## 3 Results and Discussion
### Relevance for the Production of Volatile Compounds during Alcoholic Fermentation
#### 3.3.1. Biotin Influence on Wine Volatile Compounds
This effect is to be explained by the essential role played by biotin in fatty acid synthesis and elongation, with the vitamin acting as a cofactor to Acc1 and Hfa1 in the conversion of acetyl-CoA towards malonyl-CoA, which stands as the first step of the fatty acid synthesis pathway, before malonyl-CoA enters the FAS system [48; 49; 50]. Ethyl nonanoate, however, did not present any significant effect of the initial biotin on its concentration (\(0.39\pm 0.05\) ug/L, \(0.48\pm 0.17\) ug/L and \(0.52\pm 0.16\) ug/L at 0, 0.5, and 3 ug/L of biotin, respectively), although its precursor, nonanol, did indeed show similar increases to the other impacted compounds, although to an apparently lesser degree (Figure 2). Such a difference might be solely resulting from the decreased transfer of ethyl esters to the medium when increasing the fatty acid carbon chain length [51]. However, the other longer-chained fatty acids and associated alcohols and FAEEs, such as decanoic acid (C10), decanol, ethyl decanoate, and ethyl laurate (C12), displayed no significant differences in the concentrations in regard to the initial biotin contents of the medium. Three of the impacted compounds, ethyl hexanoate, ethyl octanoate, and octanoic acid, do, interestingly, display odor activity values (OAV) above 1, detected above their olfactory thresholds of 14 ug/L, 5 ug/L, and 5 mg/L, respectively [52]. This suggests that the initial biotin in the medium might lead to notable changes in the olfactory profiles of wines. No definite conclusion, however, can be drawn on the matter without any sensory analysis to definitely assess the impact held by the vitamin on wine aromatic profiles; this, however, strikingly proves the relevance of sufficient biotin in grape must for the production of fatty acids and FAEEs. Higher Alcohols, Higher Aldehydes, Fusel Acids, and Associated Esters (Ehrlich Pathway)  
In addition to the clear impact held by the initial biotin on the production of fatty acids and derivatives that has been observed here, a lesser influence of the vitamin on the production of higher alcohols and derivatives has appeared, since 4 of the 10 higher alcohols, higher aldehydes, and associated esters, that can be linked to the Ehrlich pathway, display significant differences in their contents in regard to the initial biotin contents of the synthetic must (Figure 2B). Interestingly, two notable groups of affected compounds seemed to materialize in regard to their behaviors in the face of the initial biotin concentration: while propyl acetate and isovaleric acid increased in contents when increasing biotin, isoamyl alcohol and isoamyl acetate appeared to decrease against the vitamin (Figure 2B). Surprisingly, no consensus appears to exist in regard to the higher levels of isoamyl alcohol and its corresponding acetate observed here; as such, albeit [PERSON] and colleagues reported decreases of the isoamyl alcohol contents in case of biotin deprivation [31], a complete absence of the effect of the vitamin on the production of this compound had been previously reported by [PERSON] and associates [53]. With the yeast strains used in these various studies being different, it is not impossible that this disagreement in behavior could rather be a strain-dependent character, or reflect their individual tolerances towards low biotin availabilities. This could also result from disparities in the yeasts' physiological states, since, notably, the yeasts used by [PERSON] and colleagues were pre-starved in a biotin-deficient medium before inoculation [31]. The increase in the other compounds when increasing biotin might rather be related to their amino acid origin. Reductions in the cellular amino acid contents of yeasts have been found when grown in biotin-deficient medium [54], which might justify the limited formation of their associated higher alcohols, aldehydes, and esters in case of biotin deficiency.