## 3 Results & Discussion  
### Effect of YAN Content on Fermentation Kinetics  
The kinetics of alcoholic fermentations depended mostly on the yeast strains and secondarily on YAN concentrations. The _S. cerevisiae_ strain Sa consumed half of the reducing sugars after 4 days of fermentation. In contrast, the commercial and Sb strains were characterized by a 48 h delay, consuming half of the concentration of reducing sugars on the 6 th fermentation day. Furthermore, Sa managed to reach the levels of residual sugars (3.6 g/L) corresponding to dry wine on the 25 th day, while the Sb and the commercial strain ended up with 4.0 and 4.5 g/L, respectively. Despite the initial, somehow higher total sugar consumption rate and subsequently ethanol production rate of the commercial strain compared to the new isolates (Sa and Sb), finally, the ethanol content of the three wines produced did not show any statistically important differences (final ethanol content \(\sim\)100 g/L = 12.9% \(v/v\)) (Figure 1). The conversion yield of ethanol produced per unit of total sugars consumed was c. 0.50-0.51 g/g (the absolute value of the slope of the line) for all strains tested, that is, 98-100% \(w/w\) (Figure 2) of the maximum theoretical yield (=0.51 g/g), demonstrating the potential of the employed wild type strains for the production of wines containing high concentrations of ethanol [16]. No significant differences were observed regarding the conversion yield of ethanol between fermions of the same strain provided with different initial YAN. This outcome could indicate that the relatively low fermentation temperature and the yeast strain pose a more significant role compared to YAN [24,25]. Apart from the differences in fermentation kinetics, the strains showed variations in the consumption of fructose and glucose (Figure 3). The commercial strain presented a clear preferential uptake of glucose. Once glucose was almost depleted, there was a sharp increase in fructose uptake. As previously reported, this outcome was expected on the growth face, where actively growing cells present a preferential affinity for glucose. After this stage, the consumption rates of both glucose and fructose were noted to be similar. Furthermore, in case of the existence of two separate sugar transport systems, these results may indicate that both systems are capable of transporting either sugar, but only if the preferred sugar is not present. In the case of a common transport system, competitive inhibition of fructose uptake by glucose could have taken place [26]. However, this pathway was not confirmed in the present study, as the two sugars were consumed  
Figure 1: Production of ethanol by _S. cerevisiae_ in fermentations with (**a**) low and (**b**) high concentrations of YAN and consumption of reducing sugars by _S. cerevisiae_ in fermentations with (**c**) low and (**d**) high concentrations of YAN. Red, black and grey lines correspond to samples fermented with the commercial, Sa and Sb strains, respectively. almost simultaneously at different rates (Figure 3). This could be an indication of two separate transport systems with distinct kinetic properties. Furthermore, no significant differences concerning glucose and fructose uptake were observed between fermentations with different initial YAN, as nitrogen content in both cases was sufficient and prevented reduction in glucose uptake capacity [27], while must sugar content did not pose a limiting factor in nitrogen availability and requirements [28]. Figure 2: Conversion yield of ethanol produced per unit of total sugars consumed by the commercial strain in fermentations with (**a**) low and (**b**) high concentrations of YAN, Sa in fermentations with (**c**) low and (**d**) high concentrations of YAN, and Sb in fermentations with (**e**) low and (**f**) high concentrations of YAN. Regarding sugar utilization, differences in inorganic nitrogen requirements were observed among the different _S. cerevisiae_ strains.