# Response to Sulfur Dioxide Addition by Two Commercial _Saccharomyces cerevisiae_ Strains
## 3 Results
### Sulfur Dioxide Concentration during Fermentation  
Sulfur dioxide concentrations changed over the course of fermentation for all treatments. When no SO\({}_{2}\) was added, total SO\({}_{2}\) (free + bound SO\({}_{2}\)) was 0 mg/L on Day 1, increased between Day 1 and Day 2 due to SO\({}_{2}\) production by yeasts, and then decreased gradually over the rest of the fermentation for both yeast strains (Figure 3A). Both strains produced similar total SO\({}_{2}\) levels in the absence of added SO\({}_{2}\). The treatments that received 50 mg/L SO\({}_{2}\) contained \(\sim\)46 mg/L total SO\({}_{2}\) on Day 1 (Figure 3B). In the SO\({}_{2}\)-added fermentations, total SO\({}_{2}\) decreased consistently over the course of the nine-day experiment for both strains, resulting in similar total SO\({}_{2}\) patterns for both strains.  
Figure 1: Daily residual sugar levels (g/L \(\pm\) SEM) of controlled fermentations conducted by different yeast strains (\(n=4\) per treatment). Fermentations were conducted in the presence of either (**A**) 0 mg/L SO\({}_{2}\) or (**B**) 50 mg/L SO\({}_{2}\). Treatments with different superscript letters indicate significantly different fermentation progressions (2-factor repeated measures ANOVA, \(\alpha=0.05\)).  
Figure 2: Total yeast abundance (CFU/mL \(\pm\) SEM) during controlled fermentations conducted by different yeast strains (\(n=4\) per treatment). Fermentations were conducted in the presence of either (**A**) 0 mg/L SO\({}_{2}\) or (**B**) 50 mg/L SO\({}_{2}\). Treatments with different superscript letters indicate significantly different yeast abundance dynamics throughout fermentation (2-factor repeated measures ANOVA, \(\alpha=0.05\)).  
While total SO\({}_{2}\) is useful for confirming SO\({}_{2}\) addition levels and for monitoring SO\({}_{2}\) production by yeasts, free SO\({}_{2}\) is more useful for evaluating antimicrobial activity. Free SO\({}_{2}\) is the portion of total SO\({}_{2}\) that is available to act as an antimicrobial agent, and molecular SO\({}_{2}\), which has the most antimicrobial activity, can be calculated from the concentration of free SO\({}_{2}\) and the pH of the juice [1; 35]. In the 50 mg/L SO\({}_{2}\) treatments, approximately 28 mg/L of the SO\({}_{2}\) added was present in its free form on Day 1 (Figure 3C). The pH of the juice on Day 1 was 3.16, and molecular SO\({}_{2}\) was calculated to be ~1.2 mg/L in these fermentations. Free SO\({}_{2}\) represented approximately 60% of the total SO\({}_{2}\) concentration on Day 1. By Day 2, the fermentations inoculated with BRL97 did not contain any free SO\({}_{2}\), but the fermentations inoculated with QA23 still contained approximately 21 mg/L. By Day 4, free SO\({}_{2}\) had decreased to ~0 mg/L for both treatments and remained at this concentration until the end of the experiment. Free SO\({}_{2}\) was not produced by either strain in the control (0 mg/L SO\({}_{2}\)) treatment.  
Figure 3: SO\({}_{2}\) levels (mg/L) throughout fermentations conducted by different yeast strains and in the presence of different levels of initial SO\({}_{2}\). Wine samples from each of the four replicates were combined for SO\({}_{2}\) analysis due to volume constraints. Plots represent (**A**) total SO\({}_{2}\) produced by yeasts throughout fermentations containing 0 mg/L SO\({}_{2}\); (**B**) total SO\({}_{2}\) levels throughout fermentations containing 50 mg/L SO\({}_{2}\); (**C**) free SO\({}_{2}\) levels throughout fermentations containing 50 mg/L SO\({}_{2}\).