The enzymatic degradation of l -methionine and subsequent formation of volatile sulfur compounds (VSCs) is believed to be essential for flavor development in cheese. l -Methionine-γ-lyase (MGL) can convert l -methionine to methanethiol (MTL), α-ketobutyrate, and ammonia. The mgl gene encoding MGL was cloned from the type strain Brevibacterium linens ATCC 9175 known to produce copious amounts of MTL and related VSCs. The disruption of the mgl gene, achieved in strain ATCC 9175, resulted in a 62% decrease in thiol-producing activity and a 97% decrease in total VSC production in the knockout strain. Our work shows that l -methionine degradation via γ-elimination is a key step in the formation of VSCs in B. linens .

F. AMARITA, T. REQUENA, G. TABORDA, L. AMIGO AND C. PELAEZ. 2001. Aims: To study the ability of Lactobacillus casei and Lact. plantarum strains to convert methonine to cheese flavour compounds. Methods and Results: Strains were assayed for methionine aminotransferase and lyase activities, and amino acid decarboxylase activity. About 25% of the strains assayed showed methionine aminotransferase activity. The presence of glucose in the reaction mixture increased conversion of methionine to 4‐methylthio‐2‐ketobutanoate (KMBA) and 4‐methylthio‐2‐hydroxybutanoate (HMBA) in all strains. The methionine aminotransferase activity in Lact. plantarum and Lact. casei showed variable specificity for the amino group acceptors glyoxylate, ketoglutarate, oxaloacetate and pyruvate. None of the strains showed methionine lyase or glutamate and methionine decarboxylase activities. Conclusions: The presence of amino acid converting enzymes in lactobacilli is strain specific. Significance and Impact of the Study: The findings of this work suggest that lactobacilli can be used as adjuncts for flavour formation in cheese manufacture.

Lactic acid bacteria were screened for methional production from 4-methylthio-2-ketobutanoate. Only Lactococcus lactis IFPL730 produced high amounts