This document summarizes research on vacuum frying. It discusses how vacuum frying works by lowering the boiling points of oil and water, allowing frying at lower temperatures than atmospheric frying. This preserves quality attributes like color and nutrients compared to high-temperature atmospheric frying. The document reviews studies showing vacuum frying can reduce oil uptake and acrylamide levels compared to conventional frying. Pretreatments and process parameters like temperature, time and pressure can influence attributes of vacuum fried foods like potatoes, carrots and bananas. Vacuum frying is an alternative method that produces products similar in quality to conventional frying but with potential health benefits from lower temperatures.

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Vacuum Frying
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2. 1
VACUUM FRYING
*Masoud HASHEMI SHAHRAKI
1
, Mana MASHKOUR
1
1
Department of Food Science & Technology, Gorgan University of Agricultural Sciences and Natural Resources
Beheshti Ave., Gorgan,9381321295, Iran
Corresponding author: Masoud HASHEMI SHAHRAKI. E-mail: m.hashemi.sh@gmail.com
Keywords: Vacuum frying; Alternative Frying Technologies; Vacuum drying
ABSTRACT
During the frying process, the physical, chemical, and
sensory characteristics of foods are modified. Texture,
color and oil content are the main quality parameters of
fried products. Atmospheric deep-fat frying necessarily
occurs at high temperatures under atmospheric pressure.
Surface darkening and many adverse reactions take place
due to the high temperature treatment before the food is
fully cooked or dried Vacuum frying is a viable option to
produce high quality dried fruit and vegetable in a far
shorter processing time than conventional frying. The
sample is heated under a negative pressure that lowers the
boiling point of the frying oil and the water in the sample.
Moreover, the absence of air during frying may inhibit
oxidation reactions, including lipid oxidation, and
enzymatic browning of samples can be largely preserved.
In this study most important investigation about vacuum
frying were reviewed.
INTRODUCTION
Vacuum frying is carried out at pressures below
atmospheric levels, which lowers the boiling points of oil
and moisture in the foods. Therefore, frying can be
achieved at lower temperatures. Hence, dehydrated foods
produced by vacuum frying can have crispy texture, good
color and flavor and good retention of nutrients [1-6]. In
Europe, vacuum fryers are used to produce French fries
because it is possible to achieve the necessary moisture
content without severe darkening of these products[7].
Frying under reduced pressure (vacuum frying) is an
efficient alternative method of reducing the oil content in
fried foods while producing potato chips with the same
texture and color of those fried in atmospheric conditions
[8, 9], as well as, lower acrylamide content [10] and
enhanced organoleptic and nutritional qualities [1, 8, 11,
12]. However, vacuum frying operations are usually done
in closed vessels, and therefore suffers from the
disadvantages common to batch processes. Continuous
vacuum fryers are also available, but they are extremely
costly [13].
In vacuum frying, oil is heated by circulating it through
an external heater. The food is placed in a basket, and the
basket is placed in the vacuum chamber above the surface
of the oil. After vacuum is applied, the basket is lowered
into the hot oil. Oil temperature drops with the addition of
food, but then oil regains its initial temperature as frying
continues. After frying, fryer is vented and the vacuum is
broken slowly. Excess oil is allowed to drain and the
product is removed.
Frying can be defined as the process of drying and
cooking through contact with hot oil. There are three
distinct periods in drying of foods. These periods are also
observed in frying process. However, there are some
differences depending on the type of frying, such as
conventional and vacuum frying [9].
1. Initial heat-up period: 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 period is very short and therefore difficult to
quantify. The boiling point of water is lower in vacuum
frying due to the lower pressure. Therefore, the product is
heated-up from its initial temperature to the boiling point
of water in a very short period of time.
2. Constant rate period: The surface of the solid is
initially wet and this period continues as long as the water
is supplied to the surface as fast as it is evaporated. The
rate of mass transfer is limited by the rate of heat transfer.
Constant rate period could not be observed in vacuum
frying of potato chips [9].
3. Falling rate period: This period appears in atmospheric
and vacuum frying. The entire surface is no longer wet;
the plane of evaporation slowly recedes from the surface.
The amount of moisture removed in this period may be
relatively small, but the required time may be long.
