Formation of methanethiol by bacteria isolated from raw milk and Cheddar cheese

Abstract

SummaryStrains of coryneform bacteria which produce methanethiol from L-methionine in laboratory culture have been isolated from Cheddar cheese and raw milk Many Gram-negative raw milk isolates includingPseudomonas fluorescens, Ps. putida, Ps. fragi, Ps. aureofaciens, Acinetobacter, AeromonasandSerratia marcescensalso produced methanethiol. Representative strains of coryneforms and pseudomonads produced methanethiol only from free L-methionine; glycyl-L-methionine and D-methionine were not degraded. The aerobic nature of the methanethiolproducing bacteria and the instability and alkaline pH-dependence of their thiol producing enzymes explained their failure to increase methanethiol concentrations in experimental cheese, even when large numbers (5 × 107/g milled curd) of pregrown cells were added. Other commonly occurring cheese bacteria such as starter streptococci, micrococci or lactobacilli either did not produce methanethiol or produced only traces. It is concluded that the methanethiol normally present in Cheddar cheese is unlikely to be the product of bacteria or their enzymes.