Hydrogen sulfide formation is a chronic problem in wine production. H2S is considered
to be a defect in most wine producing regions of the world if found in the wine at the
point of consumption. Strains vary widely in the amount of sulfide released under
identical growth conditions. Sulfide production of a single strain can vary widely
depending upon the growth conditions. Yeast suppliers often advertise new strains as
being characterized as low sulfide producers, but often sulfide is produced anyway during
fermentation. In these cases it is not clear if the commercial strain dominated the
fermentation or not, or if conditions are sufficiently different from those under which the
strain was tested so that sulfide production behavior cannot be accurately predicted. Our
goal is to be able to reliably make valid predictions of sulfide producing behavior of a
given strain, and to generate strains that are true low sulfide formers regardless of the
composition of the juice or must being fermented and the fermentation conditions.
Previous research identified a strain, UCD932, that displayed virtually undetectable H2S
production under a variety of conditions. It has been our goal to define the genetic basis
of low sulfide production in this strain with the aim of then being able to better
characterize sulfide production behavior and in order to generate via genetic crossing
improved commercial isolates that no longer release sulfide.
The goal of this project was to initially screen the entire set of yeast deletion mutants to
identify those that lead to darker and lighter colony colors on BiGGY agar and to then
evaluate them for sulfide production in synthetic and natural juices. Completion of this
objective has defined the set of genes involved in both increasing and decreasing
formation of sulfide. Unfortunately, the four mutations identified as eliminating sulfide
formation all lead to a requirement for methionine for growth of the strains. If high levels
of methionine were to be added to the juice or must, off characters caused by the
methionine could develop, which would be trading one problem for another. UCD932
does not require methionine and therefore is unlikely to harbor a null or knock out
mutation that is responsible for lack of sulfide production. Rather than bad news, this is
actually good news as it means that strains can be constructed that do not produce sulfide
and that can be easily monitored by colony color to know they are dominating the
fermentation but that do not also lead to a need to supplement fermentations with sulfide.
The genetic crosses of haploid derivatives of UCD932 will now allow us to determine the
nature of the gene responsible for lack of sulfide production as proposed in the new grant.
Although there are currently valid means by which to remove sulfide from wine once it
has formed, the ideal situation would be to not have it form in the first place and thereby
not need to manipulate the wine in unnecessary ways.