Pentane, linoleic acid, autoxidation

The formation of off-flavours in beer has been reviewed [40], Autoxidation of the lipids present in beer produces carbonyl compounds with very low taste thresholds. In particular, linoleic acid is oxidized to trihydroxyoctadecenoic acids (Table 22.7) which break down into 2-/mAz.y-nonenal. This aldehyde and related compounds impart a cardboard flavour to beer at very low concentrations. Other carbonyl are formed from the lipids in beer by irradiation with light including the C9, Cjo, and Cu-alka-2,4-dienals (thresholds 0 5, 0 3 and 0 01 ppb respectively) [40]. The level of diacetyl and pentane-2,3-dione in a range of commercial beers is given in Table 22.11. Quantities in excess of 0 15 ppm impart a buttery flavour more noticeable in lagers than in ales. Bacterial contamination and petite mutants of yeast result in high levels of diacetyl. The sulphur compounds characterized in beer are listed in Table 22.19 with some threshold data. Dimethyl sulphide is the major volatile... 

From the volatile autoxidation products which contain the methyl end of the linoleic acid molecule, the formation of 2,4-decadienal and pentane can be explained by reaction 3.72. 

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. 

Other studies to elucidate the multitude of aldehydes which arise suggest that the decomposition of minor hydroperoxides formed by autoxidation of linoleic acid (cf. Table 3.28) contribute to the profile of aldehydes. This suggestion is supported by pentanal, which originates from the 14-hydroperoxide. 

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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