How does the addition of antioxidants and other sulfur compounds affect the metabolism of polyfunctional mercaptan precursors in model fermentations?

Polyfunctional mercaptans are powerful wine aroma compounds formed from cysteinylated and glutathionylated precursors present in grapes. The concentration of polyfunctional mercaptans in the wines is not directly correlated to the concentration of their precursors in the juices, which suggests that there are many aspects related to the uptake and use of precursors by yeasts which remain poorly understood. In this work we determine the influence of different sulfur sources on the release of polyfunctional mercaptans and on the consumption of their precursors. A series of fermentations of synthetic juice was prepared containing known concentrations of polyfunctional mercaptans precursors. The effect of different sulfur compounds (elemental sulfur, glutathione, methionine, cysteine and sulfur dioxide), as well as of the amino acids residues that constitute glutathione (cysteine, glycine and glutamic acid) have been studied. Significant differences were found in the concentration of polyfunctional mercaptans and in the metabolization of their precursors when varying the sulfur source. The addition of glutathione and high levels of cysteine caused a significantly lower metabolization of the glutathione precursor of 3-mercaptohexanol. In parallel, in the wines spiked with sulfur dioxide and cysteine, significantly higher levels of polyfunctional mercaptans were observed. These results suggest that the consumption of polyfunctional mercaptans precursors is dependent on the juice composition, and by modifying the sulfur source it is possible to change the concentration of polyfunctional mercaptans.