I’ve attempted to produce a simplified graphic showing the role of yeasts in producing volatile sulfur compounds, which are responsible for ‘reduction’ problems in wine. The big oval here is the yeast cell, and – grossly simplified – this is how reduction issues are caused by yeasts.
The main way in which yeasts produce volatile sulfur compounds is through the sulfate assimilatory reduction pathway. Sulfate is taken up and used in the biosynthesis of organic sulfur containing compounds such as the amino acids methionine and cysteine. Low concentrations of these two amino acids in grape must cause the induction of the sulfate assimilation pathway, in which it is taken up, reduced to sulfite and then reduced to sulfide by sulfate reductase. In addition, extracellular sulfite can also be reduced to sulfide.
The sulfide is used to form methionine or cysteine, in conjunction with O-acetylhomoserine or O-acetylserine, respectively. If these two are in short supply, then sulfide is released and combines with hydrogen to form hydrogen sulfide.
Methionine can be deaminated and reductively decarboxylated to form methional. Methanethiol can be formed from methionine by demethiolase. And sulfide can react with acetaldehyde (aka ethanal) to form ethanethiol. All these volatile sulfur compounds are quite stinky.
But there is still a lot we don’t know about how volatile sulfur compounds end up in wine and cause the problems that they do.