Nutritional and health aspects related to frying (II)

Abstract

Current consumer requirements are directed towards improving the quality of foodstuffs expecting that certain processed foods and the ones prepared by frying will exhibit in addition to sensorial attributes nutritional qualities as well. Most of the processing and preparation methods of food are based on heat treatment, which also has less desirable effects on food such as: changing in color, taste, flavor and texture. In addition, during heat treatment by frying beside the aroma compounds formed which are very appreciated by consumers other compounds which are not desirable get also accumulated in the products; those compounds are formed by partial or total alteration of thermolabile nutrients present in food and in the frying oil. Tocopherols, essential amino acids, fatty acids present in food are degraded following hydrolysis, oxidation and polymerization reactions, etc. Therefore we are trying to review the nutritional and the risk factors associated to frying.

Full text

Romanian Biotechnological Letters Vol. 15, No. 6, 2010
Copyright © 2010 University of Bucharest Printed in Romania. All rights reserved
REVIEW
5675
Nutritional and health aspects related to frying (I)
Received for publication, August 15, 2010
Accepted
, November 3, 2010
MIHAELA GHIDURUS
1*
, MIRA TURTOI
1
, GEORGE BOSKOU
2
,
PETRU NICULITA
1
, VASILICA STAN
3
1
University of Agronomical Sciences and Veterinary Medicine Bucharest, Faculty of
Biotechnology, Marasti Blv. 59,011464, Bucharest, Romania
2
Harokopio University of Athens, Department of the Science of Dietetics - Nutrition,
70 El. Venizelou av., Kallithea, 17671, Athens, Greece
3
University of Agronomical Sciences and Veterinary Medicine Bucharest, Faculty of Agronomy,
Marasti Blv. 59,011464, Bucharest, Romania
*
mihaela_ghidurus@yahoo.com
Abstract
Current consumer requirements are directed towards improving the quality of foodstuffs
expecting that certain processed foods and the ones prepared by frying will exhibit in addition to
sensorial attributes nutritional qualities as well. Most of the processing and preparation methods of
food are based on heat treatment, which also has less desirable effects on food such as: changing in
color, taste, flavor and texture. In addition, during heat treatment by frying beside the aroma
compounds formed which are very appreciated by consumers other compounds which are not desirable
get also accumulated in the products; those compounds are formed by partial or total alteration of
thermolabile nutrients present in food and in the frying oil. Tocopherols, essential amino acids, fatty
acids present in food are degraded following hydrolysis, oxidation and polymerization reactions, etc.
Therefore we are trying to review the nutritional and the risk factors associated to frying.
Keywords: food safety, food quality, frying process, oils
Introduction
A review of the literature indicates that food industry scientists and public health
authorities in several countries, especially European Union member countries, are very
worried and concerned in relation to potential health hazards generated by the consumption of
oxidized products, which most often contain lipid polymers resulted from oils used repeatedly
in frying processes.
Despite the warnings issued by nutritionists regarding the consumption of fried foods,
which contain large amount of calories, cholesterol and saturated fats, they have a growing
popularity; a moderate consumption of fat is a way to ensure a balanced and healthy diet. It is
true that fried food have unique sensorial properties, which make them very attractive for
consumers. However the changes taking place in oil due to repeated fryings are often
deteriorative and makes the food that is fried an unsuitable product in terms of nutritional
facts. During the frying process, which involves subjecting oils at temperatures of 170° -
200°C, water vapor are released from food following heat transfer and some volatile
substances present in the product and frying oil will evaporate in the atmosphere while the
rest of volatile compounds that remain in the oil are subject to changes through chemical
reactions and then absorbed in the fried food [13]. Chemical reactions that occur in oil during
frying process are not limited just to thermal oxidation and auto-oxidation, they are more
complex [16].

