Autoxidation of Fats and Oils

C. E. Swift, F. G. Dollear.

Products that contain fats and oils turn rancid and deteriorate in other ways when they are exposed to air. The action, known as autoxidation, imparts disagreeable flavors and odors to fats and foods containing them. For a long time baffled investigators spent a vast amount of effort trying to find ways to prevent autoxidation. Specifically, the basic problem was the mechanism of autoxidation how oxygen attacks a fat. On that point the investigators made several discoveries in 1943. They have developed moderately effective methods of preventing rancidification, and believe now that the solution of the problem is in sight.

It is important to prevent autoxidation because fat and fat-containing foods valued at several billion dollars are produced and marketed annually. The oil, meat, fishery, dairy, and bakery industries suffer serious losses from autoxidation of their products.

Rancidification was an important economic and military problem during the First and Second World Wars, although many modern scientific practices reduced the frequency of its occurrence. Measures that were moderately effective in preserving civilian foods availed little for food that had to be transported long distances and stored in unfavorable climates.

In different fats, and under different conditions, autoxidation produces tallowy, painty, burned, fishy, grassy, and other off-flavors and odors; senses of taste and smell can detect autoxidative deterioration quite readily. Fats in the early stages of autoxidation are edible, but unpalatable. Thereafter the degree of unpalatability increases until the odor and flavor become so repulsive that the fat cannot be eaten.

Nutritional tests indicate that autoxidation reduces or destroys the food value of fats. For example, the vitamin E content of vegetable oils and the vitamin A content of fish oils are reduced, if not entirely destroyed, during autoxidation. Furthermore, the fatty acid components of fats that are most essential in the diet are the first to be attacked during autoxidation. Rancid fat in diets adversely affects the body's utilization of vitamins from other sources. Finally, there is some evidence that autoxidized fat may exert a toxic action.

NATURAL ANTIOXIDANTS, substances that tend to prevent autoxidation, are present in small amounts in most vegetable fats. Their importance has been realized for a very long time. For instance, the American Indians added oak-bark extracts to the fat of bears to keep the fat from spoiling. Some natural antioxidants have been isolated recently from the fats in which they occur and their chemical structures have been determined. A number of fats and oils contain the same natural antioxidant; some contain one not yet found in any other fat or oil. Fats and oils also differ very markedly in the amounts of natural antioxidants originally present in them.

The most common antioxidant is vitamin E, or tocopherol. Generally speaking, vegetable fats contain appreciable amounts of tocopherols, while animal fats contain them in very low concentrations. Sesame oil contains a unique antioxidant, called sesamol. Crude cottonseed oil contains a different antioxidant, gossypol.

BESIDES NATURAL antioxidants, fats and oils generally contain phosphatides, complex substances that sometimes act as an antioxidant and sometimes also enhance the activity of other antioxidants present. A substance which has practically no antioxidant activity, but which increases the activity of a true antioxidant, is called a synergist.

Unfortunately, both the antioxidants and synergists are sometimes partly or wholly removed from crude fats and oils during the refining process to which they must be subjected to rid them of objectionable coloring matter and flavorous or odorous constituents. A refined fat or oil may, therefore, require the addition of antioxidants to increase its stability.

When refining is carefully carried out, however, most of the natural antioxidant remains in the refined oil. Under such conditions, there may be no advantage in adding more antioxidant, for it has been found that there is an optimum concentration for these substances in any fat or oil. It is still possible, however, to add synergists to assist the natural antioxidants in protecting the fat. Sometimes, of course, it is advantageous to add both antioxidants and synergists.

The phosphatides and certain other compounds form effective synergist-antioxidant combinations with natural tocopherols. The manner in which synergists act to increase the activity of antioxidants has been investigated, but is not clearly understood. Phosphoric acid, which forms an effective synergist-antioxidant combination with tocopherol, apparently is capable of regenerating tocopherol that has been oxidized. It has been shown that tocopherols in the presence of a synergist are depleted at a retarded rate during autoxidation. One present theory assumes that the antioxidants are continuously regenerated at the expense of the synergists.

