Monohydroperoxide formation

The now classic Farmer-type hydrogen-abstraction Initiation of free radical autoxldatlon accounts for a large portion of the nonenzymlc oxidations of n-3 fatty acids (45). Because fish lipids contain substantial concentrations of EPA and DHA (47-48), they provide many allowed sites (18, 22, 45, 46, 49) of hydroperoxide formations, and thus can account for a large array of decomposition products. Oxidizing model systems of unsaturated methyl esters of fatty acids yielded monohydroperoxides, but also produce dlhydroperoxldes that are formed by cycllzatlon of Intermediate hydroperoxy radicals when suitable H-donatlng antioxidants are not present to quench the free radical reaction (45, 50, 51). Decomposition of monohydroperoxides of fatty acids In model systems yields a very different profile of lower molecular weight products than observed for similar decompositions of dlhydroperoxldes of the same fatty acids (45, 46). 

Autoxidation of linolenic acid yields four monohydroperoxides (Table 3.28). Formation of the monohydroperoxides is easily achieved by H-abstraction from the bis-allylic groups in positions 11 and 14. The resultant two pentadiene radicals then stabilize analogously to linoleic... 

For this reason the 10- and 12-peroxy radicals obtained from linoleic acid readily form hydroperoxy-epidioxides. While such radicals are only minor products in autoxidation, in photooxidation they are generated as intermediary products in yields similar to the 9- and 13-peroxy radicals, which do not cyclize. Ring formation by 10- and 12-peroxy radicals decreases formation of the corresponding monohydroperoxides (Table 3.28 reaction with 02). 

Peroxy radicals interact rapidly with antioxidants which may be present to give monohydroperoxides (cf. 3.7.3.1). Thus, it is not only the chain reaction which is inhibited by antioxidants, but also P-fragmentation and peroxy radical cyclization. Fragmentation occurs when a hydroperoxidee-pidioxide is heated, resulting in formation of aldehydes and aldehydic acids. For example. 

Z)-4-heptenal), which occurs in beef and mutton and often in butter (odor threshold in Table 3.32). Also, the processing of oil and fat can provide an altered fatty acid profile. These can then provide new precursors for a new set of carbonyls. For example, (E)-6-nonenal, the precursor of which is octadeca-(Z,E)-9,15-dienoic acid, is a product of the partial hydrogenation of linolenic acid. This aldehyde can be formed during storage of partially hardened soya and linseed oils. The aldehyde, together with other compounds, is responsible for an off-flavor denoted as hardened flavor . Several reaction mechanisms have been suggested to explain the formation of volatile carbonyl compounds. The most probable mechanism is the P-scission of monohydroperoxides with formation of an intermediary short-lived alkoxy radical (Fig. 3.26). Such P-scission is catalyzed by heavy metal ions or heme(in) compounds (cf. 3.7.2. L7). 

However, in the water-free fat or oil phase of food, the homolytic cleavage of hydroperoxides presented above is the predominant reaction mechanism. Since option A of the cleavage reaction is excluded (Fig. 3.26), some other reactions should be assumed to occur to account for formation of hexanal and other aldehydes from hnoleic acid. The further oxidation reactions of monohydroperoxides and carbonyl compounds are among the possibilities. 

The occurrence of 2,4-heptadienal (from the 12-hydroperoxide isomer) and of 2,4,7-decatrienal (from the 9-hydroperoxide isomer) as oxidation products is, thereby, readily explained by accepting the fragmentation mechanism outlined above (option B in Fig. 3.26) for the autoxidation of a-linolenic acid. The formation of other volatile carbonyls can then follow by autoxidation of these two aldehydes or from the further oxidation of labile monohydroperoxides. 

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