# Response to Sulfur Dioxide Addition by Two Commercial _Saccharomyces cerevisiae_ Strains
## 4 Discussion  
The slower fermentation rate and growth delay of QA23 in the presence of SO\({}_{2}\) suggested that QA23 is more sensitive to SO\({}_{2}\) addition than BRL97. This was confirmed by a sulfite resistance assay, where BRL97 showed no decrease in colony size at any concentration of Na\({}_{2}\)SO\({}_{3}\), but QA23 showed signs of growth inhibition beginning at 1.5 mM Na\({}_{2}\)SO\({}_{3}\) (data not shown). Previous studies [52; 53; 54] have found no differences in fermentation rate in response to SO\({}_{2}\) addition, and one study [55] found an increase in fermentation rate with added SO\({}_{2}\). However, these studies were conducted using nonsterile grape must, because the objective was to follow changes in yeast community dynamics as a result of SO\({}_{2}\) addition level. In the case of [PERSON] et al. [55] it is likely that the addition of SO\({}_{2}\) reduced the growth and development of the non-_Saccharomyces_ yeast community, thereby reducing competition for _S. cerevisiae_. Contrastingly, in the case of our study, the progression of fermentation in each flask was dependent upon the SO\({}_{2}\) resistance of a single yeast strain. Furthermore, even though QA23 did perform AF at a slower rate when SO\({}_{2}\) was added, it was still able to complete fermentation successfully within 12 days, and no fermentations in this study could be considered slow or sluggish. The increased susceptibility of QA23 to SO\({}_{2}\) was also observed in the delay in cell growth, which may also have caused the slower fermentation rate in the SO\({}_{2}\) treatment as compared with the control. [PERSON] and [PERSON] [56] also observed an initial decrease in _S. cerevisiae_ cell concentration in response to SO\({}_{2}\) addition, but this initial decrease did not affect its ability to complete fermentation within a similar time frame as the fermentations conducted without SO\({}_{2}\) addition. Previous studies, conducted with nonsterilized grape must, have found that SO\({}_{2}\) addition can both stimulate yeast growth [57] or inhibit it [53; 56], depending on the must characteristics, and also likely on the yeast strain(s) used. In accordance with our QA23 results, [PERSON] et al. [54] also noticed no difference in overall yeast abundance in fermentations inoculated with QA23 and fermented with or without SO\({}_{2}\) addition. The observation of both strains being present at every stage of fermentation in the SO\({}_{2}\) treatment suggests that neither of the yeast strains entered a VBNC state. Although SO\({}_{2}\) is able to induce a VBNC response in _S. cerevisiae_[4], it is rare to have commercial _S. cerevisiae_ strains exhibiting this resistance mechanism. In addition, the concentration of SO\({}_{2}\) used in this study (50 mg/L) is commonly added to commercial fermentations and was likely much too low to induce a VBNC state in commercial _S. cerevisiae_ strains. The proportion of free SO\({}_{2}\) measured on Day 1 (\(\sim\)60%) was larger than is regularly reported in grape must after SO\({}_{2}\) addition [36; 53; 57]. This is likely because this juice was sterile filtered before the SO\({}_{2}\) was added, so there were few grape constituents, and only one microbe (as opposed to a diversity of microorganisms), present to bind the SO\({}_{2}\). The extended presence of free SO\({}_{2}\) in the fermentations conducted by QA23, but not by BRL97, suggests that these strains may have different responses to SO\({}_{2}\). It is possible that QA23 has a higher constitutive expression of _FZF1_ or _SSUI1/SSUI1-R_, which encode and regulate the sulfite efflux pump Ssu1p [9; 13]. A higher expression of these genes could lead to more free SO\({}_{2}\) being exported back outside the yeast cell. This response to SO\({}_{2}\) is desirable for winemakers, because it allows the exported SO\({}_{2}\) to act as an antimicrobial agent against other microorganisms which may cause spoilage.