## 3 Results and Discussion
### Impact of Fermentation Parameters on Volatile Compound Production
(2016) [59] observed a higher production of propanol by \(Lt\) compared to \(Sc\) in pure culture, in contrast to the observations of [PERSON] et al. (2019) [23] in a sugar-rich must. [PERSON] et al. (2021) noticed that \(L\). _thernotolerans_ usually produced low amounts of higher alcohols, but with a high strain variability [11]. Temperature had a positive effect (stand. effect 2.392) on the production of 1-propanol, contrary to the results of [PERSON] et al. (2013) [16]. All these conflicting observations highlighted the variability of propanol production, depending on strains, media, and culture conditions chosen, especially in mixed cultures [16; 23; 59; 53]. The production of 2-phenylethanol was impacted mainly by nitrogen content (positively, stand. effect 9.281) and temperature (negatively, stand. effect \(-9.895\)). [PERSON] et al. (2018) observed that nutrient conditions (nitrogen sources, glucose amount) affected the production of phenylethanol (a molecule that can play a quorum-sensing role) with other non-_Saccharomyces_ yeasts [60]. [PERSON] et al. (2021) noticed an increase in the production of 2-phenyl ethanol by \(Lt\) when the medium was supplemented with phenylalanine (its precursor) compared to a medium with an addition of ammonium only [49]. In a mixed culture \(Sc/Lt\) under anaerobic conditions compared to a pure culture of \(Lt\), [PERSON] et al. (2019) [61] observed an upregulation of genes involved in the metabolism of amino acids, and especially those involved in the conversion of phenylalanine into phenylethanol. This suggested that this upregulation, which probably was linked to interactions, could be lower in the case of aerobic conditions. This could then explain the negative impact that oxygenation seemed to have on phenylethanol production in this study (stand. effect \(-2.367\), with a lower significance: \(p\)-value 0.099). Temperature had a negative effect on 2-phenylethanol production (stand. effect \(-9.895\)). [PERSON] et al. (2013) observed no effect on this production [16]. However, they observed a higher concentration of 2-phenylethanol in the presence of \(Lt\) compared to a pure culture of \(Sc\) under all of the conditions tested (inoculation modalities, temperature of fermentation, different grape juices). They explained that this production was due to synergy between \(Sc\) and \(Lt\), as both of these yeasts are low producers of this compound. On the other hand, [PERSON] et al. (2019) observed an increase in phenylethanol production in natural must when the \(Sc/Lt\) ratio decreased in the inoculum from 0.2 to 0.02 [23]: the results here showed a slight inverse trend (stand. effect 2.350, \(p\)-value 0.1), although it did not seem to be the most impactory factor, accounting for only 2.79% of the observed variance. Both these impacts of temperature and \(Sc/Lt\) ratio on 2-phenylethanol production may, at first glance, be perceived as the sole result of the impact of temperature and \(Sc/Lt\) ratio on yeast populations shown in Section 3.1. However, metabolism modifications via yeast-yeast interactions tend to complexify the phenomenon, as discussed earlier in this study. Figure 4: Impact (standardized effects) of fermentation parameters (\(Sc/Lt\) ratio, nitrogen, oxygenation, temperature) on the production of different volatile alcohols. 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). The production of isoamyl alcohol decreased when the inoculum ratio favored \(Sc\) (stand. effect \(-\)9.407, var. contrib. 83.70%) or when temperature was increased (stand. effect \(-\)3.636, var. contrib. 12.50%). A negative impact of the \(Sc/Lt\) ratio on isoamyl alcohol production was also observed by [PERSON] et al. (2019) [23]. Temperature had a negative effect on isoamyl alcohol, but [PERSON] et al. (2013) did not observe any impact of temperature on the production of this molecule [16]. [PERSON] et al.