Volatiles sulphur compounds production by cheese-ripening micro flora: Catabolism of Lcysteine

Due to their low odour threshold values volatile sulphur compounds (VSC) significantly contribute to the cheese flavour, even at low concentrations. Until now, the studies related to the production of VSC by cheese ripening microorganisms have been focused on the methionine catabolism, without paying attention to the possible contribution of cysteine in their production. This work aimed thus to establish the contribution of cysteine in the production of VSC by cheese-ripening yeasts and bacteria. In the first part of this work, five yeasts and four bacteria were selected among 37 strains, based on their capacity to produce H2S, the main degradation product of cysteine. This selection was achieved by using a practical adaptation of the methylene blue reaction, which enabled the simultaneous growth of microorganims, the collection and accurate quantification of H2S. In the second part of this work, the production of VSC by the selected strains was then assessed with the addition of cysteine, methionine or methionine-cysteine mixtures to the microbial cultures. By adding cysteine, no new VSC was produced by yeasts and only low concentrations of VSC were quantified in the bacterial cultures. With the methionine addition, the yeast strains followed two degradation pathways of methionine, and produced methional/methionol or DMDS/DMTS. Two new volatile sulphur compounds were also characterized: one of them is a thiophenone, 2-methyl-tetrahydrothiophene-3-one, and the other one is a 1,3-oxathiane. In addition, the bacteria produced VSC (DMDS, DMTS, DMQS and some thioesters) which have been already identified as the main products of methionine catabolism. With mixtures of methionine-cysteine, the production of VSC was more or less decreased depending on the concentration of the added cysteine. This effect seems to be a consequence of the limitation of the methionine catabolism by the added cysteine. Although this effect remains strain dependant, changes in the VSC profiles were observed in all the cases. Hence, in the bacteria, we observed a major production of the polysulphides DMTS and DMQS, with a parallel decrease in the concentrations of thioesters, when they were produced. In yeasts, a similar decrease was observed in the concentration of methional/methionol and in the concentrations of thioesters, but without any increase in the production of polysulphides. The low concentration of the solubilized form of H2S (HS- and S2-) in the acid medium used for yeasts may explain this result. Preliminary sniffing tests showed that the addition of low cysteine concentrations, with the simultaneous addition of methionine, produced flavour notes very close to ripened cheeses. Thus, and in a cheese-making context, we can reasonably suppose that H2S produced by cysteine catabolism would not take part in the modification of VSC profile at the beginning of ripening, because of the acid character of curd during this period. However, the following increase in the pH, as a consequence of the deacidification promoted by yeasts development, can increase the reactivity of H2S and thus, with a simultaneous production of methanethiol by the bacterial catabolism of methionine, could provide a better balance between polysulphides and thioesters. Thus, the co-production of these sulphur molecules by ripening yeasts and bacteria could improve the development of cheese ripened flavours.