## 3 Results & Discussion
### Effect of YAN Content on Fermentation Kinetics
Sa strain consumed ammonium nitrogen (NH4+) faster, especially in the case of fermions containing 150 mg N/L. In those cases, ammonia was exhausted by the 3 rd day of fermentation (data not shown). Sb strain showed a completely different behavior regarding ammonium nitrogen consumption. In the case of musts containing 250 mg N/L, the NH4+ consumption speed was higher, even though complete exhaustion of ammonia was achieved on the 6 th day of fermentation. On the  
Figure 3: Consumption of glucose and fructose by 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. Black and grey lines correspond to glucose and fructose consumption, respectively. other hand, in musts inoculated with Sb, containing 150 mg N/L, ammonia was exhausted by the 5 th day of fermentation (data not shown). Based on these observations, Sa strain seems to exhibit higher NH4\({}^{+}\) requirements than the other strains examined, while Sb and commercial strains might be characterized by comparable NH4\({}^{+}\) demands (Figure 4a). In addition, all three _S. cerevisiae_ tested strains exhausted approximately 70% of \(\alpha\)-amino acids (FAN) by the second day of fermentation. More specifically, consumption of FAN in musts inoculated with the commercial, Sa and Sb strains, reached 69.1%, 73.7% and 59.6%, respectively. The FAN utilization followed NH4\({}^{+}\) consumption in a nitrogen catabolite repression manner. The fastest consumption of NH4\({}^{+}\) in fermentations inoculated with Sa strain compared to the Sb strain, probably lead to a faster activation of the GAP1 (general amino acid permease) and GLN3 (activated when preferred nitrogen sources are not available) proteins. The activation of the specific proteins leads the metabolic pathway to non-preferred nitrogen sources (FAN) [29]. Once again, the Sb strain was characterized by the lowest nitrogen depletion speed, indicating the lowest FAN demand (Figure 4b). In general, the utilization rate was higher in the trials containing 250 mg N/L, compared to fermentations containing 150 mg N/L. Moreover, the consumption of the different amino acids by the fermentations with lower initial YAN was not significantly different. In contrast, commercial strain musts enriched with 250 mg N/L YAN were characterized by the lowest utilization rate of leucine, valine, asparagine and glutamine, while the Sb strain presented the highest utilization rate for the same amino acids. The absence of  
Figure 4: Utilization rate of (**a**) NH4+, (**b**) FAN and (**c**) YAN by three different _S. cerevisiae_ strains. The ‘low’ and ‘high’ correspond to fermentations with 150 mg N/L and 250 mg N/L of initial YAN, respectively. The ‘com’, ‘Sa’ and ‘Sb’ correspond to fermentations carried out with the commercial and Sa and Sb strains, respectively. Values followed by different letters in each column indicate significant differences (_p_ < 0.05) among different samples. statistically important differences in ferment