Like other esters, glycerides can be hydrolyzed readily. Partial hydrolysis of triglycerides will yield mono- and diglycerides and free fatty acids. When hydrolysis is carried to completion with water in the presence of an acid catalyst, the mono-, di-, and triglycerides will hydrolyze to yield glycerol and free fatty acids. With aqueous sodium hydroxide, glycerol and the sodium salts of the component fatty acids (soaps) are obtained. This process is also called saponification. In the digestive tracts of humans and animals and in bacteria, fats are hydrolyzed by enzymes (lipases). Lipolytic enzymes are present in some edible oil sources (i.e., palm fruit, coconut). Any residues of these lipolytic enzymes (present in some crude fats and oils) are deactivated by the elevated temperatures normally used in oil processing.
1. Autoxidation. Of particular interest in the food arena is the process of oxidation induced by air at room temperature referred to as “autoxidation”. Ordinarily, this is a slow process which occurs only to a limited degree. However, factors such as the presence of light can increase the rate of oxidation. In autoxidation, oxygen reacts with unsaturated fatty acids at the double bond site. Initially, peroxides are formed which may break down into secondary oxidation products (hydrocarbons, ketones, aldehydes, and smaller amounts of epoxides and alcohols). Metals, such as copper or iron, present at low levels in fats and oils can also promote autoxidation. Fats and oils are normally treated with chelating agents such as citric acid to complex these trace metals (thus inactivating their prooxidant effect).
The result of the autoxidation of fats and oils is the development of objectionable flavors and odors characteristic of the condition known as “oxidative rancidity”. Some fats resist this change to a remarkable extent while others are more susceptible depending on certain factors, such as the degree of unsaturation.
When rancidity has progressed significantly, it becomes readily apparent from the flavor and odor of the oil. Expert tasters are able to detect the development of rancidity in its early stages. The peroxide value determination, if used judiciously, is oftentimes helpful in measuring the degree to which oxidative rancidity in the fat has progressed.
It is common practice in the industry to protect fats and oils from oxidation to preserve their acceptable flavor and to maximize shelf life. For instance, manufacturers avoid air contact by routinely blanketing oils with nitrogen during processing, storage and transportation; and may use chelating agents or antioxidants to further deter autooxidation.
2. Oxidation at Higher Temperatures. Although the rate of oxidation is greatly accelerated at higher temperatures, oxidative reactions which occur at higher temperatures may not follow precisely the same routes and mechanisms as the reactions at room temperature. Thus, differences in the stability of fats and oils often become more apparent when the fats are used for frying or slow baking. The stability of a fat or oil may be predicted to some degree by determining the oxidative stability index (OSI).
The more unsaturated the fat or oil, the greater will be its susceptibility to oxidative rancidity. Predominantly unsaturated oils (i.e., soybean, cottonseed, or corn) are less stable than predominantly saturated oils (i.e., coconut oil, palm oil). Dimethylsilicone is usually added to institutional frying fats and oils to reduce oxidation tendency and foaming at elevated temperatures. Historically, partial hydrogenation has often been employed in the processing of liquid vegetable oil to increase the stability and functionality of the oil. The trend of utilizing partial hydrogenation has been declining during the last decade due to developments in oils/fat processes and trans fat legislation.
All commonly used fats and particularly those high in polyunsaturated fatty acids tend to form larger molecules (known broadly as polymers) when heated under extreme conditions of temperature and time. Under normal processing and cooking conditions, polymers are formed. Although the polymerization process is not completely understood, it is believed that polymers in fats and oils arise by formation of either carbon-to-carbon bonds or oxygen bridges between molecules. When an appreciable amount of polymer is present, there is a marked increase in viscosity.
Glycerides are subject to chemical reactions (oxidation, hydrolysis, and polymerization) which can occur particularly during deep fat frying. The extent of these reactions, which may be reflected by a decrease in iodine value of the fat and an increase in free fatty acids, depends on the frying conditions (principally the temperature, aeration, moisture, and duration). The composition of a frying medium also may be affected by the kind of food being fried. For example, when frying foods such as chicken, some fat from the food will be rendered and blended with the frying medium while some of the frying medium will be absorbed by the food. In this manner the fatty acid composition of the frying medium will change as frying progresses. Since absorption of frying medium into the food may be extensive, it is often necessary to replenish the fryer with fresh frying medium. Obviously, this replacement with fresh medium tends to dilute overall compositional changes of the fat that would have taken place during prolonged frying.