MIHAELA GHIDURUS, MIRA TURTOI, GEORGE BOSKOU, PETRU NICULITA, SILVIA STAN
5676 Romanian Biotechnological Letters, Vol. 15, No. 6, 2010
Properties of fried food
Fats and oils play important functional and sensory roles in food products. They are
responsible for carrying, enhancing, and releasing the flavor of other ingredients, as well as
for interacting with other ingredients to develop the texture and mouth-feel characteristics of
fried foods [10].
Man has enjoyed fried foods for thousands of years, the main reason being that these
foods have unique and delicious sensory characteristics [27]. One of the fundamental
objectives of frying is to make food more acceptable. Fat is the natural palatable agent par
excellence. When frying food, the hot frying fat that has penetrated into it, replaces part of the
water it contains, making the food considerably more palatable [28]. This absorbed fat exerts
a tenderizing effect on the crust, as well as a wetting effect on the food, and thus contributes
for the popularity of deep fried foods, namely, their flavor, crispness and pleasant eating
characteristics [26]. The typical fried flavor is mainly due to lipid degradation products
originating from frying oils [21]. Foods fried at the optimum temperature and time have
golden brown color, are properly cooked, and crispy, and have optimal oil absorption.
Underfried foods at lower temperature or shorter frying time than the optimum have white or
slightly brown color at the edge, and have ungelatinized or partially cooked starch at the
center [6].
The following scheme shows a possible mechanism for the production of 2,4-
decadienal from 2-decenal [29].
Figure 1. Proposed mechanism for formation of 2,4-dienals in heated triolein by hydroperoxidation with
water/peroxide elimination reaction of 2-enals produced from decomposition of oleic hydroperoxides [29]
The volatile oxidation products of linoleic acid such as dienals, alcanals, lactones,
hydrocarbons, and various cyclic compounds are the most important flavor compounds found
in fried foods. The oxidation products of oleic acid are less important in contributing to the
fried flavor [20].
O
2
H
.
1.
2.
H
2
O
2
OH
O
O
H
.
.
O
OOH
O
O
H
2
O

Nutritional and health aspects related to frying (I)
Romanian Biotechnological Letters, Vol. 15, No. 5, 2010 5677
Table 1. Contribution of fried food to the resulting fried flavor [21]
Original food
component
Process occurring during frying Typical flavor
products
Sugars Pyrolysis (Caramelization )
Formation of Maillard products and their
thermal decomposition
Furan derivatives
Furan and pyrrole
derivatives
Amino acids
and proteins
Direct pyrolysis, deamination
Interaction with volatile aldehydes
Formation of Maillard products and Strecker
degradation
Aldehydes
Pyrazines
Aldehydic and amine
derivatives
Sulfur
compounds
Pyrolysis
Oxidation and pyrolysis
Interaction with aldehydes
Thiols
Sulfides
Thiazines
Trithiolanes
Lipid present in
food
Oxidation and cleavage
Interaction with amines
Interaction with sulfur compounds
Aldehydes
Pyrroles
Thiols, Sulfides
Phenolics Oxidation
Pyrolysis
Aromatic derivatives
Terpenes Oxidation
Condensation and polymerization
Oxygenated products
2,4-Decadienal and 2-heptenal, which are derived directly from linoleic acid
decomposition, are assumed to contribute the most to the deep-fried odor because of the high
parts per million levels produced. The effect that these compounds, when produced indirectly
and at low levels, have on the deep-fried flavor in foods is not fully understood, although
foods fried in high-oleic oils have low intensities of deep-fried flavor [30]. The compounds
may have been formed by the hydroperoxidation or hydroxylation of the allylic methylene
carbon hydrogen followed by loss of hydrogen peroxide or water from 2-alkenals produced by
decomposition of oleic hydroperoxides [29].Various specific components of fried substances
contribute to the overall flavor (table 2). Therefore, it is possible to distinguish between
flavors of different fried foods. The main chemical processes resulting in flavor substances
have been reviewed by Pokorny [21].
Nutritional aspects of fried food
There is a constant concern to pinpoint the effects that the various factors involved in a
thermal, industrial or culinary process have on the nutritive value of the food that is processed.
When the fat penetrates the food, it may selectively modify the composition of the food, as if
in a kind of chromatographic process. The changes produced depend on numerous factors,
such as the composition of frying fat and of the food, the texture, size and shape of the food
and the frying conditions such as temperature, duration, etc. All these factors influence the
changes that occur in the nutritive value of the fried food [28]. Undesirable changes may
occur concurrently with desirable modifications, one such change being the loss of nutrients,
and especially vitamins, during the frying process [8].
Deep-fat frying has significant advantages over other cooking methods: the
temperature within the product (aside from the crust region) is below 100
0
C; the frying time
is short and there is insolubility of water-soluble vitamins. Therefore, less deterioration is
expected to heat-sensitive vitamins, in comparison with baking or boiling [23].

