# Response to Sulfur Dioxide Addition by Two Commercial _Saccharomyces cerevisiae_ Strains
## 4 Discussion
Studies have shown that these genes display a high degree of polymorphism, and different yeast strains have highly variable expressions of these genes [11; 13]. However, it is also possible that the prolonged presence of free SO\({}_{2}\) seen in these treatments was simply a result of decreased yeast activity during the first days of the experiment, due to increased sulfite sensitivity; QA23 did display decreased cell growth during the first few days of fermentation when SO\({}_{2}\) was added. A detailed exploration of the transcriptome and proteome of these two strains, particularly with regards to genes and gene products involved in sulfite efflux, could help conclusively determine the reasons for the differences in sulfite response observed here. When SO\({}_{2}\) enters the yeast cell, it is converted to bisulfite (HSO\({}_{3}\)-), which may then be incorporated into the SAAB pathway and reduced to sulfide (S\({}^{2-}\)) by the sulphite reductase enzyme, encoded by the _MET5/MET10_ genes [1]. This S\({}^{2-}\) may be used to produce sulfur-containing amino acids, or may be exported outside the cell as H\({}_{2}\)S. The fate of S\({}^{2-}\) in this pathway is, in part, determined by the levels of sulfur-containing amino acids already available to the yeast, as media containing higher levels of methionine have been correlated with decreased SO\({}_{2}\) resistance in yeasts [18]. The production of H\({}_{2}\)S by yeasts is also highly strain-dependent [58], and is considered an undesirable response to SO\({}_{2}\), because H\({}_{2}\)S has a very low detection threshold (1.1-1.6 ug/L), above which it can produce a rotten egg or cooked cabbage smell in wine [14; 59]. The delay in H\({}_{2}\)S production by QA23 in response to SO\({}_{2}\) addition is possibly due to its decreased cell density at the beginning of fermentation as a result of sulfite sensitivity, since both strains eventually produced similar total H\({}_{2}\)S over the course of fermentation. H\({}_{2}\)S production by yeasts is related to the presence of free (and molecular) SO\({}_{2}\) in the environment, therefore, the higher relative proportion of free SO\({}_{2}\) in these experimental fermentations likely resulted in higher than normal H\({}_{2}\)S production by these yeast strains. Furthermore, H\({}_{2}\)S is very volatile, and our methodology in this study involved determining only the total amount of H\({}_{2}\)S produced by each strain, not the final concentration of H\({}_{2}\)S in the wines at the end of the fermentation. Anecdotally, none of these wines expressed any aromas related to H\({}_{2}\)S, and it is likely that the final concentration of H\({}_{2}\)S in the wines was below its detection threshold. Acetaldehyde has a high affinity for SO\({}_{2}\), and can bind to SO\({}_{2}\) outside the cell and effectively prevent its antimicrobial actions [1]. At low levels (\(<\)125 mg/L), acetaldehyde may benefit a wine's sensory profile, but at higher concentrations it can cause the wine to smell of bruised apple and sherry [15; 35; 60]. Therefore, high acetaldehyde production by wine yeasts is considered undesirable. In white wine, acetaldehyde levels range from 11-493 mg/L [61]. The acetaldehyde levels measured in this current study fall in the middle of this range (100-250 mg/L), and were slightly above the sensory threshold for acetaldehyde, though likely not high enough in any treatment to result in a fault. Some previous studies have found acetaldehyde production by yeasts to increase in response to SO\({}_{2}\) addition [62; 63], and others have found increased acetaldehyde production to be a general trait of sulfite-resistant strains, even in the absence of SO\({}_{2}\)[16; 17]. Since we found BRL97 to be more resistant to SO\({}_{2}\) than QA23, this could explain the higher constitutive production of acetaldehyde by BRL97.