Types of Rancidity

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.