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The effect of the carbon source and l-methionine on the ability of Geotrichum candidum to produce volatile sulphur compounds (VSC) was studied. This yeast was cultivated in a synthetic medium supplemented with various carbon sources and l-methionine at different concentrations. Both glycerol and glucose significantly increased VSC production by G. candidum. Unlike the effect on the l-methionine- and 4-methylthio-2-oxobutyric acid-demethiolating activities, the supply of a carbon source had a dramatic effect on the activity of aminotransferase, a key enzyme in l-methionine catabolism. An increase in the initial concentration of l-methionine resulted in a rise in the production of sulphur compounds (VSC, 4-methylthio-2-oxobutyric acid) but had limited effect on l-methionine-catabolising enzyme activities. Evidence for the existence of a dual effect of the carbon source and l-methionine on VSC biosynthesis was obtained in this study.

Five cheese-ripening yeasts (Geotrichum candidum, Saccharomyces cerevisiae, Kluyveromyces lactis, Yarrowia lipolytica and Debaryomyces hansenii) were compared with respect to their ability to generate volatile aroma compounds. K. lactis produced a variety of esters - ethylacetate (EA) being the major one - and relatively limited amounts of volatile sulphur compounds (VSCs). Conversely, G. candidum produced significant amounts of VSCs [with the thioester S-methyl thioacetate (MTA) being the most prevalent] and lower quantities of non-sulphur volatile compounds than K. lactis. We suspect that K. lactis is able to produce and/or accumulate acetyl CoA - a common precursor of MTA and EA - but that it produces limited amounts of methanethiol (MTL); both acetyl CoA and MTL are precursors for MTA synthesis. When supplemented with exogenous MTL, MTA production greatly increased in K. lactis cultures whereas it was unchanged in G. candidum cultures, suggesting that MTL is a limiting factor for MTA synthesis in K. lactis but not in G. candidum. Our results are discussed with respect to L-methionine catabolism.