1

Science of Oil Frying -Baskaran. G – Food Safety Manager

Introduction:

There has been an increased in the consumption of frying fats and oils over the last few decades.

This change is driven majorly due to the development of a wide range of new products, most notably

frozen pre-fried foods and fast foods. Did you know? The economy of oil production in India

estimated around 26.68 million MT (source: DFPD). Global oil consumption is likely to increase 50%

by 2025 (Fig-1- show world consumption pattern of oil). One of the key demanding sectors for the oil

production increments is Food Service, Packaged foods and Retails

Fig-1 – World Consumption of fats and oils -2015

What is oil?

Oil belongs to a group of biological substances called lipids. It serves various functions in organisms

like regulatory messengers, structural components of the membrane and as energy store

warehouses. Fats differ from oils only in that they are solid at room temperature. Fats and oil share a

common molecular structure called as triacylglycerol. Most fats and oils used for human

consumption are plant- derived. The vast commodity of fats and oils are derived from soybean,

cottonseed, canola, sunflower, corn, peanut, palm kernel, and coconut oils, olive, almond, cashew

nut, hazelnut, avocado and sesame (Fig-2 shows- World consumption for major fats and oils).

Fig-2 – World Consumption for Major fats and oil -2015

2

Application of oil:

Oil & Fats used in variety food products to add flavour, texture. Lubricity, and satiety to foods. Some

of the primary constituents are:

Margarines,

Butterfat,

Shortenings,

Oils for salad and

Cooking

In addition to the visible fat contained in food, fats, and oils are found in high quantities in many

bakery goods, infants’ formula, dairy products, and some sweets.

Cooking / deep fat frying:

Deep fat frying is one of the favourites and most common method of food preparations. Frying is a

process of immersing food in hot oil with a contact among oil, air, and food at a high temperature

(E.CHOE, AND B.B MIN). The results have desirable flavour, colour, and crispy texture which

eventually make very popular to the consumer. The amount of absorbed oil depends on various

factors such as frying time, food surface area, the moisture content of food, types of breading, and

oil. Most of the food items observed oil tends to accumulate on the surface of fried food and move

into the interior of foods during cooling (Fig3- show minimum oil contents of various foods)

Fig-3 Minimum oil contents of various foods Source: E.CHOE AND D.B MIN

Food fried at the optimum temperature and time have desirable effects, However, on other side

food fried at low temperature has undesirable effects like the slightly brown colour on edge and

partially cooked food.

Chemical reactions of oil:

During the course of frying oil undergo various chemical reactions such as hydrolysis, oxidation, and

polymerization produces desirable and undesirable compounds. This eventually has changed in

flavour stability, quality, colour and texture of fried foods.

Compounds formed differ in polarity from the major compounds initially present in the fresh oil. The

content of total polar compounds and acid value are the most important indicators of oil quality. For

33%

30%23%

20%

10%

Minimum oil contents of various foods

Potato Chips Corn Chips Tortilla chips

Doughnuts French Fries

3

public health concerns, the content of total polar compounds and the acid value in frying oil are

regulated at not more than 25% and 2.0mg KOH/g respectively.

Hydrolysis of Oil:

It is the well-known reaction affecting fats and oils due to the action of moisture. When food is fried

in oil, the moisture forms steam which evaporates with a bubbling action and gradually subsides as

the foods are fried (E.CHOE & D.B MIN). Hydrolysis is the only reaction breaking down the

triacylglycerol to diacylglycerides &free fatty acid.

Free fatty acids contents in oil gradually increase depends on with a number of frying. Hydrolysis of

oil with short and unsaturated fatty acid than with oil with long and saturated fatty acids. Reason

being short and unsaturated fatty acids are more soluble in water. Frequent replacement of frying

oil with fresh oil slow down the hydrolysis of frying oil (Romero and others 1998- Fig-4 shows

physical and chemical changes of oil during deep fat frying)

Fig-4 - Physical and chemical changes of oil during deep fat frying

Oxidation of oil:

The chemistry of lipid oxidation at the high temperature of food processes like baking and frying

highly complex because oxidative and thermal reactions are involved simultaneously. As

temperature increases, the solubility of oxygen decreases drastically, although all oxidations are

accelerated. The chemical mechanism of thermal oxidation is principally the same as the

autoxidation. It is well known that autoxidation is an important degradation reaction which is

attributed to the rancidity of oil and fats.

Table: 1 Main Groups of compounds formed during frying

Alteration Causative Agent New compounds Hydrolysis Moisture Fatty acids,

Diacylglycerols Oxidation & polymerization Air Oxidised Monomeric,

Triacylglycerol Oxidised dimeric and oligomeric triacylglycerol volatile compounds

Thermal Alteration Temperature Cyclic Monomeric triacylglycerol, Isomeric monomeric triacylglycerol nonpolar dimeric and oligomeric triacylglycerol

4

Source: Navas (17).

Polymerization

The development of polymerization reactions accounts for the major and most complex group of

degradation products found in used frying fats and oils. Significant information on the mechanism of

polymerization has been limited to the formation of the dimers obtained in the first steps of

polymerization. Polymers formed in deep fat frying are rich in oxygen .Yoon and others -1988

reported that oxidised polymers compounds accelerated the oxidation of the oil. Polymers

accelerated further degradation of the oil, increase the viscosity (Tseng and others -1996), reduce

the heat transfer, produce foam during deep fat frying and develop undesirable colour in food. A

polymer as well causes the high oil absorption to foods.

Polymers are highly conjugated dienes and produce a brown resin-like residue along the sides of the

fryer, where the oil and metals come in contact with oxygen from the air. Resin-like residue often

produced when the oil does not release the moisture but keeps it trapped while also incorporating

air (Lawson 1995; Moreria and others 1999c).

Interactions between the food and the frying oil

The condition to which food are immersed during the frying process initiates physical and chemical

changes that depend completely on the composition of food. This eventually affects the

development of colour, flavour, and taste, besides changing food texture.

The table below summarises the physical and chemical changes in food during the frying process.

Components Changes during frying Fat Increased concentration and change in composition Water Significant loss

Reducing sugars Maillard reactions Starch Gelatinization Proteins Alterations of the compositions Amino Acids Formations heterocyclic flavouring substances

Flavouring substances Formed by oxidative and Maillard reactions Vitamins Moderate loss Minerals Small loss Antioxidant Moderate loss

Table -2 physical and chemical changes in food during the frying process Source: Adapted from

Pokomy (66)

Factors affecting the quality of oil:

The turnover rate of oil, frying time and temperature, type of heating, frying oil composition, initial

oil quality, and composition of food to be fried, type of fryer, antioxidant, and oxygen content affects

the deterioration of oil during deep fat frying.

Reference:

http://www.fao.org/docrep/v4700e/V4700E0b.htm

https://www.ihs.com/products/fats-and-oils-industry-chemical-economics-handbook.html

Chemistry of deep-fat frying oils- E Choe and B.B. Min

5

http://lipidlibrary.aocs.org/OilsFats/content.cfm?ItemNumber=39209

http://www.fda.gov.tw/upload/133/Content/2013050913435287631.pdf

Chemical alterations took place during deep-fat frying based on certain reaction products: A review -

Qing Zhang, Ahmed S.M. Saleh, Jing Chen, Qun Shen∗

http://www.scielo.org.ve/scielo.php?script=sci_arttext&pid=S0004-06222013000100001

Advances in deep-fat frying of foods - Professor Da-Wen Sun, Director