MIHAELA GHIDURUS, MIRA TURTOI, GEORGE BOSKOU, PETRU NICULITA, SILVIA STAN
5678 Romanian Biotechnological Letters, Vol. 15, No. 6, 2010
Fat intake-uptake during frying
The intake of too much saturated fat, too many calories and cholesterol from animal
fats used in frying is a problem that has been already addressed by many medical and
nutritional bodies. The public is now fully aware of the risks in relation to atherosclerosis and
other diseases [4]. Total fat intake may influence some of the major risk factors for coronary
heart disease, particularly through its impact on obesity and type II diabetes. Recent studies
have shown that a high-fat meal may also impair vaso-activity and transiently impair
endothelial function [15].
Fat intake is a much smaller contributor to coronary heart disease than the type of fat
[12]. It is evident that the fatty acid composition of a particular fat is more important that its
absolute concentration regarding these diseases [22]. High intakes of saturated fats and trans
fatty acids have been recognized as a risk factor for coronary heart disease [12]. The degree of
saturation of fat used for deep-frying will be reflected in the fat content of the chips, and this
will significantly influence the tendency to artheroscrelosis and thrombosis [15].
The study of oil uptake during the frying of foods is complex and depends
significantly on the substrate to be fried [14].
The fat content increases with increasing frying time. Foods of plant origin absorb more than
foods of animal origin. The highest fat uptake has been observed in fried vegetables, such as
peeled eggplant, tomatoes, onion, mushrooms and flesh pineapple. White bread also absorbs a
lot of fat. The fat intake of peeled potatoes was 19.4% after 70 seconds, which is higher than
the fat intake of fried chicken, meat, fish or prawns. Low fat changes have been determined in
frying beef, pork, lamb or sausages [21].
Frying oils and fats are absorbed by cooked food, and so become part of our diet. The
uptake of absorbed oil in food ranges in percentages from 4% to 14% of the total weight,
depending upon the food and the type of the frying medium [1]. Frying oil quality influences
the oil uptake. The interfacial tension between frying oil and potato surface is high in fresh
oil. During repeated frying the polarity of oil increases and the interfacial tension decreases.
Therefore, the oil intake increases during repeated potato frying [19].
Vitamin changes during frying
Several vitamins are sensitive to higher temperatures and oxidation, but high
temperatures are reached only in surface layers of fried food, where their loss is certainly very
high. Total losses depend mostly on internal temperature, which usually varies between 70
and 90
0
C. In this range, vitamin retention depends much more on the internal temperature
than on the temperature of the frying oil [21].
Vitamin E
Vitamin E is lost along with the oxidation of unsaturated fatty acids during heating.
Frying oil is absorbed by the food during cooking and the absorbed quantity depends on the
quality of the cooking oil, which affects the net intake of vitamin E [2].
A remarkable resistance has been demonstrated for tocopherol homologues during
domestic frying simulation with virgin olive oil, sunflower oil or vegetable shortening oil for
eight successive frying operations. Fifty percent of Vitamin E was retained after four to five
consecutive frying sessions, depending on the oil type [1].
Vegetable frying oils are an excellent source of vitamin E [5]. All vegetable oils used
for frying contain vitamin E at a concentration of between 15 and 49 mg a-tocopherol
equivalents/100 g. Fried foods, due to oil uptake, are enriched with considerable amounts of
the vitamin. For instance, a portion of 100 g homemade French fries provides up to 50% of
the RDA (Recommended Daily Allowance) of vitamin E [23].
An increase in vitamin E loss during frying was observed by Carlson and Tabacchi [5].
Vitamin E (tocopherols) from the frying oil participates in free radical reactions, thus

