## 3 Results and Discussion
### Impact of Fermentation Parameters on Fermentation Time and Physicochemical Variables
with a low significance (\(p\)-value of 0.063 and 0.065, respectively), by the \(Sc/Lt\) ratio and oxygenation. An increase in the \(Sc/Lt\) ratio led to an increase in \(Sc\) at the expense of \(Lt\) (see Section 3.1.) and to an increase in ethanol production (stand. effect 2.899). On the contrary, oxygenation, which led to a decrease in the \(Sc\) compared to \(Lt\) population (see Section 3.1), led at the same time to a decrease in ethanol production (stand. effect \(-\)2.857). As the Crabtree effect, which was weaker in \(Lt\) than in \(Sc\), favors respiration and leads to lower ethanol production by \(Lt\)[11], this lower ethanol production could be explained by the modulation of this metabolic pathway. Moreover, the ethanol yield of \(Lt\) has also been described as being significantly lower than for \(Sc\)[12]. Although the respective impacts of these two hypotheses were impossible to properly determine within the scope of this study, the overall negative impact of oxygenation on ethanol production was still observable, and could be of interest to winemakers willing to decrease the ethanol content of their wines. The final pH was not significantly impacted by temperature, oxygenation, or nitrogen content. The \(Sc/Lt\) ratio had a positive effect on pH (stand. effect 2.575, var. contrib. 57.90%). The pH values observed when the inoculum ratio was 0.1 (predominance of _Lachancea thermotolerans_) were lower than those obtained with a ratio of 1. This pH reduction in mixed cultures \(Sc/Lt\) compared to \(Sc\) pure culture is often observed whatever the conditions (modalities of inoculation, temperature, medium) [16]. This decrease in pH could be linked at least partially to the production of lactic acid. _Lachancea thermotolerans_, as discussed above, is specifically known amongst non-_Saccharomyces_ for its ability to transform pyruvate into this acid through lactate dehydrogenases [11; 50]. As \(S\). _cerevisiae_ is not able to produce lactic acid [51] or, depending on the strain, can produce it in trace amounts only [50], the final amount of this acid can be directly related to the population dynamics, and specifically to the \(Lt\) population. Conditions either favoring \(Lt\) development compared to \(Sc\) or inhibiting antagonistic interactions from \(Sc\) that limit \(Lt\) growth or persistence will therefore indirectly favor lactic acid production during the fermentation process. In the different conditions tested, the production of lactic acid varied between 0.82 and 3.52 g\(\cdot\)L\({}^{-1}\). These values are classical ones as the production of this acid is highly dependent on the strain (\(<\)0.1 to \(>\)15 g\(\cdot\)L\({}^{-1}\)) and on conditions [53; 50; 11; 52]. Lactic acid concentration was considerably impacted by the \(Sc/Lt\) ratio (var. contrib. 83.30%, stand. effect \(-\)7.322). A decrease in this ratio indeed favored the predominance of \(Lt\) in the earlier stages of the fermentation, and this species' lactic acid production has previously been shown to occur predominantly at the beginning of fermentation, during exponential growth [2; 6; 16; 54]. This hypothesis was further supported by the pH dynamics shown in Figure 2, in which a rapid pH decrease during the first 72 h can be observed, with this falloff being on average significantly greater for conditions with an \(Sc/Lt\) ratio of 0.1. Using an \(Sc/Lt\) ratio of 0.1 in mixed culture in natural must, [PERSON] et al. (2013) [16] showed that the effects of the interaction between \(Lt\) and \(Sc\) on fermentation (decreases in ethanol and pH, increase in lactic acid compared to a pure culture of \(Sc\)) were more significant at 20 \({}^{\circ}\)C than at 30 \({}^{\circ}\)C, but this could have been related to the better persistence of \(Lt\) in these conditions (as also observed earlier in this study--see Section 3.1). [PERSON] et al. (2018) also observed that a higher lactic acid production was correlated with a more numerous \(Lt\) population and a higher nitrogen content (up to 500 mg/L) [54].