Monohydroperoxide linolenic acid

The hypothesis presented in Fig. 3.19 is valid only for the initiation phase of autoxidation. The process becomes less and less clear with increasing reaction time since, in addition to hydroperoxides, secondary products appear that partially au-toxidize into tertiary products. The stage at which the process starts to become difficult to survey depends on the stability of the primary products. It is instructive here to compare the difference in the stractures of monohydroperoxides derived from linoleic and linolenic acids. 

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... 

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). 

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. 

Monohydroperoxides are the primary products of lipid oxidation. A variety of hydroperoxides with positional and geometrical isomers are formed depending on the position and number of double bonds of the unsaturated fatty acids and the oxidation mechanism. A number of reviews have been published on the composition of isomeric hydroperoxides formed from oxidation of oleate, linoleate, and linolenate (286, 287-291). The hydroperoxides formed are odorless, but they are relatively unstable and are the precursors of a variety of volatile and nonvolatile scission products that are important to the oxidized flavor. 

The hydroperoxy epidioxides formed from photosensitized oxidized methyl linoleate are important precursors of volatile compounds, which are similar to those formed from the corresponding monohydroperoxides. Thus, 13-hydroperoxy-10,12-epidioxy-tra 5 -8-enoic acid produces hexanal and methyl lO-oxo-8-decenoate as major volatiles (Figure 4.24). The 9-hydroperoxy-10,12-epidioxy-rrans-13-enoic acid produces 2-heptenal and methyl 9-oxononanoate. Other minor volatile products include two volatiles common to those formed from the monohydroperoxides, pentane and methyl octanoate, and two that are unique, 2-heptanone and 3-octene-2-one. The hydroperoxy epidioxides formed from autoxidized methyl linolenate produce the volatiles expected from the cleavage reactions of linolenate hydroperoxides, and significant amounts of the unique compound 3,5-octadiene-2-one. This vinyl ketone has a low threshold value or minimum detectable level, and may contribute to the flavor impact of fats containing oxidized linolenate (Chapter 5). 

As observed with the monohydroperoxides (Section D), thermal decomposition of the hydroperoxy bicyclo-endoperoxides from methyl linolenate produced more complex mixtures of volatile compounds than acid decomposition with acidic methanol. The thermal decomposition products included methyl 9-oxononanoate, propanal, 2,4-heptadienal, methyl octanoate, methyl 13-0X0-9,11-tridecadienoate and ethane (Figure 4.25X The acid decomposition products, analysed as the di- and tetramethyl acetals, comprised only propanal, methyl 9-oxononaoate, and malonaldehyde. As with the monohydroperoxides, by thermal decomposition the bicyclo-endoperoxides are cleaved on either side of the hydroperoxide group, whereas by acid decomposition they are cleaved only between the hydroperoxide group and the... 

Figure 6.12. Analytical normal phase HPLC separation of monohydroperoxide isomers of autoxidized triUnolenin, as described in Figure 6.10. Ln, Unolenic acid residue etc 12(13)OOH, cis,trans,cis linolenic 12+13-hydroperoxides cct 16-OOH, cis,cis,trans linolenic 16-hydroperoxide tcc 9-OOH, trans,cis,cis linolenic 9-hydroperoxide. From Frankel et al. (1990). Comtesy of the American Oil Chemists Society.

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