## 3 Results and Discussion
### Impact of Fermentation Parameters on Volatile Compound Production
As the \(Sc/Lt\) ratio significantly impacted the proportion of each yeast in the media over time (as seen in Section 3.1), and as these two species did not produce the same aroma compounds [16; 20; 23], the aroma profile of a given mixed culture was indeed expected to be very different from either pure culture. However, interactions between yeasts can also change their metabolism and their aroma production, resulting in an apparent synergy (higher amount in mixed culture compared to pure cultures), as observed previously [16; 23]. The amount of nitrogen had a significant effect (stand. effect 4.742) only on alcohol production. [PERSON] et al. (2021) highlighted the link between nitrogen sources and aroma production in cocultures of \(Sc\) with several non-_Saccharomyces_ yeasts [56]. Higher alcohols can be produced from both sugars and amino acids: differences in the consumption kinetics of nitrogen sources and in metabolism flux according to the yeast can then lead to various productions of aroma compounds. Some authors also found that modifying nitrogen levels can lead to the production of different volatile compounds in pure \(Sc\) cultures [22], thus correlating with this conclusion. Oxygenation had no significant impact on the production of the different families of volatile compounds. This was in contradiction with the results of [PERSON] et al. (2017), who noticed an increase in the production of alcohols and volatile acids and a decrease in medium-chain fatty acids and corresponding esters [20]. However, the differences in oxygenation protocols already discussed in Section 3.1 could explain this lack of a significant effect in the conditions of our study. Temperature had a negative impact on the production of alcohols and ketones (stand. effect \(-10.175\) and \(-3.825\), respectively), a positive effect on ester and acid production (stand. effect 4.514 and 2.685, respectively), and no significant effect on aldehyde production. As temperature was previously shown to significantly impact population dynamics in this study, such shifts in aroma production were to be expected, as the media was fermented by very different yeast populations depending on the temperature chosen. An increase in aroma production, notably esters, was observed previously at low temperature with some strains of \(S\). _cerevisiae_, and this increase seemed to be related to a change in membrane lipid composition in response to the stress linked to the temperature decrease [57; 58]. More specifically, regarding the volatile molecules produced during fermentation, several alcohols are produced: primary alcohols with 3 to 10 carbons, isobutyl alcohol, isoamyl alcohol, 2-phenylethylanol, etc. Table 6 shows the impacts of fermentation parameters on the production of these alcohols. These results are also illustrated in Figure 4, which presents the standardized effects of each fermentation parameter on this production. A significant impact of environmental factors was observed only on three molecules: 1-propanol, isoamyl alcohol, and 2-phenylethylanol. For the other alcohols, the contribution of residuals to total variance was high, but no significant impact of interactions between factors could be observed, even when examined in more detail. This furthered the hypothesis that this lack of observed effects was due to a real absence of effects of the conditions studied on the production of these molecules. The production of 1-propanol was significantly influenced by nitrogen content (negatively, stand. effect \(-3.271\)) and to a lesser extent (\(p\)-value 0.097) by temperature (positively, stand. effect 2.392). The negative influence of nitrogen content showed the complexity of forecasting the production of an aroma compound in a coculture. Propanol is indeed produced from amino acids (while other alcohols can come from both amino acids and sugars) [56]: an increase in nitrogen content should have favored its production. [PERSON] et al. (2013) [16] and [PERSON] et al.