Phenolic acids in beer are important because they can be decarboxyiated to phenols, which usually impart off-flavours. An improved high performance liquid chromatographic system was used to monitor phenolic acids and phenols during the brewing process. Ferulic acid was the most significant phenolic acid found in beers prepared from malted barley. Extraction of ferulic acid from malt involved an enzymatic release mechanism with an optimum temperature about 45°C. Mashing-in at 65°C significantly decreased the release of free ferulic acid into the wort. Wort boiling produced 4-vinyl guaiacol by thermal decarboxylation, in amounts (0.3 mg/L) close to its taste threshold, from worts that contained high contents of free ferulic acid (> 6 mg/L). The capacity of yeasts to decarboxylate phenolic acids (Pof* phenotype) was strong in wild strains of Saccharomycoa and absent in all lager brewing yeast and most ale brewing yeasts. Some top-fermenting strains, especially those used in wheat beer production, possessed a weak decarboxylating activity (i.e. Pof1). During storage of beers there were appreciable temperature-dependent losses of 4-vinyl guaiacol. These results indicated that the production of 4-vinyl guaiacol is amenable to close technological control.
Isoamyl acetate is a natural flavour ester, widely used as a source of banana flavour by the food industry. Fusel alcohols such as amyl alcohol are produced in significant quantities as a waste product, sometimes referred to as "lees oil" or "fusel oil", of the alcohol distilling industry. By manipulation of brewing yeast fermentation conditions, a significant portion of added amyl alcohol was shown to be converted to isoamyl acetate. This was achieved by the addition of L-leucine and amyl alcohol in fermentations carried out by a high ester-producing brewing yeast strain of Saccharomyces cerevisiae and by the use of alkaline fermentation conditions coupled with high gravity media. Mutant strains selected on 5,5,5 trifluoro-DL-leucine produced substantially high levels of isoamyl acetate. The adjustment of fermentation conditions outlined in this paper may act as a stepping stone for the potential use of Saccharomyces cerevisiae and other yeasts to produce high levels of natural flavour esters.
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