The mass transfer in atmospheric frying can be analyzed
by dividing the process into two periods: during frying and
during cooling. During frying, product temperature rises to
the boiling point of water and water within the sample
vaporizes. Temperature and pressure within the sample
increase at a fast rate. During this period, capillary
pressure is negligible. During cooling period, oil adhered
to the surface of fried product penetrates into the pores
because the pressure inside the pores changes as a
consequence of capillary pressure rise[14]. In the case of
vacuum frying, there is a pressurization period between
frying and cooling periods [9]. During the frying period,
capillary pressure is negligible, as in the case of
atmospheric frying, therefore oil absorption in this period
is negligible. When the product is removed from the frying
oil and the vessel is vented, the pressure in the sample
The 1st
Middle-East Drying Conference (MEDC2012)
February 19-20, 2012, Mahshahr, Iran

3. 2
rises to atmospheric levels. During this stage, air and
surface oil are carried into the empty pores. As the sample
is removed from the oil, the adhered oil intends to flow
into the pores during pressurization period. An increase in
the surrounding pressure at constant temperature, during
the pressurization period, causes the vapor inside the pores
to condense. As the water vapor condenses, oil penetrates
into the pores due to the pressure difference. However, the
penetration of oil into the pores may be obstructed if the
diffusion of gas is faster. During the cooling period, part
of the adhered oil penetrates into the pores, as in
atmospheric frying. In vacuum frying, the pressurization
period has an important role in oil absorption. Oil
absorption may decrease or increase depending on the
amount of surface oil and free water in the product.
RECENT STUDIES ABOUT VACUUM FRYING
Several studies have shown that less oil is absorbed during
the vacuum frying process using different pre-treatment
and de-oiling steps [4, 9, 11, 15, 16], found that oil
absorption at the surface increases during vacuum frying
processes because of the higher heat and mass transfer
rates and the existence of a pressurization step, thus
increasing the final oil content compared to traditional
frying for the same working temperature.
The pressurization process plays an important role in
the oil absorption mechanism. It can increase or decrease
oil absorption depending on the amount of surface oil and
free water present in the product [9]. Furthermore, Moreira
et al. [16] considered the amount of surface oil present at
the moment of pressurization as a determining aspect for
the final oil content of the product, and established that a
de-oiling process must be used to remove surface oil under
vacuum after the product is fried. They determined the
internal and structural oil absorption kinetics during
vacuum frying of potato chips, and found that 14% of the
total oil content was located in the core (internal oil) and
the remaining 86% of the oil content was surface oil. The
de-oiling mechanism (centrifuging system) used in the
study removed surface oil before the pressurization step
and was able to reduce the total oil content in about 80–
90%.
In order to understand and decrease oil absorption
during the pressurization step, Mir-Bel et al. [17]
investigated the influence of various parameters of the
pressurization and cooling stage on the final oil content of
fried potato using different geometries, and explained that
oil absorption during the cooling stage is greatly
influenced not only by the difference in temperature, but
also by the vacuum break conditions as the system
recovers atmospheric pressure. They found that the
volume of oil absorbed by the product is inversely
proportional to the pressurization velocity meaning that
lower velocities favors oil absorption showing an increase
of 70% for potato chips compared to the oil content when
the vacuum breaks abruptly.
Dueik et al [11] studied the most important quality
parameters of vacuum and atmospheric fried carrot slices
in order to identify the specific advantages of vacuum
technology. Said parameters include oil uptake, color
changes, and trans a and b-carotene degradation.
Equivalent thermal driving forces were used (DT =60 C
and 80 °C) to compare the processes, maintaining a
constant difference in temperature between the oil and the
boiling point of water at the working pressure. The results
showed that vacuum frying can reduce oil content by
nearly 50% (d.b.) and preserve approximately 90% of
trans a-carotene and 86% of trans b-carotene. This process
also allowed for the raw carrot color to be preserved,
which was reflected by good correlations between a* and
trans b-carotene content, b* and trans a-carotene content,
and hue and total carotenoid content.
Vacuum frying was suggested as an alternative process
to produce chips with lower acrylamide levels [18].
Acrylamide content of chips fried under vacuum was
reduced by 94% as compared with those fried to the same
moisture content under atmospheric conditions. Different
trends were observed in the acrylamide formation during
vacuum frying and conventional frying. During
conventional frying, the acrylamide content in potato chips
increased with frying time and eventually leveled off. The
leveling off was explained by the degradation of
acrylamide with increasing time and temperature. In
vacuum frying, the acrylamide content in potato chips
continued to increase with frying time.
