Hexanal linoleic acid, autoxidation

Volatile decomposition products from autoxidized linoleic acid and methyl linoleate were characterized for their intense aroma and flavor impact by capillary gas chromatography-olfactometry. This technique involves sniffing the gas chromatograph effluent after stepwise dilution of the volatile extract. The most intense volatiles included hexanal, c/ -2-octenal, /ra. s-2-nonenal, l-octene-3-one, 3-octene-2-one and trans-l-ociQmX (Table 4.2). This analytical approach does not, however, consider the effects of complex interactions of volatiles occurring in mixtures produced in oxidized food lipids. 

Odor-Active Monocarbonyl Compounds. Model expriments showed that the volatile fractions formed during the autoxidation of oleic, linoleic and linolenic acid contain mainly aldehydes and ketones (Table 3.31). Linoleic acid, a component of all lipids sensitive to autoxidation, is a precursor of hexanal that is predominant in... 

Also the delayed appearance of hexanal during the storage of linoleic acid containing fats and oils compared to pentane and 2,4-decadienal, supports the hypothesis that hexanal is not directly formed by a P-scission of the 13-hydroperoxide. It is mainly produced in a tertiary reaction, e. g., during the autoxidation of 2,4-decadienal. 

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). 

F g- 4.13. Chromatograms of autoxidized fatty acid methyl esters (FAME) kept in glass vials, exposed to O2 and ambient light for 8 d for (A) methyl linoleate, (B) c9,f1l-18 2, and (C) tl0,cl2-18 2. Chromatogram A indicates labeled peaks for pentanal (C -al), hexanal (Cg-al), f2,f4-decadienal (C Q-dienal), methyl stearate (is), methyl 9-oxo-nonanoate (9-oxo-FAME) and methyl linoleate (c9,cl2-C g.2-FAME). Chromatogram B also has labeled heptanal (C -al), 2-heptenal (C7.-,-al), 2-nonenal (C. -al), methyl 9,12-epoxy-9,11-octadecadienoate (Fg 2)/ chromatogram C has additionally labeled methyl nonanoate (Cg-FAME), 2-octenal (Cg. -al), methyl 10-oxo-decanoate (10-oxo-FAME) and methyl 10,13-epoxy-10,12-octadecadienoate (F q 3). 

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