The following article has been condensed and adapted from Lipid Oxidation by Edwin N. Frankel, Chapter 12, “Frying Fats.” For more information, see this article's end.—Eds.

Deep-fat frying is perhaps the most complex of food processing operations because of the multiplicity of reactions that occur and the vast quantity of chemical products that are generated. Understanding the chemical processes that occur during frying and the methods available to control oil degradation can have a big impact on finished product flavor and shelflife.

Frying is usually done in the 180°-200°C range. There are three major chemical reactions that occur: oxidation, polymerization and hydrolysis. Each of these reactions produces a wide range of compounds which contribute to the formation of both desirable and undesirable products in fried foods.

Thermal lipid oxidation and hydrolysis produce complex mixtures of volatile and non-volatile monomeric and polymeric substances. Polar materials are polymers, which are useful indicators of oil quality. The food itself also releases proteins, carbohydrates, fats, oils and metals into the frying medium that may promote reactions affecting oil performance as well as its health and nutritional properties.

The rapid decomposition of hydroperoxides and aldehydes during frying produces an intricate assortment of volatile compounds. However, they are found in very low concentrations because the frying process acts like a large steam distillation operation. Those that remain are important because many at extremely low levels (in parts per million) contribute to the distinctive flavors and aromas so typical of fried foods. The chart “Various Volatiles” shows some of the many compounds found in frying oils and/or French fries. One of the compounds listed in this table, 2,4-decadienal, a decomposition product of linoleate that produces rich, fried food flavor, was a significant flavor noted during sensory evaluation of French fries. The presence of this linoleate in vegetable oils contributes to the formation of 2,4-decadienal. When frying potato chips, research indicates that a certain level of linoleate is essential for desirable flavor development. The downside is that the presence of linoleate also increases the probability that rancidity will occur. Because decadienal imparts a desirable “fried flavor” at low concentrations, but too much can cause an undesirable “rancid” flavor, the frying process also must be carefully controlled by a turnover process to introduce fresh make-up oil during frying.

Oil Sources and Evaluations

Both fats and oils of animal and vegetable origin are used for frying. General considerations involved in the selection of frying oil include the turnover rate (i.e., oil added to a fryer to replace that removed by the fried products), the amount of fat absorbed by the food and the shelflife of the food. For high-volume frying, oils should not exceed 2%-3% linolenic acid, a polyunsaturated fatty acid that is readily oxidized and polymerized. The process of partial hydrogenation was aimed at reducing the linolenic acid content of soybean and canola oils. However, partial hydrogenation has the problem of producing trans isomers that are nutritionally undesirable. Many frying fats are developed by blending oils with much lower degrees of hydrogenation to meet the recent FDA requirement of labeling the content of trans fats in foods. In response to this issue, plant breeders have produced zero-trans oils with higher oleic acid content and reduced linolenic acid to improve stability during frying without hydrogenation.

There are many ways to measure the changes that occur in oils during frying. As frying progresses, the oil darkens, becomes more viscous and tends to foam, and the degree of unsaturation drops. Chemical methods which estimate the amount of polar materials, total polymeric materials and/or free fatty acids also are routinely used to monitor oil degradation.

Polar materials are measured by column chromatography. This is a time-consuming, expensive method and consequently is not suitable for quality control. To reduce costs and minimize the amount of solvent used, rapid micro chromatographic methods have been developed.

The rate of polar material formation varies with the type of frying operation. Oils used for restaurant frying can contain up to 30% polar materials after 40 hours of frying. The reason for this rapid buildup is that batch frying is done intermittently and oil turnover is slow (i.e., oil is not frequently replaced). Turnover in industrial frying operations is much greater due to the volume of food being prepared. The frying fat generally reaches a steady-state condition after a few hours of continuous operation. The amount of polar material produced by the three common frying operations can be compared in the chart “Frying Operations and Amount of Polar Compounds.”

There also are a number of rapid test methods available. These include a range of commercial colorimetric tests based on redox indicators, carbonyl compounds, free fatty acids and alkaline materials. Unfortunately, these methods are not specific, and [this author suggests they are not to be] recommended as tools for quality monitoring. The ultimate arbiter of frying, however, is in the taste and quality of the fried food. The most sensitive and reliable methods for monitoring frying oil degradation are based on sensory evaluation of the foods and gas chromatographic analysis of the volatile flavor compounds in the foods, preferably potatoes fresh and after storage.

Control Measures

A number of control mechanisms are available to enhance food quality and extend the usable frying life of the cooking oil. These include monitoring and controlling frying oil temperatures, increasing turnover rates, filtering the oil, using additives such as antioxidants and polymerization inhibitors and controlling fat uptake by the foods.

Optimal frying is conducted at temperatures ranging from 160°-190°C, and frying at lower temperatures will minimize thermal degradation of the oil. However, sufficiently hot oil is essential to ensure that food is thoroughly cooked. The continuous emanation of bubbles from frying food is proof that moisture is escaping as steam. Steam release helps strip volatile decomposition products from the oil, which delays fat deterioration. To reduce the potential for thermal degradation and oil breakdown, the temperature of the oil can be lowered when not in use.

Filtration is used to remove charred food particles and batter that accumulate in the oil. Removal of these materials can slow oil degradation, maintain oil color and minimize the development of off-flavors. Metal screens, paper and plastic are used as filter materials. Filtration can be augmented by using filter aids such as diatomaceous earth.

The use of additives can enhance both oil life and the shelflife of fried foods. Antioxidants such as BHA, BHT, propyl gallate and TBHQ may be included in commercially used frying fats and oils. These oils also may include naturally occurring or added tocopherols, which generally are depleted during the frying process. Natural antioxidants such as rosemary and sage also may be added to frying oils. Like the synthetic antioxidants noted above, these also can protect the oil during frying and enhance shelflife of fried food.

The amount of fat absorbed by the food during frying affects its sensory perceptions and acceptability. The chemical makeup of the frying oil and that picked up by the food are not significantly different, and minimizing the rate of oil degradation can reduce the amount of oil picked up by the food.

Deep-fat frying is perhaps the most complex processing operation known. The frying oil is a dynamic entity that changes depending upon the food being fried and the time of frying. And although oil degradation helps produce some of the delicious flavors that make fried foods so palatable, understanding its processes and the means to control them is essential to producing healthy and nutritious food.

This article was condensed and adapted, in part, by Rick Stier from Lipid Oxidation, 2nd ed., written by Edwin N. Frankel, University of California, USA, and published by The Oily Press, PJ Barnes & Associates. Published March 2005. 486 pages, 152 tables, 148 figures, 87 equations, 849 references. Volume 18 in The Oily Press Lipid Library. For more information, call: + 44-1823-698973; e-mail: sales@pjbarnes.co.uk. See also www.pjbarnes.co.uk/op/lo2.htm.