Shyu and Hwang [1] studied the effects of processing
conditions and immersing the blanched apple slices in
fructose solution as a pretreatment on the quality of
vacuum fried apple chips at < 98.66 kPa. Moisture
content, lightness (L) and breaking force of apple chips
decreased as frying temperature and time increased, while
the color difference (ÄE) and oil content increased during
vacuum frying. Moisture content, oil content, color and
texture of apple chips were signii cantly affected by
frying temperature, time and concentration of fructose
(p≤0.05). The optimal conditions were found to be
vacuum frying at 100–110°C, for 20–25 min after
immersing the apple chips in fructose solution of 30–40%
concentration when the breaking force was considered as a
quality indicator. Fan et al, [3] studied the effects of
processing conditions (frying temperature, vacuum
pressure and frying time) on the moisture, oil contents and
texture of fried carrot chips. The optimal conditions were
found to be vacuum frying at a temperature of 100–110°C,
at a vacuum pressure of 0.010–0.020 MPa for 15 min
using the oil content and breaking force as quality
indicators. Frying temperature, vacuum pressure and
frying time signii cantly affected moisture content, oil
content and texture (p < 0.05). Moisture content and the
breaking force decreased, but the oil content increased
with decrease in vacuum pressure and increase in frying
time and temperature [3]. Lower vacuum pressure
increased the rate of moisture removal and oil uptake
during frying (p < 0.05). Moisture and oil contents of the
carrot chips were found to be affected by the vacuum
temperature and frying time. Color was not affected from

4. 3
frying temperature and vacuum level [3]. The effects of
pretreatment and vacuum frying conditions on the quality
of carrot chips were studied by Shyu, Hau, and Hwang
[12].
Blanched carrot slices were studied at different
pretreatment methods before frying: immersion into
fructose solution at 50°C for various times (1, 30, 60, 180
min); immersion in fructose solution and then freezing at –
30°C overnight; immersion in fructose solution, freezing
and then thawing; freezing, thawing and then immersion in
fructose solution. In vacuum frying, vacuum pressure was
kept constant at 98.66 kPa and the effects of frying
temperature (70, 90, 100 and 110°C) and frying time (5,
10, 15, 20, 25 and 30 min) were studied. It was observed
that moisture and oil contents of carrot chips were signii
cantly reduced when carrot slices were immersed in
fructose solution for 30 min, frozen and then vacuum fried
(p < 0.05). It was possible to produce carrot chips with
lower moisture and oil contents as well as with good color
and crispy texture by means of vacuum frying at 90–
100°C for 20–25 min.
Fan, Zhang, and Mujumdar [19] also studied the effects
of pretreatments on quality of carrot slices in vacuum
frying. Carrot slices were subjected to different
pretreatments prior to vacuum frying, which were
blanching; blanching and air drying; blanching and
osmotic dehydration; and blanching, osmotic dehydration
followed by freezing. These pretreatment methods
significantly affected the total yield, carotenoid and
vitamin C contents, color, moisture and oil contents and
water activity of carrot chips (p < 0.05). There was no
significant difference in the breaking force of carrot chips
produced with different pretreatment methods. The highest
yield and lowest oil content was observed in the carrot
chips vacuum fried after blanching and osmotic
dehydration.
Moisture sorption isotherms of vacuum-fried carrot
chips were prepared at 10, 25 and 40°C [20]. Peleg,
Halsey and GAB models showed good agreement with the
experimental data over the entire range of water activity.
Smith model was shown to give good fit to the
experimental data in the water activity range of 0.32–0.95.
The experimental data with water activity values of < 0.50
could be well-fitted to the BET model.
Vacuum frying was also used in the frying of donuts
and the effects of initial moisture content of the dough,
vacuum pressure (3, 6 and 9 kPa) and frying temperature
(150, 165, 180°C) on the required frying time and physical
properties of donuts (volume, color, texture, moisture and
oil contents) were studied [21]. Higher initial moisture
content provided the product with lower volume and oil
content, higher moisture content and lighter color. Frying
time changed depending on the frying temperature, but it
was not affected by vacuum pressure.
Donuts with higher volume were obtained when fried
under higher vacuum and lower temperatures. Oil uptake
increased with increase in vacuum and decrease in frying
temperature. The total color change increased with
increase in frying temperature, but was not affected by
vacuum level. Higher vacuum level and lower frying
temperature provided a less compact and firm product.
Frying at 180°C under a 3k Pa vacuum was considered to
be the desired conditions for vacuum frying of donuts. Oil
uptake was higher in vacuum frying of donuts as
compared with that in atmospheric frying [21]. However,
lower oil uptake was observed in vacuum-fried potato
chips as compared with conventionally fried chips [9].
This may be due to the difference in the amount of oil
adhered to the surface of the product.
The effects of vacuum frying on product structural
were studied [22]. Effects of oil temperature, frying time,
and ripeness on dimensional changes of vacuum fried
bananas were studied. Banana slices with cross section
diameters of 25–30 mm and a thickness of 3.5–4.5 mm
were fried at temperatures of 100, 110, and 120 °C and 8
kPa for 20 min to determine which temperature produced
the highest degree of expansion. Using this temperature,
the width and thickness of the product were measured at 0,
5, 10, 15, and 20 min to model the dimensional changes as
a function of moisture ratio. Sensory evaluation was
conducted using a 7-point hedonic scale test to determine
the effect of ripeness on acceptability of the product.