The tocopherols, which are only moderately active as compared with certain antioxidants, have some advantages. They impart no color, odor, or flavor to fats. During the autoxidation of fats that contain tocopherols, red substances, known as chroman-5, 6-quinones, are produced, but the color formed is usually not objectionable. Because the tocopherols are not soluble in water and are relatively stable to heat, they generally are carried over into finished food products, as, for example, baked goods, such as piecrust or cakes. Finally, the antioxygenic activity of a tocopherol may be greatly increased when any of a number of synergists is used to increase its activity. These properties indicate that the tocopherols are singularly well adapted to their role as natural antioxidants.

Several effective antioxidants and antioxidant combinations have been suggested for use in artificially stabilizing fats. Only a few are important. The addition of antioxidants to foods is subject to Federal regulations. Their acceptability must be established by toxicological and nutritional tests before their addition is permitted. A long time may elapse between the laboratory discovery and the commercial utilization of an antioxidant.

MANY INVESTIGATIONS of the effect of various antioxidants on fat stability have provided a great deal of information on the properties of antioxidants. Most compounds with appreciable antioxygenic activity belong to the phenols, a group of complex substances including the natural tocopherols--gum guaiac obtained from a tropical tree and nordihydroguaiaretic acid obtained from the creosotebush of the southwestern United States and the synthetic substance known as hydroquinone.

Adding phenolic antioxidants to vegetable fats which already contain natural tocopherols improves the stability of the fat only slightly. On the other hand, addition of the phenolic antioxidants to fats naturally lacking in antioxidants, such as animal fats, improves stability appreciably. Moreover, a relatively large group of synergists can greatly enhance the antioxygenic activity of the phenolic antioxidants. The effective members of this group include citric, ascorbic, gallic,# and phosphoric acids and the phosphatides, especially the commercial lecithins, such as the soybean, peanut, and rapeseed lecithins.

Propyl gallate, a chemical compound that acts synergistically with tocopherols, is particularly effective in vegetable fats. It is one of the few compounds that are both phenolic antioxidants and synergists. Animal fats and fish oils can be effectively stabilized by the addition of phenolic antioxidants, and even more effectively by synergist-antioxidant combinations. The antioxidants approved for use, subject to change and with certain limiting restrictions, are the tocopherols, gum guaiac, nordihydroguaiaretic acid, butylated hydroxyanisole, thiodipropionic acid, thiopropionic acid plus dilaurylthiodipropionate, and propyl gallate, also the lecithins, and citric and phosphoric acids, which are synergists. The addition of natural food substances is permissible, but generally only mildly effective.

There are several other effective antioxidants whose use in food products has not yet been approved. At the Eastern Regional Research Laboratory work has been done to improve the utility of gallic and ascorbic acids as antioxidants by converting them to esters, which are more soluble in fats than the acids themselves. One recently discovered phenolic antioxidant is norconidendrin, which was derived from the waste liquor from pulping western hemlock by workers at the Southern Laboratory. This compound has antioxygenic activity approximately equal to that of nordihydroguaiaretic acid.

IN PRESENT PRACTICE, autoxidation is prevented mainly by methods of handling, processing, and packaging fats and fat-containing foods. Care is taken to minimize exposure to light, heat, and air (oxygen) and contamination with metallic pro-oxidants.. or promoters of autoxidation, such as copper and iron. These precautions call for attention to the design of equipment, to the materials from which the equipment is fabricated, to packaging materials, and to numerous other details. If suitable precautions are observed, many fats, particularly vegetable fats, are sufficiently stable for most uses without added antioxidants.

A large portion of commercially important liquid vegetable oils are hydrogenated, or hardened, to produce plastic fats like shortening and oleomargarine. Although the main objective of hydrogenation is to produce plastic fats, hydrogenated fats are generally many times more stable than the unhardened oils from which they are produced.

Lard and other animal fats are the main ones that require stabilization with antioxidants. Much of the lard marketed today is stabilized with approved antioxidant-synergist combinations selected from those we have mentioned.

An appreciable quantity of fat is marketed in the form of animal-vegetable fat combinations. The animal fat ingredients in such products are stabilized to a marked extent by the natural tocopherols present in the vegetable fats.

THE MECHANISM of autoxidation, or how fats become rancid, has held the interest of chemists for many years.

M. E. Chevreul, a French chemist, discovered a century ago that fats are compounds formed by the reaction of fatty acids and glycerol, or are, as the chemist says, glycerol esters of fatty acids. Scientists also learned that some fatty acids contain carbon-to-carbon double bonds (>C=C<) and are therefore chemically related to other compounds that undergo autoxidation.