Nutritional and health aspects related to frying (I)
Romanian Biotechnological Letters, Vol. 15, No. 5, 2010 5679
decreasing their rate [21]. However, no significant change in the vitamin E content of the French
fries during four days of commercial frying was observed, due to an increase in the fat intake
of the fries that compensated for the vitamin E reduction resulting from frying the oil [5].
Simonne et al. [24] studied the changes in Vitamin E content of chicken nuggets and
breaded shrimp during frying. After frying in soybean and corn oils, an increase in total vitamin
E was noted from 4.6 mg/100 g before frying to 4.9 mg/100 g after frying in chicken nuggets
and in breaded shrimp from 0.6 mg/100 g before frying to 5.1 mg/100 g after frying [24].
Vitamin A
Vitamin A active carotenes are present in plant foods. If the frying process is short,
losses of beta-carotene, the most important representative of this group of pro-vitamins, are
kept low. In deep fried vegetables, losses of beta-carotene were twice as high as in shallow-
fried foods. Some beta-carotene could migrate into the frying oil. During the frying of
cabbage, 29% of beta-carotene was destroyed [21].
The effect of processing on the carotenoid content of Thai vegetables was studied by
Speek et al. [25]. He found average losses of vitamin A activity of 14% for boiling, 24% for
frying, 29% for fermenting, 44% for sun-drying and 60% for sun-drying followed by boiling.
Some carotenoids were lost in the cooking water but losses during frying were greater due to
leaching into frying oil [25].
Vitamin B and C
Vitamins B
1
, B
2
, B
6
and C are better retained in frying than in boiling, steaming, or
stewing [4].
Thiamin is one of the most important vitamins of the B group. Fortunately, thiamin
losses are lower in the case of frying than when food is prepared using other methods [21].
The largest loss of thiamin occurred in boiling (70%), followed by steaming (40%), parching
(35%) and frying (30%). This can be attributed to the water-soluble nature of the vitamin
being leached out into the water [11].
Riboflavin is another important vitamin of the B group, and is often deficient in human
diet. Riboflavin was better retained when frying chicken meat than thiamin was, when frying
dark meat. Riboflavin losses in frying calves liver and swine liver were 42.5 and 43.5%,
respectively [21]. The losses during the boiling of potatoes, vegetable, meat and fish occur
mainly because of leaching. The increase in riboflavin content during frying is significant.
This is caused probably by the generation of riboflavin from riboflavin precursors during
frying [3].
Niacin is relatively stable. Still 45% losses have been reported in frying pork muscle,
beef and chicken meat. In peanuts, niacin content increased during frying [21].
Vitamin C (expressed as ascorbic acid) retention in green vegetables under different
cooking methods such as boiling, microwave cooking and stir-frying has been studied by
Eheart and Gott [7]. Stir-frying resulted in a better retention of vitamin C than cooking with a
lot of water or using a microwave [8].
Table 2. Vitamin C retention of broccoli and green beans cooked using four methods [8]
Cooking
Method
Vitamin C % retention in
broccoli
Vitamin C % retention in
green beans
Stir-frying
Microwave
Much water
Little water
76.6
56.8
44.8
72.2
57.5
58.9
59.6
76.0

