food Engineering 55 (2002) 181–191
Table 5
Comparison of textures values of potato chips fried under atmospheric
and vacuum conditions (Toil ¼ 144 °C, Pvac ¼ 3:115 kPa)
Toil (°C)
Pvac (kPa)
FT (s)
Hardness (N)
165
165
165
165
165
101.3
101.3
101.3
101.3
101.3
30
150
180
240
300
0:65 0:11a
2:65 0:59b
2:89 0:91b
3:45 0:88c
2:88 0:54b
144
144
144
144
3.115
3.115
3.115
3.115
30
150
240
360
3:78 1:83a
3:31 0:55b
3:17 0:49b
2:71 0:79b
191
Comparing atmospheric frying to vacuum frying the
following conclusions were obtained. The drying rate for
potato chips fried under atmospheric and were the same
for the two treatments compared in this study. The final
oil content was higher for potato chips fried under atmospheric conditions than for potato chips fried under
vacuum conditions. The frying behavior was different
for the chips fried under vacuum and atmospheric
conditions.
Potato chips fried under vacuum pressure had higher
volume shrinkage, were lighter, and softer than potato
chips fried under atmospheric conditions.
Means with the same letter are not significantly different ðp < 0:05Þ.
Toil ¼ oil temperature, Pvac ¼ vacuum pressure, FT ¼ frying time.
References
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4. Conclusions
The main purpose of using vacuum frying in this
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Oil absorption rate during vacuum frying of potato
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