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.

Two families of peroxidases-lignin peroxidase (LiP) and manganese-dependent lignin peroxidase (MnP)are formed by the lignin-degrading white rot basidiomycete Phanerochaete chrysosporium and other white rot fungi. Isoenzymes of these enzyme families carry out reactions important to the biodegradation of lignin. This research investigated the regulation of LiP and MnP production by Mn(II). In liquid culture, LiP titers varied as an inverse function of and MnP titers varied as a direct function of the Mn(II) concentration. The extracellular isoenzyme profiles differed radically at low and high Mn(II) levels, whereas other fermentation parameters, including extracellular protein concentrations, the glucose consumption rate, and the accumulation of cell dry weight, did not change significantly with the Mn(II) concentration. In the absence of Mn(II), extracellular LiP isoenzymes predominated, whereas in the presence of Mn(II), MnP isoenzymes were dominant. The release of "'Co2 from "4C-labeled dehydrogenative polymerizate lignin was likewise affected by Mn(II). The rate of "'CO2 release increased at low Mn(II) and decreased at high Mn(II) concentrations. This regulatory effect of Mn(II) occurred with five strains of P. chrysosporium, two other species of Phanerochaete, three species of Phkbia, Lentinula edodes, and Phellinus pini. Phanerochaete chrysosporium is the most extensively studied lignin-degrading white rot basidiomycete. It has been used to elucidate lignin biodegradation with model compounds (17, 37), and it was the first organism for which lignin peroxidases (LiPs) were described (8, 21, 33, 35). More recently, P. chrysosporium has been shown to produce two other types of extracellular enzymes, Mn(II)-dependent peroxidases (2, 3, 11, 29), and glyoxal oxidase, an enzyme involved in extracellular H202 production (14). Both LiPs and manganese-dependent lignin peroxidases (MnPs) are glycosylated heme proteins containing protoporphyrin IX, and both are believed to play roles in the biodegradation of lignin (30). LiP and MnP differ in their catalytic mechanisms. LiP acts by abstracting single electrons from aromatic rings of lignin and lignin model compounds, leading to the formation of a cation radical (15) and subsequent cleavage reactions (10). MnP acts by generating Mn (III), a highly reactive intermediate, which, when stabilized by chelators (7), can diffuse from the enzyme active site to attack and oxidize the lignin structure in situ (9, 30). LiP and MnP have been demonstrated in a number of white rot fungi (7, 20, 27, 36). Previous studies on the lignin-degrading basidiomycete P. chrysosporium have shown the importance of nutritional factors in the appearance of ligninolytic activity (13, 19). Using assays based on the release of 14CO2, Keyser et al. (16) demonstrated that the ligninolytic enzyme system of P. chrysosporium is synthesized under nitrogen starvation. Nitrogen repression of ligninolytic activity has been well studied in P. chrysosporium (5, 6, 31), and this regulatory effect has been observed...

Aryl metabolite biosynthesis was studied in the white rot fungus Bjerkandera adusta cultivated in a liquid medium supplemented with L-phenylalanine. Aromatic compounds were analyzed by gas chromatographymass spectrometry following addition of labelled precursors ( 14 C-and 13 C-labelled L-phenylalanine), which did not interfere with fungal metabolism. The major aromatic compounds identified were benzyl alcohol, benzaldehyde (bitter almond aroma), and benzoic acid. Hydroxy-and methoxybenzylic compounds (alcohols, aldehydes, and acids) were also found in fungal cultures. Intracellular enzymatic activities (phenylalanine ammonia lyase, aryl-alcohol oxidase, aryl-alcohol dehydrogenase, aryl-aldehyde dehydrogenase, lignin peroxidase) and extracellular enzymatic activities (aryl-alcohol oxidase, lignin peroxidase), as well as aromatic compounds, were detected in B. adusta cultures. Metabolite formation required de novo protein biosynthesis. Our results show that L-phenylalanine was deaminated to trans-cinnamic acid by a phenylalanine ammonia lyase and trans-cinnamic acid was in turn converted to aromatic acids (phenylpyruvic, phenylacetic, mandelic, and benzoylformic acids); benzaldehyde was a metabolic intermediate. These acids were transformed into benzaldehyde, benzyl alcohol, and benzoic acid. Our findings support the hypothesis that all of these compounds are intermediates in the biosynthetic pathway from L-phenylalanine to aryl metabolites. Additionally, transcinnamic acid can also be transformed via ␤-oxidation to benzoic acid. This was confirmed by the presence of acetophenone as a ␤-oxidation degradation intermediate. To our knowledge, this is the first time that a ␤-oxidation sequence leading to benzoic acid synthesis has been found in a white rot fungus. A novel metabolic scheme for biosynthesis of aryl metabolites from L-phenylalanine is proposed.Consumer preferences for products with a natural origin have led to the exploitation of microbial sources that produce natural aroma compounds (16,28). Among the potential aroma producers, white rot basidiomycetes are probably the most versatile microorganisms. These fungi are able to produce a wide variety of volatile aryl metabolites of commercial interest, such as vanillin, benzaldehyde (bitter almond aroma), and cinnamaldehyde (1,7,12). Therefore, fermentation of natural substrates, such as L-phenylalanine or tyrosine, by white rot fungi can offer alternative routes for biosynthesis of a wide spectrum of aryl metabolites (11, 21). Other biosynthetic precursors, like aromatic acids, stimulate the production of aryl metabolites in Bjerkandera adusta BOS55 (21).The metabolism of L-phenylalanine has been studied in several white rot fungi (13, 17). Among the extracellular aromatic compounds that these organisms produce, veratryl alcohol has received special attention because it is known to be a substrate and possibly a mediator in lignin biodegradation (6,15). This compound is the major aryl metabolite formed in Phanerochaete chrysosporium cultures supplemented wi...

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