Origin and formation of 2-nonenal in heated beer.

Stenroos, L., Wang, P., Siebert, K. and Meilgaard, M.

Abstract
Beer produces large amounts of 2-nonenal on heating at 100 degrees C and pH 2, and the same is true of a suspension of barley and linoleic acid. Three isomeric trihydroxy octadecenoic acids were identified by Esterbauer et al. in the latter system and were shown to be the major source of nonenal production in both barley and beer. Two of the compounds were identified as Gravelands compounds G1 and G2 while the third is a new compound which has the three hydroxy groups on adjacent carbon atoms. In pilot brewing, addition of linoleic acid to the mash greatly increased the amounts of pH 2 nonenal precursors in the resulting beer. Brewing under nitrogen, treatment of the wort with active carbon, and combinations of these treatments caused a reduction in the concentration of nonenal precursors, but this did not result in any improvement in flavour stability. It was shown that the trihydroxy acids, although they produce large amounts of 2-nonenal when boiled at pH 2 are too stable to break down in finished beer at its natural pH.
Keywords: barley beer carbonyl compound fatty acid heat off-flavour 2-nonenal