Nonenal, -3-, linoleic acid, autoxidation

Z)-2-Nonenal is also formed during autoxidation of linoleic acid. 

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... 

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. 

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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