Autoxidation, of lipids

POKORNY j (1987) Major factors affecting the autoxidation of lipids , in Chan El W S, Autoxidation of unsaturated lipids , London, Academic Press, 141-206. 

SCHEME 1. Autoxidation of lipids (RH) is afree radical process yielding hydroperoxides as primary products and many other secondary derivatives... 

It has been known for many years that Maillard Reaction products can behave as antioxidants in food systems (13,14), and they have been shown to inhibit warmed-over flavor development in cooked meat which is caused by the autoxidation of lipids, especially phospholipids. There has been a significant amount of research examining the Maillard reaction products and intermediates from model systems which may have antioxidative properties. 

Ketones Aliphatic ketones formed by autoxidation of lipids also contribute to the flavor of oils and food products. For example, Guth and Grosch (13) identified l-octen-3-one as one of the odor-active compounds in reverted soybean oil. This compound was described as metallic and mushroom-like. The reaction pathway for the formation of l-octen-3-one from the linoleate-10-hydroperoxide via the p-scission route is illustrated in Figure 2. 10-Hydroperoxide of linoleate is not the usual hydroperoxide formed by autoxidation of linoleate however, it is one of the major hydroperoxides formed by the photosensitized oxidation (singlet oxygen reaction) of linoleate (14). 

Pokorny, J. 1987. Major Factors Affecting the Autoxidation of Lipids, in Autoxidation of Unsaturated Lipids. Chan, H.W.-S. (ed.) London, UK Academic Press Ltd., pp. 140-233. 

The carbonyl compounds formed due to autoxidation of lipids and in catabolic processes postmortem in muscles, or introduced with wood smoke, can bring about undesirable effects by interacting with a.a. residues. On the other hand, some carbonyl compounds are used intentionally to modify the proteins. Formaldehyde can harden the collagen dope in the manufacturing of sausage casings, protect fodder meals against deamination by the rumen microflora, and bind immobilized enzymes on supports. 

Negative catalysis may also occur, since several compounds are known that inhibit reactions. For example, some cations, notably Cu2+, catalyze the autoxidation of lipids chelating agents like citrate may greatly lower the activity of divalent cations, thereby decreasing the oxidation rate. Inhibition of enzymes is frequently observed. 

Ingold, K.U., Bowry, V.W., Stocker, R., and Walling, C., Autoxidation of lipids and antioxidation by alpha-tocopherol and ubiquinol in homogeneous solution and in aqueous dispersions of lipids unrecognized consequences of lipid particle size as exemplified by oxidation of human low density lipoprotein, Proc. Natl. Acad. Sci. U.S.A. 90 (1), 45-49, 1993. 

Autoxidation is the process in foods and bulk lipids, which leads to rancidity. Rancidity is the spoiled off-flavor obtained by subjective organoleptic appraisal of food. Autoxidation is the oxidative deterioration of unsaturated fatty acids via an autocatalytic process consisting of a free radical mechanism. This indicates that the intermediates are radicals (odd electron species) and that the reaction involves an initiation step and a propagation sequence, which continues until the operation of one or more termination steps. Autoxidation of lipid molecules is briefly described by reactions 1-3. ... 

Pokomy, J. Major factors affecting the autoxidation of lipids. In Autoxidation of Unsaturated Lipids, Chan, H.W.S., Ed., Academic Press London, 1987 pp. 159-206. 

Radicals are continuously produced in milk and dairy products by various mechanisms, such as autoxidation of lipids and other components as a result of processing, light exposure, and enzyme action. 

Because the sample size is usually small, the solvents used to prepare lipid samples can be easily evaporated under a nitrogen stream. However, when it is necessary to use a heated bath to facilitate removal of solvents, the temperature of the bath, in general, should not exceed about 40 C to avoid any accelerated autoxidation of lipids. Moreover, the more volatile lipids may also be lost by evaporation. 

Unsaturated fatty acids or their triacylglycerols, as well as unsaturated aldehydes obtained through autoxidation of lipids (cf. 3.7.2.1.5), can be separated by argentation chromatography . The separation is based on the number, position and configuration of the double bonds present. The separation mechanism involves interaction of the r-electrons of the double bond with Ag" ions, forming a reversible TT-complex of variable stability ... 

Volatile aldehydes and ketones are the most important odour-and taste-active substances. They occur in foods as primary substances, as components of various essential oils and also result from enzymatic and chemical reactions from various precursors as secondary substances. They are often desirable flavour-active components of foods, but in some cases may also carry undesirable odour and taste. Then they serve as indicators of unwanted changes in sensory or nutritional value of foods (such as autoxidation of lipids). 

Lipid components associated with meat fat, especially unsaturated aldehydes, play a significant role in species-characterization flavors. For example, ( ,Z)-2,4-decadienal exhibits the character impact of chicken fat and freshly boiled chicken (66). ( , )-2,6-Nonadienal has been suggested as the component responsible for the tallowy flavor in beef and mutton fat (63). 12-Methyltridecanal was identified as a species-specific odorant of stewed beef and provides a tallowy, beeflike flavor character (67). Aldehydes provide desirable flavor character to cooked meat, but they can contribute rancid and warmed-over flavors at high concentrations, resulting from autoxidation of lipids (68). 

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