Heptadienal. 2.4-, autoxidation

Flavors and aromas commonly associated with seafoods have been intensively investigated in the past forty years ( l-7), but the chemical basis of these flavors has proven elusive and difficult to establish. Oxidized fish oils can be described as painty, rancid or cod-liver-oil like (j ), and certain volatile carbonyls arising from the autoxidation of polyunsaturated fatty acids have emerged as the principal contributors to this type of fish-like aroma ( 3, 5, 9-10). Since oxidized butterfat (9, 11-12) and oxidized soybean and linseed oils (13) also can develop similar painty, fish-like aromas, confusion has arisen over the compounds and processes that lead to fish-like aromas. Some have believed that the aromas of fish simply result from the random autoxidation of the polyunsaturated fatty acids of fish lipids (14-17). This view has often been retained because no single compound appears to exhibit an unmistakable fish aroma. Still, evidence has been developed which indicates that a relatively complex mixture of autoxidatively-derived volatiles, including the 2,4-heptadienals, the 2,4-decadienals, and the 2,4,7-decatrienals together elicit unmistakable, oxidized fish-oil aromas (3, 9, 18). Additionally, reports also suggest that contributions from (Z -4-heptenal may add characteristic notes to the cold-store flavor of certain fish, especially cod (4-5). 

The descriptions of aromas produced in these systems are shown in Table IV. The principal compounds which contribute to the initial fish-like aromas of surimi appear to be the enzymically-derived eight-carbon carbonyls and alcohols in combination with some oxidized fishy aroma undertones that are caused by very low levels of autoxidatively-derived carbonyls, including the 2,4-heptadienals and the 2,4-decadienals (unpublished data). When geranium leaves were macerated before addition to surimi, the six-carbon volatile compounds dominated the overall aroma, and the desirable contributions associated... 

Hydroxycurcumin [l-hydroxy-l,7-bis(4-hydroxy-3-methoxyphenyl)-(6 -6-heptene-3,5-dione] m 84-88 was isolated from the same source, had UV at 372nm in MeOH and had a weaker antioxidant activity than the other curcumins towards the autoxidation of linoleic acid in a water-alcohol system [Masuda et al. Phytochemistry 31 3645 1992]. An optically active form, (l -l-hydroxy-l,7-bis(4-hydroxy-3-methoxyphenyl)-(lE,6E)-l,6-heptadiene-3,5-one, has m 92.0-96.0 , with [a]o +12.2 (c 0.06, EtOH, configuration unknown), M 386.4. Its UV has Imsix nm (e)(MeOH) at 230 (sh), 260 (21000), 283 (18000),... 

With autoxidized methyl linolenate, decatrienal and methyl octanoate are derived from the 9-hydroperoxide 2,4-heptadienal from the 12-hydroperoxide ... 

The main volatile compounds detected from autoxidized trilinolein monohydroperoxides include pentane, hexanal, 2-heptenal and 2,4-decadienal. The volatiles from trilinolenin and monohydroperoxides include propanal, 2,4-heptadienal and 2,4,7-decatrienal. Mixtures of 1 1 trilinolein and trilinolenin autoxidized at low peroxide values (PV 34) show equal contribution of volatiles derived from linoleate and linolenate hydroperoxides. 

Low molecular weight fraction separated by gel permeation chromatc raphy (or size exclusion chromatography) after decomposition of autoxidized methyl linolenate with iron-asctxbate at room temperature (mean molecular weight 150, mixture of methyl heptanoate and octanoate, methyl 9-oxo-nonanoate, 2,4-heptadienal, and 2,4,7-decatrianal). 

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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