WORT ENTEROBACTERIA-A REVIEW

Abstract

Of the family Enterobacteriaceae, Citrobacter freundii, Enterobacter aerogenes, Ent. cloacae, Hafnia alvei, Klebsiella aerogenes and Serratia species have been detected in fermenting wort. Escherichia coli and animal parasites have not been isolated. As shown by G.C. ratio, DNA base sequence comparison, numerical taxonomy and phage typing, Obesumbacterium proteus shares the same family and is placed in the genus Hafnia as H. protea, with two subspecies. H. protea survives brewery fermentations better than other members of the family and is therefore common in pitching yeast. However, all wort enterobacteria are sensitive to pH values below 4.4 and to a lesser degree to ethanol concentrations over 2% (w/v). Rates of brewery fermentations may be retarded by enterobacteria and the beer pH elevated. Other wort bacteria isolated, species of Achromobacter, Acinetobacter and Pseudomonas, are sensitive to pH and ethanol, and present in smaller numbers in wort than the enterobacteria. Beer flavour with respect to fusel alcohols and esters, volatile sulphur compounds, carbonyl compounds and volatile phenols is seriously influenced by wort enterobacteria. Isolation and enumeration of the enterobacteria in fermenting wort are conveniently carried out on MacConkey agar medium; H. protea colonies grow in 48–72 h at 30°C but the other enterobacteria produce colonies in 20–30 h.