The formation of volatile sulfur compounds (VSC) in fermented food is a subject of interest. Such compounds are essential for the aroma of many food products like cheeses or fermented beverages, in which they can play an attractive or a repulsive role, depending on their identity and their concentration. VSC essentially arise from common sulfur-bearing precursors, methionine being the most commonly found. In the first section of this paper, the main VSC found in cheese, wine, and beer are reviewed. It is shown that a wide variety of VSC has been evidenced in these food products. Because of their low odor threshold and flavor notes, these compounds impart essential sensorial properties to the final product. In the second section of this review, the main (bio)chemical pathways leading to VSC synthesis are presented. Attention is focused on the microbial/enzymatic phenomena-which initiate sulfur bearing precursors degradation-leading to VSC production. Although chemical reactions could also play an important role in this process, this aspect is not fully developed in our review. The main catabolic pathways leading to VSC from the precursor methionine are presented.

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Cheese rinds host a specific microbiota composed of both prokaryotes (such as Actinobacteria, Firmicutes and Proteobacteria) and eukaryotes (primarily yeasts and moulds). By combining modern molecular biology tools with conventional, culture-based techniques, it has now become possible to create a catalogue of the biodiversity that inhabits this special environment. Here, we review the microbial genera detected on the cheese surface and highlight the previously unsuspected importance of non-inoculated microflora--raising the question of the latter's environmental sources and their role in shaping microbial communities. There is now a clear need to revise the current view of the cheese rind ecosystem (i.e. that of a well-defined, perfectly controlled ecosystem). Inclusion of these new findings should enable us to better understand the cheese-making process.

Although a large number of key odorants of Swiss-type cheese result from amino acid catabolism, the amino acid catabolic pathways in the bacteria present in these cheeses are not well known. In this study, we compared the in vitro abilities of Lactobacillus delbrueckii subsp. lactis, Lactobacillus helveticus, and Streptococcus thermophilus to produce aroma compounds from three amino acids, leucine, phenylalanine, and methionine, under mid-pH conditions of cheese ripening (pH 5.5), and we investigated the catabolic pathways used by these bacteria. In the three lactic acid bacterial species, amino acid catabolism was initiated by a transamination step, which requires the presence of an ␣-keto acid such as ␣-ketoglutarate (␣-KG) as the amino group acceptor, and produced ␣-keto acids. Only S. thermophilus exhibited glutamate dehydrogenase activity, which produces ␣-KG from glutamate, and consequently only S. thermophilus was capable of catabolizing amino acids in the reaction medium without ␣-KG addition. In the presence of ␣-KG, lactobacilli produced much more varied aroma compounds such as acids, aldehydes, and alcohols than S. thermophilus, which mainly produced ␣-keto acids and a small amount of hydroxy acids and acids. L. helveticus mainly produced acids from phenylalanine and leucine, while L. delbrueckii subsp. lactis produced larger amounts of alcohols and/or aldehydes. Formation of aldehydes, alcohols, and acids from ␣-keto acids by L. delbrueckii subsp. lactis mainly results from the action of an ␣-keto acid decarboxylase, which produces aldehydes that are then oxidized or reduced to acids or alcohols. In contrast, the enzyme involved in the ␣-keto acid conversion to acids in L. helveticus and S. thermophilus is an ␣-keto acid dehydrogenase that produces acyl coenzymes A.Amino acid catabolism by the microflora is a major process for the formation of a large number of key aroma compounds in Swiss-type cheeses such as gruyere and emmental (7,14,17). However, the amino acid catabolic pathways in the different bacteria present in these cheeses are not well known. This knowledge could lead to the development of cultures with optimized aromatic properties.Amino acid catabolism by Lactococcus lactis and mesophilic lactobacilli has been extensively studied recently (2,8,11,19,26; S. Gummalla and J. R. Broadbent, Abstract, J. Dairy Sci. 79(Suppl.