Science of Oil Frying


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

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:

stability, quality, colour and texture of fried foods.

For 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,


Oxidation  & polymerization Air
Thermal Alteration Temperature Cyclic Monomeric triacylglycerol, Isomeric monomeric triacylglycerol nonpolar dimeric and oligomeric triacylglycerol

Source: Navas (17).


The development of polymerization reactions accounts for the major and most complex group of degradation products found in used frying fats and oils.

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.


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

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

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