Rancidification
Rancidification is the chemical decomposition of fats, oils and other lipids. There are three basic types of rancidity. Hydrolytic rancidity occurs when water splits fatty acid chains away from the glycerol backbone in glycerides. Oxidative rancidity occurs when the double bonds of an unsaturated fatty acid react chemically with oxygen. Microbial rancidity refers to a process in which micro-organisms such as bacteria use their enzymes, including lipases, to break down chemical structures in the fat. In each case, these chemical reactions result in undesirable odors and flavors.
Free fatty acids produced by hydrolysis can undergo auto-oxidation. Oxidation primarily occurs with unsaturated fats by free radical-mediated processes. These chemical processes can generate highly reactive molecules in rancid foods and oils which are responsible for producing unpleasant and noxious odors and flavors. These chemical processes may also destroy nutrients in the food. Under some conditions, rancidity and the destruction of vitamins occurs very quickly.
Antioxidants are often added to fat-containing foods in order to delay the onset or slow the development of rancidity due to oxidation. Natural antioxidants include flavonoids, polyphenols, ascorbic acid (vitamin C) and tocopherols (vitamin E). Synthetic antioxidants include butylated hydroxyanisole (BHA), butylated hydroxytoluene (BHT), propyl gallate and ethoxyquin. The natural antioxidants tend to be short-lived, so synthetic antioxidants are used when a longer shelf-life is preferred. The effectiveness of water-soluble antioxidants is limited in preventing direct oxidation within fats, but is valuable in intercepting free-radicals that travel through the watery parts of foods. A combination of water-soluble and fat-soluble antioxidants is ideal, usually in the ratio of fat to water.
In addition, rancidification can be decreased, but not completely eliminated, by storing fats and oils in a cool, dark place with little exposure to oxygen or free-radicals, since heat and light accelerate the rate of reaction of fats with oxygen. The addition of antimicrobial agents can also delay or prevent rancidification due to the growth of bacteria or other micro-organisms.
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