Autoxidation lipid influence

Interestingly, early examples of carotenoid autoxidation in the literature described the influence of lipids and other antioxidants on the autoxidation of carotenoids." " In a stndy by Budowski et al.," the influence of fat was fonnd to be prooxidant. The oxidation of carotenoids was probably not only cansed by molecnlar oxygen bnt also by lipid oxidation products. This now well-known phenomenon called co-oxidation has been stndied in lipid solntions, in aqueons solntions catalyzed by enzymes," and even in food systems in relation to carotenoid oxida-tion." The inflnence of a-tocopherol on the antoxidation of carotenoids was also stndied by Takahashi et al. ° who showed that carotene oxidation was snppressed as... 

Food lipids possess an inherent stability to oxidation, which is influenced by the presence of antioxidants and pro-oxidants. After a period of relative stability (induction period), lipid oxidation becomes autocatalytic and rancidity develops. Thus, the typical time-course of autoxidation, as measured by the concentration of hydroperoxides, consists of a lag phase (induction) followed by the rapid accumulation of hydroperoxides, which reaches a maximum and then decreases as hydroperoxide decomposition reactions become more important. The longer the induction period, the more stable the food to oxidation (Lundberg, 1962). 

Presence and concentration of hemoproteins and free iron in meats from different species may also influence the rate of lipid autoxidation/degradation during the cooking and subsequent storage periods (13). Thus, development of off-flavors and unpleasant odors referred to as "warmed-over flavor" (2) depends primarily on the degree of unsaturation of lipid components of meats and somewhat on the level of iron-porphyrin materials present in the muscle. 

Scheme 1. Possible reactions of the autoxidation process. R is an alkyl group of an unsaturated lipid molecule. H is an a-methylenic hydrogen atom easily detachable because of the activating influence of the neighboring double bond or bonds. RO is alkoxy radical, ROO" is peroxy radical, and is an initiator.

Foods of animal origin are suspected to contain some amount of COP formed by autoxidation. Cholesterol autoxidation is a well-established free radical process that involves the same chemistry that occurs for the oxidation of unsaturated lipids. Cholesterol contains one double bond at the carbon-5 position therefore, the weakest points in its structure are at the carbon-7 and carbon-4 positions. However, due to the possible influence of the hydroxyl group at carbon-3 and the tertiary carbon atom at carbon-5, the carbon-1 position is rarely attacked by molecular oxygen, and therefore the abstraction of an allylic hydrogen predominantly occurs at carbon-7 and gives rise to a series of A and B ring oxidation products. In the chain reaction,... 

Oxidation of unsaturated acyl chains of lipids is a major route to volatile compounds during cooking of fat-containing food of either animal or vegetable origin. The unsaturated fatty acids, readily susceptible to the attack by oxygen, form hydroperoxides which in themselves are odorless and tasteless. The compounds that influence the flavor of the product result from a further breakdown of these hydroperoxides, and, normally, include saturated and unsaturated aldehydes, alcohols, and ketones. The carbonyl compounds resulting from autoxidation impart specific flavors that are normally detrimental to food products (Table 9.3). It should be pointed out, however, that they may also contribute to the desirable characteristic flavor of foods [48]. 

The carotenoid activity during oxidation is strongly influenced by the oxygen pressure (PO2) of the experimental conditions. Kiokias and Oreopoulou have shown that certain natural carotenoid mixtures (paprika, bixin and tomato, and palm-oil preparations) inhibited the azo-initiated oxidation of sunflower oil-in-water emulsions (operated rapidly under low pOj) in terms of both primary and secondary oxidation products. However, other studies " concluded that carotenoids not only did not inhibit aerial lipid autoxidation (high PO2) but even exerted a prooxidant character, a phenomenon also observed at high carotenoid concentrations that could be due mainly to a more increased formation of carotene-peroxyl radicals, promoting the propagation of autoxidation. 

Semwal, A.D. Sharma, G.K. Arya, S.S.Factors influencing lipid autoxidation in dehydrated precooked rice and Bengalgram dhal. J. Food Sci. Technol 1994, 31, 293—297. 

The autoxidation of acyl lipids is also influenced by the moisture content of food. The reaction rate is high for both dehydrated and water-containing food, but is minimal at a water activity (aw) of 0.3 (Fig. 0.4). The following hypotheses are discussed to explain these differences The high reaction rate in dehydrated food is due to metal... 

The methods used for determination of fat or oil in food are often based on extraction with either ethyl ether or petroleum ether and gravimetric determination of the extraction residue. These methods may provide unreliable or incorrect results, particularly with food of animal origin. As shown in Table 14.20, where a corned beef sample was analyzed, the amount and composition of fatty acids in the fat residue were influenced greatly by the analytical methods used. In addition to the accessible free lipids, the emulsifiers present and the changes induced by autoxidation affect the amount of extractable lipids and the lipid-to-nonlipid ratio in the residue. The use of a standard method still does not eliminate the disadvantages shown by analytical methods of fat analysis. Therefore, in questionable cases, quantitative determination of fatty acids and/or glycerol is recommended. 

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