Decarboxylation of Substituted Cinnamic Acids by Enterobacteria: the Influence on Beer Flavour

Many species within the Enterobacteriaceae decarboxylate the substituted cinnamic acids p -coumaric acid and ferulic acid. The enzyme responsible is principally associated with the 'free-living' genera, Klebsiella, Enterobacter and Hafnia and is absent from Proteus mirabilis, Escherichia coli, Serratia and Salmonella spp. Some strains of Hafnia protea , a common bacterial contaminant of brewers' yeast display decarboxylase activity towards hydroxycin-namic acids. These H. protea strains (members of taxonomic group 1) produce higher concentrations of steam-volatile phenolic compounds when grown in wort than strains lacking the decarboxylase. The addition of ferulic acid or p -coumaric acid to wort prior to the growth of H. protea group 1 strains increases the concentration of steam-volatile phenolic compounds thus implicating this route as a source of phenolic off-flavours in beer.