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-␥-lyase (MGL) can convert Lmethionine to methanethiol (MTL), ␣-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 thiolproducing activity and a 97% decrease in total VSC production in the knockout strain. Our work shows that L-methionine degradation via ␥-elimination is a key step in the formation of VSCs in B. linens.Due to their low detection threshold and diversity, volatile sulfur compounds (VSCs) are of prime importance in the overall flavor of cheese and make a significant contribution to the typical aromas of different cheeses (12,14,33). VSCs arise primarily from the degradation of L-methionine to methanethiol (MTL) by the cheese microflora. This thiol is a common precursor for a variety of other sulfur-bearing compounds including the auto-oxidation products (11), dimethyl disulfide (DMDS), dimethyl trisulfide (DMTS), and S-methylthioesters, primarily arising from chemical reaction of MTL with acyl coenzyme A (acyl-CoA) (22). Numerous studies have therefore been done to control and/or diversify VSC synthesis during the ripening process by the use of properly selected microorganisms (4, 6, 15, 43). Many cheese microorganisms are capable of producing VSCs from L-methionine. Some of them, such as brevibacteria, especially Brevibacterium linens (17), are known to be very good VSC producers while others, such as lactic acid bacteria (LAB), can produce only limited amounts of VSCs (14).The most direct route for MTL biosynthesis, is the L-methionine ␥-elimination that directly produces MTL, ␣-ketobutyrate, and ammonia from L-methionine. This L-methionine ␥-elimination activity is quite high in B. linens and corynebacteria (17) and is also suspected in several other cheese surface bacteria, such as Micrococcus luteus, Arthrobacter sp., and Staphylococcus equorum (8). In contrast, such activity is quite low in LAB (14). In B. linens, the methionine ␥-elimination is catalyzed by a L-methionine-␥-lyase (MGL), a pyridoxal phosphate (PLP)-dependent enzyme for which L-methionine is the best substrate (16). In contrast, in LAB the reaction is catalyzed by a cystathionine ␤-lyase (CBL) and a cystathionine ␥-lyase (CGL) which are only slightly active towards L-methionine (1, 10, 18). In LAB, another pathway for L-methionine conversion to VSCs also exists but produces limited amounts of MTL (7,35).Coryneform bacteria are generally found on the surface of smear cheeses and give the typical sulfur notes to cheeses such as Limburger, Tilsiter, Livarot, Epoisses, and Munster. To date, B. linens is the only food-grade bacterium from which MGL has been purified and characterized (16,26,31,38,39), but neither its protein sequence nor its gene...

Aims: To study the ability of Lactobacillus casei and Lact. plantarum strains to convert methonine to cheese¯avour 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 speci®city 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 speci®c. Signi®cance and Impact of the Study: The ®ndings of this work suggest that lactobacilli can be used as adjuncts for¯avour formation in cheese manufacture.

Lactic acid bacteria were screened for methional production from 4-methylthio-2-ketobutanoate. Only Lactococcus lactis IFPL730 produced high amounts of methional. It was demonstrated that production of this compound was an exclusively enzymatic reaction. The present work describes for the first time that L. lactis can convert enzymatically methionine to methional in a process mediated by aminotransferase and K-ketoacid decarboxylase activities. The activity seems to be strain dependent. ß

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