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.