MIHAELA GHIDURUS, MIRA TURTOI, GEORGE BOSKOU, PETRU NICULITA, SILVIA STAN
5680 Romanian Biotechnological Letters, Vol. 15, No. 6, 2010
The cooking of potatoes under different conditions shows less vitamin C loss for deep
frying (5-35 %) a deep frying followed by boiling (30%) and stewing (76%) [3].
Mineral component changes during frying
Mineral components show great changes during cooking operations, such as boiling,
because of their solubility in water. Their changes are almost negligible during frying, as they
are soluble only in trace amounts in frying oil. Due to water loss, the weight of fried food
decreases during frying. Most mineral components are non-volatile, therefore the content of
minerals, on wet weight, would be expected to rise. There occurs, however, another process at
the same time, i.e., the uptake of frying oil. The weight of fried material increases, and if the
metal content is expressed on a dry weight basis, a moderate decrease of mineral content
would be found [21].
Mineral losses in deep fried foods vary from 1% in potatoes to 26% in beef, being
significantly lower than in boiled foods of the same type. The mineral losses in breaded meat
and fish vary after deep-frying from 2% and 8% and are significantly lower than in deep fried
meat and fish fillet without coating. The breadcrumbs seem to absorbe the minerals solved in
the gravy of meat [3]. Gall et al. [9] reported no changes in the mineral composition of
mackerel fatty fish after baking or frying. In the case of low fat fish, grouper and red snapper,
small losses of major minerals of up to 20% during baking were found but losses after frying
were very limited: slight losses in Na, K and Mg were recorded, although Zn, Cu, Fe and Mn
did not vary in any case.
Frying influence on mineral metabolism
The influence of the consumption of olive oil, sun-flower oil and palm-olein, unused
and used in frying, on the bioavailability of magnesium, calcium and phosphorus in growing
rats has been studied by Perez-Granados et al. [17]. The type of oil consumed did not
influence the bioavailability of magnesium, but the three oils used in frying enhanced
magnesium absorption and urinary magnesium without affecting the body retention of this
mineral. Calcium absorption efficiency increased in animals consuming both forms of
sunflower oil (unused and used in frying), but no effect of the amount absorbed on calcium
retention, urinary calcium, serum calcium or carcass calcium was observed. The intake of oils
used in frying did not induce any significant changes in Ca bioavailability [17, 18].
Protein changes during frying
If the food is fried without any additional ingredients, as is normally the case, frying
does not change the digestibility of the protein. When reducing substances are added to the
food that is fried, for instance, carbohydrates (meatballs and fishballs which contain flour),
protein digestibility is lowered slightly, albeit significantly [28].
Bognar [3] investigated the effects of certain cooking methods on the protein content
of selected foods. The retention of protein in the investigated cooked food varied from 90% in
boiled meat and 96% to 100% in deep fried potatoes, meat and fish. The results obtained by
Bognar [3] were in concordance with the results of Gall [9] and Varela et al. [28]; frying did
not change the digestibility of protein.
Table 3. Protein digestibility coefficient of raw and fried foods [28]
State Hake Beef Pork Swordfish Meat balls Fish balls
Raw
Fried
0.92
0.91
0.93
0.93
0.92
0.92
0.94
0.96
0.90
0.88
0.92
0.89

Nutritional and health aspects related to frying (I)
Romanian Biotechnological Letters, Vol. 15, No. 5, 2010 5681
Carbohydrate changes during frying
The change in carbohydrate content during frying has been investigated in potatoes,
potato products and breaded meat and fish. The results showed that the retention of
carbohydrates varied from 95% to 100%, depending on the kind of food, indicating that the
frying method has no influence [3]. In comparison with raw samples, the backing of frozen
French fries has no effect on starch composition, while deep-frying significantly increases the
percentage of resistant starch. This is partially attributed to the formation of amylose –lipid
complexes. Although frying decreases the amount of digestible starch in potato, the dietary
fibre content is increased. Dietary fibres are playing an important role in the prevention of
diseases such as colonic cancer, cardiovascular disease and diabetes [8].
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