In our daily diets, carbohydrates, fats, proteins and water are the main components in our food. However, if the food is left untreated, oxidative deterioration of the flavor and odor of fats and fatty constituents in it will be resulted. Hence, antioxidant are used widely in the production of food which fats and oils are used as raw materials and in the marketing of foods containing fats under modern conditions.
The major factor in quality degradation of fats and fatty portions of foods is oxidation. In the oxidative deterioration of fats and fat-like substances, off-flavors and off-odors are usually reported. Rather than the breakdown of the unsaturated fat molecules, there are four types of fat deterioration. Hydrolysis is the formation of free fatty acids and glycerol, which will give Â¡Â§soapyÂ¡Â¨ flavor. Nevertheless, triglycerides with fatty acids having shorter chain lengths normally produce off-flavors upon hydrolysis. The autoxidation of unsaturated fatty acids resulting in a mixture of volatile components leading to off-flavors is rancidity. By the oxidation of linolenic-type acids, reversion causes flavor and odor degradation associated with vegetable, fish and other unsaturated oils. Oxidation of fats also results in polymerization that occurs between two fatty acid chains. Oxygen bonding then can be found at the unsaturated site. Although antioxidants are effective in reducing rancidity and polymerization, they do not affect hydrolysis and reversion.
Lipids are hydrocarbons that are found in living systems in the environment. The main classes of lipids are triglycerides, waxes, steroids, phospholipids, gylcolipids, and sphingolipids (Glanze). The simplest lipid, which makes up the backbone of all of these, is the fatty acid (see page 2). The main characteristics that separate the different kinds of lipids are the derivatives, such as acids, ...
Unsaturated and saturated fatty acids can be oxidized by the usual chemical oxidizing agents, for instance nitric acid, ozone and potassium permanganate. However, these are not the concern for the food technologist. Autoxidation (atmospheric oxidation) under the mild processing and storage conditions of the food industry is of utmost importance due to the resultant malodor- and malflavor-producing aldehydes and ketones. The oxidation of the highly unsaturated fats resulting in polymeric end products and the oxidation of the moderately unsaturated fats resulting in rancidity, reversion and other types of off-flavor and off-odors are two general areas studied for the autoxidation of unsaturated lipid substances.
The path of lipid autoxidation and the resulting end products are dependent to the various conditions during the oxidation. The conditions include temperature, catalysts, fatty acids type, the distribution and geometry of double bonds and the amount of oxygen available. The oxidation of fatty substances is autocatalytic and has the characteristics of a Â¡Â§chain reactionÂ¡Â¨. Three stages: initiation, propagation and termination are involved. It is believed that the hydroperoxides generated during propagation give rise to the additional chain-propagating radicals that further increase the reaction. In most cases, the primary products from autoxidation reaction are odorless and tasteless. Secondary direct addition of oxygen may take place at double bonds of acids under higher temperatures or at higher oxidation levels. Various non-volatile oxygen-containing compounds will be formed and these secondary products are generally odorless but may develop taste. When they become ketones or aldehydes, the flavor and odor will be extremely intense.
A number of studies on the inhibition of lipid oxidation reveal several antioxidation mechanisms, which work, in different conditions and types of system. Four possible mechanisms by which an inhibitor may function as a chain stopper for the free radical chain mechanism of lipid oxidation are suggested. They are hydrogen donation and electron donation by the antioxidants, addition of the lipid to the aromatic ring of the antioxidant and the formation of a complex between the lipid and the aromatic ring of the antioxidant.
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Some researchers believe that the electron or hydrogen donation is a primary reaction, and the formation of a loose complex between the antioxidant and the fat chain is a secondary reaction. However, it is also possible that a combination of the various reactions occurs, with the antioxidant itself being completely oxidized or inactivated. The inactive antioxidant radical thus produced must be one that cannot initiate further reaction. This can be shown as the following equation.
RÂ¡E+ AH à RH + AÂ¡E where RÂ¡E is the fat containing a free radical and AH is the antioxidant. Antioxidants useful in commercial food application usually are the phenols under steric hindrance with short side chains.
Metal, hematin compounds, lipoxidases, etc. can catalyze the oxidation reactions. It is believed that these catalysts increase the rate of chain generation to a point where normal concentrations of antioxidants are not effective in stabilizing the free radical. Some reports, in contrast, show that in some occasion, heavy metals tend to reduce the reactivity of the free radicals which would lead to a stabilization of the fat breakdown.
Certain compounds tend to enhance the effectiveness of various antioxidant systems although the opposite may be true under some circumstances. For example, water acts as an inhibitor of the oxidation reaction, but in dehydrated foods, the moisture content has an active bearing on oxidative on oxidative stability. The possible reasons may be that the mono-molecular layer of absorbed water on various food products provides the oxidative stability.
Data do exist that show the effectiveness of amines, amino phenols and phenol-type antioxidants in food fat. However, most amine-type compounds are toxic, can produce colored bodies, and hence not satisfactory as food-grade antioxidants. Phenols that exist naturally or are synthesized artificially are used extensively as food antioxidants. The antioxidants usually are structurally similar that they have the unsaturated aromatic ring with either an amine or hydroxyl group for free radical satisfaction.
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In order to achieve high effectiveness, antioxidants must be used in conjunction with good raw materials, correct processes, and proper packaging and storage conditions. Moreover, a good antioxidant system will impart no flavor, odor, of color to a finished food when used under the proper conditions.