Naturally occurring antioxidants include vitamins E and C, lecithin, flavonoids, and gum guaiac. Many vegetable oils naturally contain vitamin E. Commercial antioxidants permitted by the FDA include butylated hydroxyanisole (BHA), butylated hydroxytoluene (BHT), propyl gallate, and tertiary butyl hydroquinone (TBHQ).

In the past, cereal manufacturers incorporated predominantly saturated fatty acids such as coconut and palm oils into their products to reduce the risk of rancidity. Public concern over saturated fat and its relationship to blood cholesterol levels has contributed to increasing use of unsaturated oils and new ways of deterring rancidity. One method of inhibiting rancidity consists of packing such items as potato chips, tortilla chips, and other foods high in unsaturated fatty acids in vacuum packs or nitrogen to prevent contact with oxygen.

To prevent rancidity of the oils and fats themselves, there are several protective measures that can be taken. Vegetable oil bottles should be recapped immediately after use to minimize exposure to oxygen. They are best kept in cool, dry places away from air, light, high temperatures, and exposure to metals such as iron and copper. In warmer climates, they fare better in the refrigerator. Storing a bottle of oil on the shelf near the range, where heat is constantly being generated, is not recommended.

There are two basic types of rancidity: hydrolytic rancidity, which occurs when water breaks larger compounds into smaller ones; and oxidative rancidity, in which the double bond of an unsaturated fatty acid reacts chemically with oxygen to result in two or more shorter molecules.

Hydrolytic Rancidity

Fats become rancid through the addition of water because water hydrolyzes the bonds in the triglyceride, causing it to break down into smaller compounds. Catalyzing this reaction are lipase enzymes and heat. This hydrolytic rancidity has implications for deep-fat frying, because placing cold, wet food in heated frying oil introduces water and makes the oil prone to hydrolytic rancidity. Conversely, fats that have not been heated are more prone to hydrolytic rancidity because the lipase enzymes have not yet been destroyed by heat. Room temperature is ideal for the lipase enzyme, which is why butter left out at room temperature quickly decomposes. Butter’s volatile short-chain fatty acids, such as butyric and caproic acids, create a rancid odor and off-flavor when released into the air. The long-chain fatty acids are also freed, but they are not volatile and therefore do not contribute to the odor of rancid butter.

Oxidative Rancidity

Fats can also become rancid when they are exposed to the oxygen in the air. The higher the degree of unsaturation, the more likely it is that the fat will be subject to oxidative rancidity. This is why saturated and hydrogenated fats used to be popular with some food manufacturers and food service establishments.

Unlike hydrolytic rancidity, the rancidity due to oxygen occurs in a series of steps. The initiation period is slow and is triggered by light, high temperatures, table salt, food particles in the frying oil, and certain metals such as iron, copper, and nickel. This initial stage is followed by a quicker, irreversible, and self-perpetuating chain reaction. Oxygen atoms attach to the carbons next to the double bond of the fatty acid, creating very reactive and unstable molecules called free radicals. These free radicals contribute to the further breakdown of fats into smaller compounds, resulting in unpleasant odors and off-flavors. Once this process starts, it is difficult to stop, because the free radicals generated by the reaction create more free radicals, which in turn keep producing free radicals until all the double bonds have been used in the process. Antioxidants, found naturally in the fat or commercially added, inhibit oxidative rancidity and extend shelf life.

Although saturated fats resist rancidity better than unsaturated fats, it is best to protect both from this inevitable deterioration process. Fats such as butter and margarine are best stored in the refrigerator. Butter will keep for months in the freezer, but margarines do not freeze as well because their emulsions may separate. Shortenings and most oils should be kept tightly covered in a dark spot on the cupboard shelf; however, they will keep longer if refrigerated. Olive oil has a shorter shelf life than most vegetable oils and should be refrigerated fairly soon after opening.

Rancidity of Fats

Rancidity is the chemical deterioration of fats, which occurs when the triglyceride molecule and/or the fatty acids attached to the glycerol molecule are broken down into smaller units that yield off-flavors and odors. The longer a fat is stored, the greater the possibility of its becoming rancid. Fats and oils used in cooking can also become rancid as they are exposed to oxygen, heat, or light. For this reason, they should be checked frequently for rancidity, since using a rancid fat or oil to make cakes, cookies, or other baked goods will adversely affect their flavor. Rancid fat will also ruin the flavor of sauteed or fried foods and cause problems during heating because of its lower smoke point.