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Abstract
The formation of trans-2-nonenal is one of the main causes of beer staling. The main mechanism contributing to the generation of trans-2-nonenal in beer is the enzymatic or nonenzymatic oxidation of lipids and oxidized free fatty acids. This hypothesis has been examined by looking for relations between lipoxygenase activity in malt, nonenal potential in wort, and nonenal values in fermenting wort and beer in relation to the sensory flavor stability of the finished beer. New methods were developed to determine nonenal potential and trans-2-nonenal values. It was concluded that within one experimental series of brews, lipoxygenase activity and nonenal potential were useful to predict susceptibility of beer to staling. Moreover, there was a high degree of correlation between nonenal potential and trans-2-nonenal developed in beer after natural aging. A test in which three oxidation products of linoleic acid, all suspected precursors of trans-2-nonenal in beer, were added to beer, did not give evidence of whether these components were precursors or not. Other aldehydes, besides trans-2-nonenal, were formed that do not appear during natural or forced staling of beer. In fermentation trials with an excess of trans-2-nonenal in wort, rapid reduction of trans-2-nonenal to a nondetectable level was demonstrated. At the same time, 2-nonen-1-ol and 1-nonanol were formed. Different crown corks showed remarkable differences in the diffusion velocity of oxygen through the plastic liner. The choice of a liner with a low diffusion velocity leads to a lower oxygen content and thus better flavor stability of the finished beer during storage. Finally some suggestions are given for the reduction of staling.