Key enzymes involved in methionine catabolism by cheese lactic acid bacteria

Cheese microbiota and their enzymatic conversion of l-methionine to volatile sulphur compounds (VSCs) play an important role in aroma formation during cheese ripening. Here, lactic acid bacteria (LAB) strains isolated from raw goats' milk cheeses were screened for the major enzymes critical to the formation of VSCs from l-methionine. A large natural biodiversity in enzyme capabilities and high inter- and intra-species variability was found among the LAB isolates investigated. From those isolates tested, lactococci displayed higher C–S lyase specificities towards the sulphur-containing compounds examined than did Lactobacillus and Leuconostoc, in some cases generating higher levels of VSCs than B. linens, known to be an efficient producer of methanethiol (MTL) and related VSCs. Moreover, these differences in C–S lyase activities (determined spectrophotometrically by measuring the formation of free thiol groups) were shown to correspond with the enzymatic potential of the isolates as determined by visualization of enzymatic activities. This technique could therefore prove valuable for the detection and preliminary characterization of C–S lyase activities among LAB isolates. Lactococci were also found to possess higher aminotransferase activities than lactobacilli and leuconostocs, while glutamate dehydrogenase activities were observed to be highest among Leuconostoc and Lactobacillus spp. Meanwhile, α-keto acid decarboxylase activities were highly variable and were measurable in only a limited number of isolates, mainly lactobacilli. From these data, combining indigenous isolates showing high VSCs-producing capabilities with those that facilitate the completion of the metabolic pathway responsible for degrading l-methionine into volatile compounds may provide an efficient approach to enhance cheese aroma development.