Autoxidation of unsaturated fatty acids

Autoxidation of unsaturated fatty acids is well known. Modifications of the extraction and separation procedures were made to investigate the possibility that these oxygenated fatty acids (OFAs) arise as artefacts. When exposure to light and air were minimized, no changes were noted in TLC and HPLC. 

The hydroperoxides formed in the autoxidation of unsaturated fatty acids are unstable and readily decompose. The main products of hydroperoxide decomposition are saturated and unsaturated aldehydes. The mechanism suggested for the formation of aldehydes involves cleavage of the isomeric hydroperoxide (I) to the alkoxyl radical (II), which undergoes carbon-to-carbon fission to form the aldehyde (III) (Frankel et al. 1961). 

Badings, H. T. 1970. Cold-storage defects in butter and their relation to the autoxidation of unsaturated fatty acids. Ph.D thesis, Vageningen, The Netherlands. 

Milk is characterized as having a pleasing, slightly sweet taste with no unpleasant after-taste (Bassette et al., 1986). However, its bland taste makes it susceptible to a variety of flavor defects. Autoxidation of unsaturated fatty acids gives rise to unstable hydroperoxides, which decompose to a wide range of carbonyl products, many of which can contribute to off-flavors in dairy products. The principal decomposition products of hydroperoxides are saturated and unsaturated aldehydes (Frankel et al., 1961), with lesser amounts of unsaturated ketones (Stark and Forss, 1962), saturated and unsaturated hydrocarabons (Forss et al., 1961), semialdehydes (Frankel et al., 1961) and saturated and unsaturated alcohols (Hoffman, 1962 Stark and Forss, 1966). 

Some volatile aldehydes formed by autoxidation of unsaturated fatty acids are listed in Table 1. The aromas of aldehydes are generally described as green, painty, metallic, beany, and rancid, and they are often responsible for the undesirable flavors in fats and oils. Hexanal has long been used as an index of oxidative deterioration in foods. Some aldehydes, particularly the unsaturated aldehydes, are very potent flavor compounds. Table 2 fists aroma characteristics of some common aldehydes found in fats and oils (8). 

TABLE 1. Some Volatile Aldehydes Obtained from Autoxidation of Unsaturated Fatty Acids (6). 

Autoxidation of unsaturated fatty acids is initiated by a free radical, which is formed in the oil when an unsaturated fatty acid is exposed to oxygen in the... 

In Colombian green coffee, Boosfeld et al. (1994) and Boosfeld and Vitzthum (1995) elucidated the structure of two pairs of unsaturated aldehydes, (E,E)-2,4- and (2F,4Z)-2,4-nonadienal as well as (E,E)-2,4- and (2 , 4Z)-2,4-decadienal, probably generated via autoxidation of unsaturated fatty acids and contributing particularly to the typical green-coffee odor. The products were identified by mass spectrometry, GC-FTIR and NMR spectroscopy. The authors used particularly mild extraction techniques in order to avoid artifact formation and isomerization of the conjugated double bonds. The sensory impressions of these aldehydes at the sniffing port varied from metallic, fried, and flowery to oily notes. 

The autoxidation of unsaturated fatty acids is a chain process occurring autocatalytically through free radical intermediates. Although autoxidation implies that this reaction occurs spontaneously under mild conditions, it is generally initiated by trace metals and peroxides or hydroperoxides present as ubiquitous impurities in food and biological lipid systems (see Chapter 1). 

C. Relative rates of autoxidation of unsaturated fatty acids... 

Hancock, R. A., Leeves, N. J., Nicks, P. F. (1989). Studies in autoxidation 1. The volatile byproducts resulting from the autoxidation of unsaturated fatty-acid methyl-esters. Progress in Organic Coatings, 17(3), 321-336. 

Also, the monocarbonyl compounds derived from autoxidation of unsaturated fatty acids readily condense with protein-free NH2 groups, forming Schiff bases that can provide brown polymers by repeated aldol condensations (Fig. 3.34). The brown polymers are often N-free since the amino compound can be readily eliminated by hydrolysis. When hydrolysis occurs in the early stages of aldol condensations (after the first or second condensation cf. Fig. 3.34) and the released aldehyde, which has a powerful odor, does not... 

Conjugated unsaturated fatty acids also form at the beginning of hydrogenation and during autoxidation of unsaturated fatty acids. In these processes, even isolated double bonds isomerise. The partially hydrogenated oils, which originally contained linoleic and linolenic acids, in addition to isomers with double bonds at positions C-9, C-12 and C-15, thus also contain other isomers of octadecenoic acids with double bonds in positions C-5 to C-16. 

Terpenic hydrocarbons are stable in the absence of air, but are easily oxidised in air, especially at higher temperatures. Their autoxidation proceeds by similar mechanisms as autoxidation of unsaturated fatty acids and depends greatly on the hydrocarbon structure. The primary autoxidation products are hydroperoxides. In branched hydrocarbons, the hydroperoxyl group mainly occurs in the secondary or tertiary carbon adjacent to the quaternary carbon of the double bond. The final autoxidation products are usually epoxides, alcohols and ketones. The primary site of attack in olefins is the carbon adjacent to the double bond, as in monounsaturated... 

Polyunsaturated fatty acids, such as linoleic acid, a-linolenic acid, arachidonic acid and docosahexaenoic acid have characteristic physiological activities and are essential for human health. However, when these polyunsaturated fatty acids are exposed to oxygen, they are oxidized to form conqjlex oxidation products. Since autoxidation of unsaturated fatty acids decreases their biological availability in foods and produces offensive odors and... 

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