Frying conditions do not saturate the unsaturated fatty acids, although the ratio of saturated to unsaturated fatty acids will change due to degradation and polymerization of the unsaturated fatty acids. The frying operation also results in an increase in the level of “polar compounds” (mono- and diglycerides, free fatty acids, and other polar transformation products) formed during frying/heating of foodstuffs in the oil.
It is the usual practice to discard the frying medium when (1) prolonged frying causes excessive foaming of the hot oil, (2) the medium tends to smoke excessively, usually from prolonged frying with low turnover, or (3) an undesirable flavor or dark color develops. Any or all of these qualities associated with the frying medium can decrease the quality of the fried food.
The smoke, flash, and fire points of a fatty material are standard measures of its thermal stability when heated in contact with air. The “smoke point” is the temperature at which smoke is first detected in a laboratory apparatus protected from drafts and provided with special illumination. The temperature at which the fat smokes freely is usually somewhat higher. The “flash point” is the temperature at which the volatile products are evolved at such a rate that they are capable of being ignited but not capable of supporting combustion. The “fire point” is the temperature at which the volatile products will support continued combustion. For typical non-lauric oils with a free fatty acid content of about 0.05%, the “smoke”, “flash”, and “fire” points are around 420°F, 620°F, and 670°F respectively. The typical smoke, flash and fire points of commercially available oils and fats used for food purposes in the U.S. are given in Table VIII.
The degree of unsaturation of an oil has little, if any, effect on its smoke, flash, or fire points. Oils containing fatty acids of low molecular weight such as coconut oil, however, have lower smoke, flash, and fire points than other animal or vegetable fats of comparable free fatty acid content. Oils subjected to extended use will have increased free fatty acid contents resulting in a lowering of the smoke, flash, and fire points. Accordingly, used oil freshened with new oil will show increased smoke, flash, and fire points. For additional details see Bailey’s Industrial Oil and Fat Products.1
It is important to note that all oils will burn if overheated. This is why most household fat and oil products for cooking carry a warning statement on their labels about potential fire hazards. Accordingly, careful attention must be given to all frying operations. When heating fat, do not leave the pan unattended. The continuous generation of smoke from a frying pan or deep fryer is a good indication that the fat is being overheated and could ignite if high heating continues. If smoke is observed during a frying operation, the heat should be reduced. If, however, the contents of the frying pan ignite, extinguish the fire by covering the pan immediately with a lid or by spraying it only with an appropriate fire extinguisher. Do not attempt to remove a burning pan of oil from the stove. Allow the covered frying container to cool. Under no circumstances should burning fat be dumped into a kitchen sink or sprayed with water.
Furthermore, if a consumer wishes to save the fat or oil after cooking, the hot fat or oil should never be poured back into its original container. Most containers for cooking oils are not designed to withstand the high temperatures reached by the oil during cooking. Pouring hot oil into such containers could result in breakage or melting of the container and possible burns to the user.
*The values in this table represent typical smoke, flash and fire points for commercially available edible fats and oils. The values are based on a single test for each fat and oil source, thus they do not represent a statistically valid mean or indicate the range of values attributable to each of the source oils. Smoke, flash and fire points may vary within a source oil due to such factors as processing techniques and/or seasonal variations. In addition, there can be analyst subjectivity when using this test procedure (i.e. AOCS Cc 9a-48 method, Cleveland Open Cup). Therefore, to the extent practicable, ISEO recommends that individual companies conduct independent testing that accounts for such variability within source fats and oils unique to their business practices. Further, to the extent any company chooses to rely upon the accompanying data, ISEO strongly urges the employment of a prudent margin of safety below the ISEO test based smoke, flash, and fire points. Commercial samples were tested after deodorization and had a free fatty acid content of 0.05% or less.
1 Hui, Y.H., ed. Bailey’s Industrial Oil and Fat Products, vol. 2, Edible Oil and Fat Products: Oils and Oilseeds, 5th ed., New York, John Wiley & Sons, Inc., p. 214, 1996.