Online-Based Kinetic Analysis of Higher Alcohol and Ester Synthesis During Winemaking Fermentations

The production of the major volatile carbon compounds corresponding to the predominant higher alcohols and esters synthesized during wine alcoholic fermentation was monitored using an online GC system. The accuracy and frequency of measurements made it possible to calculate kinetic parameters, in particular rates and specific rates of production. Using this original GC system and a model describing the evolution of the gas-liquid ratio during the fermentation process, kinetic gas-liquid balances of the synthesis of volatile compounds could be followed. This showed that, for esters-substantial proportions of which are lost in the off-gas (up to 40 % of the total production at 20 A degrees C)-measurements of the liquid concentration, rather than of the total production (liquid content + losses), can lead to misinterpretations of yeast metabolism. The specific production rate of individual higher alcohols reached their maximum values before the exhaustion of the corresponding precursor amino acids. Isobutanol and isoamyl alcohol were formed from carbon metabolites and nitrogen metabolites and consequently were produced continuously throughout the fermentation process. In contrast, propanol synthesis was strongly correlated with the presence of assimilable nitrogen, during both the growth and stationary phases. Therefore, propanol appears to be a highly pertinent marker of nitrogen metabolism. Acetate ester concentrations correlated linearly with the concentrations of the corresponding higher alcohols, indicating that the availability of the precursors is the main limiting factor for the production of these esters. These results open possibilities for innovative approaches based on metabolic flux analysis and taking their dynamics into account.