Edible fats and oils, alone or when in foods, face the threat of oxidation — an irreversible, adverse chemical reaction — from the moment they are exposed to the atmosphere. Antioxidants, by slowing oxidative degradation, give fats a chance to not bear the blame for foods’ demise.
“Guarding against oxidation is a constant battle and frequently requires protective ingredients (antioxidants) to be added directly into food products,” says Jeff Hunsicker, global vitamin E product manager, Cognis Nutrition & Health, LaGrange, IL. The harm of oxygenLipid oxidation occurs when atmospheric oxygen combines with unsaturated fatty acids (those with one or more double bonds), producing peroxides that further break down into aldehydes, ketones, acids and alcohols. These compounds can have a negative effect on food’s appearance, odor and taste. This is described as rancidity. Rancidity manifests itself in an array of ways, including making a food taste fishy, “painty” or cardboard-like. It can dull the color of food and change its nutritional composition. In meat products, it can result in a condition referred to as warmed-over flavor (WOF). WOF is a rancidity phenomenon that occurs in cooked refrigerated and frozen meats. It can arise in either the four-minute, microwave-ready packaged beef tips in gravy or Sunday’s leftover pork roast. WOF develops during the very early stages of lipid oxidation and is very challenging to control. Most fats contain a mixture of fatty acids, so fats are categorized according to their predominant fatty acids, i.e., saturated fats have high levels of saturated fatty acids. However, even saturated fats contain some unsaturated fatty acids, which are susceptible to oxidation. In addition, prepared foods described as low in fat, or even fat-free, most likely contain small amounts of fatty acids, some of which are unsaturated. Thus, food formulators are rarely off the hook when it comes to preventing fat oxidation. A small amount of oxidized fat, and subsequent development of rancidity, can ruin a food. For example, skim milk — which is virtually free of fat — contains trace amounts of phospholipids that are rich in polyunsaturated fatty acids. When these polyunsaturated fatty acids oxidize, the resulting off-flavors can make a consumer throw out a gallon of milk days before its expiration date. “In addition to reducing the quality and nutritional value of foods, some end products of lipid oxidation — such as peroxides, aldehydes and ketones — have been shown to be harmful to human health,” says Stephen Byrd, technical service technologist, Eastman Chemical Co., Kingsport, TN. Antioxidants, which block formation of free radicals by donating electrons or hydrogen ions to halt the oxidative process, can slow oxidation and, hence, rancidity development. Thus, antioxidants lengthen the shelf life of foods. In general, the more double bonds, or the higher the degree of unsaturation, the greater the probability that the fatty acid will be oxidized if left unprotected. However, Byrd says it is important to note that “rancidity can also be caused by enzymatic changes in fats and oils, which differs from oxidative rancidity and is not prevented by the use of antioxidants.” Several factors catalyze oxidation; thus, controlling these variables also slows oxidation. For example, heat, light, trace metals (e.g., copper, nickel, iron and other pro-oxidant metals), various pigments, alkalinity and degree of unsaturation all can affect oxidation rate. Oxygen availability also affects oxidation, with more available oxygen increasing susceptibility to the reaction. Metal barsThe addition of sequestering and/or chelating agents — such as citric acid; ethylenediaminetetraacetic acid (EDTA), which occurs as calcium disodium EDTA and disodium dihydrogen EDTA; and phosphates — can control trace metals. These ingredients inhibit metals from initiating undesirable lipid oxidative reactions by forming strong complexes with the metal ions. The use of citric acid is well-established in meat products with high iron contents. USDA allows manufacturers to add citric acid to meat products at levels up to 100 ppm based on final product weight. EDTA is effective in blocking copper and iron from catalyzing oxidative chain reactions. It is FDA-approved for use in various vegetable-oil-based products, such as margarine, mayonnaise and salad dressing, as well as a variety of canned vegetables and fish products. Average approved use levels range from 75 to 300 ppm.
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