# Yeast Diversity during Spontaneous Fermentations and
## 3 Results and Discussion
### Viiiification Using Indigenous S. cerevisiae Strains with Differentiated Genotypes  
It has been reported that the genotype of _S. cerevisiae_ has a crucial impact on wine characteristics [54]. Here, to investigate how genetic differences of indigenous _S. cerevisiae_ influence vinification profiles, 14 strains with varying genotypes were subjected to fermentations with Cabernet Sauvignon and Chardonnay. Nine representative genotypes found from uninoculated red grape fermentations (NX2, NX2, NX3, NX4, NX5, NX12, NX22, NX23 and NX24) (Table 1) were inoculated into Cabernet Sauvignon must whilst NX15 and NX21 obtained from white grape spontaneous fermentations (Table 1) were evaluated with Chardonnay juice. NX1, NX13 and NX16 commonly seen in spontaneous fermentations with multiple grape varieties (Table 1) were tested in both Cabernet Sauvignon and Chardonnay fermentations. All tested strains fermented to dryness (residual sugar \(<\) 4 g/L), and had little to no impact on alcoholic fermentation duration in any juice/must. The resultant wines were analysed for ethanol, pH, titratable acidity (TA), volatile acidity (VA), free SO\({}_{2}\) and total SO\({}_{2}\). All parameters were within the acceptable ranges referring the National Standard of the People's Republic of China (GB/T 15038-2006); however, they varied by genotypes (Figure 4). For instance, NX1 yielded higher amount of ethanol but low to intermediate TA (Figure 4). In contrast, NX13 constantly yielded the highest level of TA and decreased ethanol, suggesting partial diversion of carbon from ethanol production to organic acids. Acidification traits of indigenous _S. cerevisiae_ can be affected by the interaction of grape varieties, e.g., NX16 resulted in high TA in Cabernet Sauvignon wines whilst yielding a significantly lower TA in Chardonnay wines (Figure 4). Empirically, _S. cerevisiae_ strains would not lead to large variations in TA or pH after alcoholic fermentation. Microbial solutions to wine acidification are more commonly associated with the application of non-_Saccharomyces_, e.g., _Ladunca Hemtolerans_[55]. Nonetheless, [PERSON] et al. [16] had reported that several indigenous _S. cerevisiae_ strains obtained from Qilian vineyards, China, were capable of producing significantly higher total acidity compared to the commercial strain EC1118. This acidifying trait has particular oenological significance since increasingly common inadequate acidity found in high sugar must/juice can be corrected by inoculation with such _S. cerevisiae_ strains. The impact of _S. cerevisiae_ genotypes on wine volatile compounds was further examined using HS-SPME-GC-MS. A total of 49 volatile compounds were identified and quantified in Cabernet Sauvignon wines, and 50 volatiles for Chardonnay wines (Figures 5A and 6A, Tables S5 and S6). For Cabernet Sauvignon fermentations, 46 out of 49 volatile compounds (except nonanal, geranyl acetate and ethyl tetradecanoate) were significantly different between _S. cerevisiae_ genotypes (Table S5). 15 volatiles, including seven esters, five higher alcohols, two fatty acids and one aldehyde had odour activity values (OAV) >1 (Table 2), indicating their direct contribution to wine aroma formation. Although most volatile compounds did not surpass aroma thresholds, their interactions with other compounds may indirectly lead to global changes in wine aroma [56]. Here, we focused on volatiles with concentrations surpassing their sensory thresholds. Among the ethyl esters of medium chain fatty acids with OAV > 1, ethyl hexanoate and ethyl isovalerate, which were documented as contributing to fruity aromas [57], were richer in wines produced by all tested indigenous strains except NX2, NX4 than XR wines (Figure 5A). NX2 and NX4 wines had lower concentrations of ethyl caprate compared to XR wines whilst the highest value of ethyl caprate was observed in NX24 wines (Table 2). Ethyl caprylate and ethyl heptanoate were also 1.4-fold and 1.8-fold higher in NX24 wines than XR wines, respectively (Table 2).