## 3 Results and Discussion
### Impact of Fermentation Parameters on Volatile Compound Production
(2021) noticed an increase in the production of isoamyl alcohol by \(Lt\) when the medium was supplemented with the amino acid precursors of this alcohol (leucine, isoleucine) compared to a medium with an addition of ammonium only [49]: such an impact of nitrogen was not observed here. [PERSON] et al. (2017) [20] observed, in mixed \(Sc/Lt\) cultures, that oxygenation resulted in an increase in higher alcohols, particularly isoamyl alcohol, 2-phenylethanol, and isobutanol: the absence of an effect of oxygenation observed here can be linked to different conditions of temperature and medium composition (yeast extract as a nitrogen source versus individual amino acids), to the differences in oxygenation levels mentioned previously, or to a strain-dependence factor. Table 7 shows the impacts of fermentation parameters on the production of various volatile compounds (aldehydes, ketones, and acids). These results are also illustrated in Figure 5, which presents the standardized effects of each fermentation parameter on this production. During fermentation in the different conditions tested, aldehydes (acetaldehyde, benzaldehyde) and some ketones were produced. Concerning the global production of aldehydes, none of the four factors tested had a significant effect on their production. As far acetaldehyde is concerned, the \(Sc/Lt\) ratio had a positive effect on its production according to [PERSON] et al. (2019) [23], but this was not observed here. The results concerning the production of acetaldehyde in mixed \(Sc/Lt\) cultures indeed seemed to be variable and strain-dependent [11]. A slight effect of the \(Sc/Lt\) ratio on benzaldehyde production (stand. effect \(-\)2.424, \(p\)-value 0.094) was nevertheless observed. Regarding ketones, the production of 6-methyl-5-hepten-2-one was negatively impacted by an increase in temperature (stand. effect \(-\)3.838), but not by the other factors. [PERSON] et al. 2019 [23] showed that this compound was produced more by \(Sc\) than \(Lt\) in pure culture, and that in mixed culture with different \(Sc/Lt\) ratios, it led to lower quantities than in \(Lt\) culture, leading to the assumption that it may be regulated by an interaction between the two species. Figure 5: Impact (standardized effects) of fermentation parameters (\(Sc/Lt\) ratio, nitrogen, oxygenation, temperature) on the production of different volatile compounds (aldehydes, ketones, acids). 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. The fermentation of the different synthetic musts in tested conditions led to the production of various organic acids: isovaleric acid, octanoic acid, decanoic acid, and acetic acid. Octanoic acid was not affected by the factors studied. Decanoic acid production was impacted by the \(Sc/Lt\) ratio as isovaleric acid (stand. effects 3.250 and 6.826, \(p\)-value 0.048 and 0.006, respectively) and also by temperature (stand. effect 3.100, \(p\)-value 0.053). Regarding acetic acid, its final content was managed by temperature and oxygenation (stand. effect 23.103 and 11.375, respectively). Its production also increased when the \(Sc/Lt\) ratio increased (stand. effect 20.805), supporting previous results [23; 47]. \(Lt\) is indeed known to produce acetic acid at low concentrations, leading to a lower volatile acidity of wine [2; 14]; however, high strain variability was observed in previous studies [11]. The positive impact of oxygenation on acetic acid production was unexpected, as this parameter favors the overall presence of \(Lt\) over time. \(Lt\) does not produce as much acetic acid as \(Sc\)[20; 23]. However, it has also been shown in previous works that oxygenation significantly modulated acetic acid metabolism and increased its production for both species in both pure and mixed cultures.