## 3 Results and Discussion
### Impact of Fermentation Parameters on Volatile Compound Production
Moreover, this modulation was linked to a decrease in ethanol production [20], a tendency noted in Section 3.2 (albeit with low significance), which further corroborated this hypothesis. Table 8 shows the impacts of fermentation parameters on the production of esters. These results are also illustrated in Figure 6, which presents the standardized effects of each fermentation parameter on this production. After fermentation, the media contained different esters derived from different acids and fatty acids: ethyl acetate, isoamyl acetate, ethyl lactate, ethyl hexanoate, octanoate, etc. Although some factors influenced the production of fatty acids such as decanoic acid, no significant effect was observed on esters from these acids. The environmental factors studied significantly impacted only four molecules: ethyl acetate, isoamyl acetate, ethyl hexanoate, and ethyl lactate. Ethyl acetate production increased with an increase in the \(Sc/Lt\) ratio, oxygenation, and particularly temperature (stand. effects 5.914, 6.856 and 21.058, respectively), but with a decrease in nitrogen content (stand. effect \(-6.856\)). These results were in contradiction with previous works [16; 23] in which an increase in ethyl acetate was observed in conditions favoring the development of \(Lt\) (low temperature, low \(Sc/Lt\) ratio). However, other studies have also shown that the population dynamics were  
Figure 6: Impact (standardized effects) of fermentation parameters (\(Sc/Lt\) ratio, nitrogen, oxygenation, temperature) on the production of different esters. Data are identified as statistically almost significant (0.05 \(<\)\(p\)-value \(<\) 0.1, light grey), significant (\(p\)-value \(<\) 0.05, grey), or very significant (\(p\)-value \(<\) 0.01, dark grey). Compounds in italics and in grey were only present in trace amounts, with values too low to perform a relevant statistical test. not the only factor in play here, and that synergy effects could occur as mixed \(Sc/Lt\) cultures can end up with higher ethyl acetate content than either of the pure ones [20; 59]. Isoamyl acetate production was negatively impacted by temperature (stand. effect \(-5.746\)), while ethyl hexanoate was influenced mostly and positively by the \(Sc/Lt\) ratio (stand. effect \(8.486\)), and negatively by oxygenation and temperature (stand. effects \(-4.185\) and \(-4.766\), respectively). Ethyl lactate production was negatively influenced by the \(Sc/Lt\) ratio (stand. effect \(-4.883\)). This result should be linked to results concerning lactic acid, since it is a precursor of ethyl lactate. [PERSON] et al. (2021) [53] showed that sequential inoculation, which allows \(Lt\) to develop better at the beginning of fermentation, led to an increase in ethyl lactate, but these authors also noted that the production of this aroma compound was strain-dependent. Overall, the production of aroma compounds was impacted by fermentation parameters, but in very different ways depending on both the modulated factor and the volatile molecule studied. Some impacts could be explained by the population dynamics, but interactions between \(Lt\) and \(Sc\) often occurred. These interactions led to metabolism changes and consequently to volatile production, which was different from what would be expected. As far as the production of higher alcohols is concerned, nitrogen content seemed to be a key factor to control. As was shown in Section 3.1 nitrogen content did not significantly impact the population dynamics; this suggested an interaction between \(Sc\) and \(Lt\), and more specifically competition for amino acids known to be aroma precursors [22; 35].