12 frying

12
Frying
J. Pokorn´y, Prague Institute of Chemical Technology

12.1

Introduction

Frying, especially deep fat frying, has become the most popular food preparation technology during the last five decades. The reason is that the preparation is
easy even for less experienced cooks, the procedure is rapid, and the finished
product is highly palatable. In the frying procedure, fat is the medium of heat
transfer. Two main frying methods exist, namely shallow frying and deep
frying.
In case of shallow frying, the layer of frying oil is about 1–10 mm thick in a
pan and the fried food is only partially immersed; it has to be turned during the
process to obtain an evenly cooked product. The frying takes about 5–10 min, and
frying oil is used for greasing the food as it is cooking. The oil is not reused.
In case of deep frying, the layer of frying oil is 20–200 mm thick or greater
and the fried material is immersed in oil or floats on the surface. The frying again
takes about 5–10 min, depending on the dimensions of the food being fried and
on the temperature. After frying, the food is removed and the frying oil is used
again for the next frying. The duration of use depends mainly on the frying
medium, the technical equipment and on the food.
The increasing consumption of fried foods contributes to a high intake of
fats and oils. Because consumers wish to reduce their consumption of fats and
oils pans are offered on the market that do not require any fat. When these are
used the heat transfer medium is not oil and therefore the process should not be
regarded as frying but as roasting. During frying, fat or oil is preheated to temperatures of 150–180 °C. In contact with oil, fried food is heated rapidly in the
surface layers to the temperature of the frying oil. The temperature reaches only
80–100 °C in inner layers.

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12.2

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Changes in frying oil

12.2.1 Types of reaction
The oil is subject to three types of reaction during deep frying:

• hydrolytic reactions;
• oxidation reactions;
• pyrolysis of oxidation products.
Triacylglycerols in frying oil are hydrolysed by steam produced from water in
the fried product when it is in contact with the hot frying oil. As the two reacting partners are not miscible, the reaction is relatively slow, resulting in the formation of diacylglycerols and free fatty acids. Diacylglycerols are more polar
and therefore their contact with water vapour is better; monoacylglycerols and
free fatty acids are formed by further hydrolysis. Monoacylglycerols are rapidly
hydrolysed into fatty acid and glycerol. Under deep frying conditions, glycerol
is dehydrated into acrolein, which is very volatile and its vapours irritate the eyes
and mucosa.
The rate of oxidation reactions depends on the concentration of oxygen.
Oxygen present in the original frying oil is rapidly consumed, usually before the
temperature of oil reaches the frying temperature. Additional oxygen can enter
frying oil only through diffusion from air (Fujisaki et al, 2000). When contact
with air is moderate the oxidation of the frying oil is slow. It is consumed for
the destruction of natural antioxidants, and only when they are destroyed, triacylglycerols are oxidised, too. Hydroperoxides are formed as primary reaction
products, but they are very unstable at high temperature so that their content rarely
exceeds 1%.

Some components present in fried food affect the oxidation rate of frying oil
(Pokorny´, 1998). The oxidation rate could be reduced by addition of antioxidants
even when they are less efficient than under storage conditions. Most synthetic
antioxidants, such as BHT and BHA, are too volatile under frying so that they
have only moderate activity. Gallates are more efficient in frying oils. Currently
it is considered preferable to use natural antioxidants. Tocopherols are present in
most frying oils, and their addition is efficient (Gordon and Kourimska, 1995).
Ascorbyl palmitate, citric acid and its esters are useful as synergists. Rosemary
and sage resins were also found to be active in frying oils (Che Man and Tan,
1999). Oxidation reactions can be inhibited by polysiloxanes, which form a very
thin layer on the surface of the frying oil, preventing the access of oxygen (Ohta
et al, 1988). Because they are not resorbed in the intestines they are considered
safe for human consumption.
The third group of reactions are secondary reactions of hydroperoxides. They
are decomposed in three ways during frying:

• Decomposition into nonvolatile products with the same number of carbon
atoms, such as epoxides, ketones or hydroxylic compounds. When the con-


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centration of these products (known as polar products) exceeds 25–27%, the
frying oil has to be replaced by fresh oil. At still higher levels of polar products, foaming takes place, which increases the contact area of oil with air, and

thus the rate of oxidation.
Decomposition into volatile low-molecular weight compounds, such as aldehydes, alcohols, ketones or hydrocarbons. Some products possess a typical
fried flavour, e.g. 2,4-decadienals or unsaturated lactones. They are formed
from linoleic acid bound in frying oil.
Decomposition into high molecular weight compounds, usually dimers or
trimers with fatty acid chains bonded by C–C, C–O–C or C–O–O–C bonds.
The content of polymers is a good indicator of the degree of frying oil degradation. When their content reaches 10%, used oil should be replaced by
fresh oil.