Scanning electron microscopy (SEM) was used to analyze
the structure of the vacuum fried bananas.
The experimental results under this vacuum pressure
revealed that frying temperature of 110 °C on bananas at
the second day of ripeness yielded the highest volume
expansion. Sensory evaluations did not unveil any
significant difference (p > 0.05) in acceptability of the
products based on ripeness. Results from SEM exhibited,
as a function of frying time, a dramatic increase in the
pore size of the bananas, while the Heywood shape factor
indicated an overall increase in the product volume.
Modeling water loss and oil uptake during vacuum
frying of pre-treated potato slices were studied [4]. In this
research determined the kinetics of water loss and oil
uptake during frying of pre-treated potato slices under
vacuum and atmospheric pressure. Potato slices (diameter:
30 mm; width: 3 mm) were pre-treated in the following
ways: (i) raw potato slices ‘‘control’’; (ii) control slices
were blanched in hot water at 85 °C for 3.5 min; (iii)
blanched slices were dried in hot air until reaching a
moisture content of w0.6 g water/g dry basis. The slices
were fried under vacuum (5.37 kPa, absolute pressure, at
120, 130 and 140 °C) and atmospheric conditions (at 180
°C). Two models based on the Fick’s law were used to
describe water loss: (i) with a constant effective diffusive
coefficient; and (ii) with a variable effective diffusive
coefficient. Oil uptake data were fitted to an empirical
model, with a linear behavior for short times whereas the
model was time independent for long times. The variable
diffusivity model better fitted experimental water loss,
giving values of effective diffusivity between 4.73×10-9
and 1.80×10-8
m2
/s. The proposed model for the study of
the kinetics of oil uptake fitted the experimental data
properly. Control and blanched vacuum fried potato chips

5. 4
increased their final oil contents to 57.1% and 75.4%
respectively, when compared with those fried at
atmospheric pressure. However, the oil absorption of dried
vacuum fried potato chips diminished by w30%.
Edible coating and post-frying centrifuge step effect on
quality of vacuum-fried banana chips were studied [23]. A
high oil content in fried banana chips shortens the shelf
life of the product and causes a decrease in product
acceptability to consumers. The oil absorption problem
associated with fried products might be reduced by using
hydrocolloids as edible coatings and modifying the frying
process during the oil centrifuge step of vacuum frying.
The objective of this study was to determine the effect of
edible coating materials and the speed of the oil centrifuge
step on the amount of oil absorption and the physical
properties of vacuum-fried banana chips. Compared with
regular vacuum-fried products (control samples), banana
chips coated with either guar gum or xanthan gum
solutions at 1.5% or centrifuged at a higher speed than
standard conditions (from 140 to 280 rpm) reduced oil
absorption by 25.22%, 17.22% and 17.31%, respectively.
Moreover, the combination of an edible coating and the
higher centrifugation speed resulted in a greater reduction
of oil absorption (33.71%) compared with control
samples. Therefore, banana chips coated with an edible
coating and produced using the higher speed during the oil
centrifuge step in the vacuum-frying process maintained a
good quality with low oil content, representing a healthier
snack for consumers.
Phenolic compounds in Chinese purple yam and
changes during vacuum frying were studied [24]. Phenolic
compounds and their changes during vacuum frying were
investigated for a Chinese purple yam. Three cyanidin
derivatives and one peonidin derivative were tentatively
identified by HPLC–DAD–ESIMS analysis; sinapic acid
and ferulic acid were identified by HPLC–DAD analysis
with authentic chemicals. There were 31.0 mg/100 g (dry
weight, DW) of total anthocyanin (ACN) and 478 mg/100
g DW of total phenolic content (TPC) in the fresh yam.
Sinapic acid and ferulic acid were 135 and 31.3 mg/100 g
DW respectively. The blanching process caused about
60% of ACN, and 30–50% of phenolic acids and TPC to
be lost, which showed that anthocyanins were most
vulnerable during blanching. The retention rate of the
phenolic compounds during vacuum frying was 60–69%,
indicating it was a practical technology for purple yam
processing, on account of its impact on the phenolic
compounds stability.
Conclusions
Alternative technologies can be used to improve the
quality of fried products. Vacuum frying increased
significantly oil content of products and had a significant
effect on the instrumental and sensory parameters of color.
Vacuum frying is an efficient method to reduce the
acrylamide content of product. In vacuum frying, the
pressurization period has an important role in oil
absorption. Oil absorption in vacuum-fried products may
be lower or higher as compared with conventionally fried
products, depending on the type of the product.
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