Several methods are used for monitoring oil degradation during frying (Wu
and Nawar, 1986). Used oil can be analysed with use of HPLC (for polar compounds) or HPSEC (for polymers); this is best done in combination with column
chromatography (Sánchez-Muniz et al, 1993). Among other methods, the spectrophotometry, determination of permittivity (dielectric constant), specific gravity
or different colour tests can be used (Xu, 2000).
Frying oil can be used for a longer time if it is purified from insoluble particles and polar substances by using a suitable adsorbent, such as magnesium silicate (Perkins and Lamboni, 1998). Commercial products for this pupose are
available (Gertz et al, 2000). Their combination with antioxidants is recommended (Kochhar, 2000). Membrane processes have been proposed for purification of frying oil (Miyagi et al, 2001).

12.2.2 Choice of frying oil
Frying oil should contain some bound linoleic acid to generate a fried flavour
(Warner et al, 1997). Some oils, such as soybean, sunflower or rapeseed oils are
rich in linoleic acid, but are rather unstable under frying conditions and should
be replaced very often by fresh oil, which is expensive (Gertz et al, 2000). Low
polyunsaturated oils, such as olive oil, are highly priced. Hydrogenated vegetable oils are more stable but are objectionable because of the content of transunsaturated fatty acids. Pork lard is an excellent frying medium from the
standpoint of sensory value, but there are objections because of its high content
of saturated fatty acids and of cholesterol. The best choice are high-oleic lowpolyenoic modified vegetable oils, such as fractionated palm oil, i.e. the palm
olein fraction (Che Man and Hussin, 1998), modified soybean, sunflower, rapeseed, peanut, and even linseed oil. If they contain 3–10% linoleic acid, they still
produce an attractive fried flavour and are sufficiently stable on frying. A problem
is their availability on the market.


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12.3

The nutrition handbook for food processors

Impact of deep frying on nutrients

The following main changes occur in the frying process (Fillion and Henry,
1998):

• Mass transfer between frying oil and fried food;
• Thermal decomposition of nutrients and antinutritional substances in fried
food;

• Interaction between fried food components and oxidation products of fried oil
(Dobarganes et al, 2000).

12.3.1 Impact of frying on main nutrients
The main change in the food composition during frying is the loss of water and
its replacement with frying oil. Most foods (other than nuts) contain water as their
major component. In contact with hot frying oil, water is rapidly converted into
steam, at least in the surface layer of fried material. The temperature of inner
layers does not exceed the boiling point of water so that water losses are only
moderate.
Spaces left in fried food after water evaporation are filled with frying oil
(Pinthus et al, 1995). This process increases the available energy content of the
product and because the energy intake in the diet is too high in many countries,
it is desirable to reduce the absorption of frying oil. This may be achieved by
drying pieces of food on the surface before immersion into oil (Baumann and
Escher, 1995). Another way is to produce a crust on the surface of fried pieces,

which prevents water losses and oil uptake, and preserves juiciness in fried material (Ateba and Mittal, 1994). It is possible to cover the surface with batter or
various other preparations, such as cellulose derivatives (Priya et al, 1996). The
oil absorption can be reduced by half using these procedures. The oil removed
by absorption into fried food should be replenished by fresh oil from time to time
in order to keep the volume of frying oil constant.
Changes in nutritional value depend not only on the amount of absorbed frying
oil, but also on its composition. If fresh edible oil is used, the contents of essential fatty acids and tocopherols in fried food rise. If food is fried in oil used for
a longer time, the content of essential fatty acids and tocopherols becomes low,
so that the increase in nutritional value, due to absorbed oil, is not significant. On
the contrary, such antinutritional products as polar lipids and polymers are
absorbed with used frying oil.
If fried food is stored, even under refrigeration, the thin layer of frying oil on
the surface is autoxidized, especially in case of oil rich in polyunsaturated fatty
acids (Warner et al, 1994). Fried food should be stored either in vacuum or an
inert gas or protected by antioxidants.
If fat-rich food is fried, such as bacon, sausages or fat fishes, some fat originally present in food is released into frying oil. Eicosapentaenoic and docosahexaenoic acids were detected in oils used for frying fish like sardines
(Sánchez-Muniz et al, 1992). Cholesterol may also be extracted into frying oil.


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If plant foods are subsequently fried in the same oil, cholesterol or fish fatty acids
may be absorbed.
Lipids present in food are decomposed only to a small extent, including high
unsaturated fish oils. It is due to short frying time and limited access of oxygen.
Based on dry matter content, the concentration of most nutrients is reduced during frying, as the original nutrients are diluted with absorbed frying oil. Starch
and non-starch carbohydrates are partially destroyed during frying, and starchlipid complexes are formed (Thed and Phillips, 1995). The fraction of resistant
(undigestible) starch changes during the operation (Parchure and Kulkarni, 1997).

Sucrose is hydrolysed into glucose and fructose, which are destroyed by heating,
mostly by Maillard or caramelisation reactions.
Proteins are rapidly denaturated in surface layers of food particles, more
slowly in inner layers than on the surface. Enzymes get nearly completely
deactivated. The availability of proteins in humans is usually reduced by frying
(Fukuda et al, 1989), especially on the surface (Pokorny´ et al, 1992). Some essential amino acids are destroyed, such as lysine or tryptophan (Ribarova et al, 1994).
If protein comes into contact with the hot walls of the frying pan above the oil
level, it is dehydrated and pyrolysed into polycyclic aromatic compounds
(Övervik et al, 1989).
12.3.2 Impact of frying on micronutrients
Vitamins are relatively labile substances. Tocopherols are decomposed by oxidation reactions so that frying oil used for repeated frying contains only traces
of tocopherols. Ascorbic acid is also destroyed by mechanisms similar to those
of reducing sugars. The Vitamin B complex is also substantially damaged by
frying (Kimura et al, 1991; Olds et al, 1993). Carotenes and carotenoid pigments
are easily oxidised and polymerised (Speek et al, 1988), which is visible from
colour changes.
Mineral components are also affected. Iron and other heavy metals are mostly
bound in complexes, which are partially decomposed during frying, and metal
ions may contaminate frying oil by decreasing its resistance to oxidation. Ferric
ions are less digestible than iron in haem complexes. Sodium and potassium chlorides present in food are very slightly dissociated, and sodium and potassium ions
react with free fatty acids forming soaps (Blumenthal and Stockler, 1986). Soaps
are surface active agents, increasing foaming and thus accelerating oxidation.
Volatile mineral components, such as selenium or mercury derivatives, are partially lost at high frying temperatures. Many foods contain antinutritional or
even toxic substances, which are often partially decomposed or evaporated during
frying.
12.3.3 Changes in sensory characteristics
Frying imparts a distinctive flavour to fried products; some flavours are common
to all fried foods and some are additional and, specific for particular products,
e.g. french fries (Wagner and Grosch, 1998).

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The colour of the fried product often differs substantially from that of the original food material. The most important reactions are nonenzymic browning reactions between reducing sugars and free amino acids, called Maillard reactions.
Colourless premelanoidins are a group of intermediary products with very low
nutritional value. They are rapidly polymerised into macromolecular deep brown
melanoidins, which are completely unavailable for human nutrition. To obtain
light-coloured potato chips, it is necessary to adjust the concentration of reducing sugars to low values (Califano and Calvelo, 1987). A side reaction is the
Strecker degradation of free amino acids under attack of dicarbonylic sugar
degradation products. Heterocyclic products, such as pyrazines or furans, have
typical fried or roasted flavour notes (Chun and Ho, 1997). Similar browning
reactions are caused by interactions of frying oil oxidation products (mostly
hydroperoxides or unsaturated aldehydes) with the free amine group of
bound lysine (Pokorny´, 1981). Lysine thus becomes unavailable for human
nutrition.
Oxidised frying oil also contributes in significant degree to the flavour of fried
food (Chang et al, 1978). Moderate amounts are necessary to produce the typical
fried flavour but greater amounts are objectionable. For this reason the frying oil
should not be too fresh for high quality fried foods. The flavour improves by
repeated use for frying, but if use is too long then products are of lower quality.
The producer’s aim is to maintain frying oil for the longest time possible at the
stage of optimum performance (Blumenthal and Stier, 1991).

12.4

Future trends

Deep frying will probably become an even more important process of food preparation, and its conditions and the equipment will be improved gradually. Fried

foods contain high amounts of fat, and are therefore rich in available energy. Since
energy intake is too high in most countries, frying technology will be modified
in order to reduce the absorption of frying oil during the operation; however, the
intensity and pleasantness of fried food should remain unaffected.
Frying oil is oxidised during heating. Criteria for replacing used oil by fresh
oil are arbitrary (Firestone, 1993), e.g. 25% polar products and 10% polymers,
and are not based on experimental evidence. The composition of oxidation products and their impact on food safety should be determined more precisely. Frying
oil deterioration depends on its degree of unsaturation and on the content of
natural and/or added antioxidants. New oils will be introduced on the market with
better stability during frying and intervals between necessary replacing of used
oil will get longer. The flavour quality of fried products should not be affected
by new, more stable frying oils. New flavourings will be developed for flavouring products fried in oils free of polyunsaturated fatty acids, or obtained by
heating in pans designed for frying without oil. The flavour of such products
should be that of deep fried products.


Frying

12.5

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Sources of further information and advice

blumenthal m m (1987), Optimum frying: theory and practice, Piscataway, NJ, Libra
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boskou d and elmadfa i (1999), Frying of food, Lancaster, PA, Technomic Publishing
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application, Champaign, IL, AOCS
pokorny´ j (1989), ‘Flavor chemistry of deep fat frying in oil’, in Min D B, Smouse T H,

(eds), Flavour Chemistry of Lipid Foods, Champaign, IL, AOCS, 113–55
A special issue on deep frying (1998), Grasas Aceites, 49, No. 3/4
A special issue on deep frying (2000), Eur J Lipid Sci Technol, 102, No. 8/9

12.6

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