Abstract
In Cognac, the musts are rich in grape solids and fermentations are usually run with turbidities ranging between 500 and 1500 NTU (nephelometric turbidity unit). These conditions, considered favourable for generating the desired organoleptic profiles of the final Eaux-de-vies, are unusual in winemaking, and, consequently, their impact on yeast metabolism is poorly understood. This study aims to better describe and understand the synthesis of fermentative aromas in such lipid-excess conditions, while integrating the effect of two other very important parameters: the initial concentration of assimilable nitrogen and the temperature of fermentation. To reach this objective, a Box-Behnken design was implemented to describe and model the simple effects of these factors as well as their interactions. Although the lipid concentration was very high, impacts on the production of fermentative aromas were observed. Indeed, high lipid levels promoted the synthesis of higher alcohols. Observing this effect was surprising because there is no metabolic connection between the anabolic pathways of production of these alcohols and the lipid pathway. This effect may be partly explained by impairment in the activity of alcohol acetyl transferases in the presence of lipids, which catalyse the conversion of higher alcohols into the corresponding esters. Therefore, in this study, the negative impact of turbidity was very significant on acetate esters related to the production of acetyl-CoA, which was the main molecule disturbed by the strong presence of lipids. Finally, and more surprisingly, lipid intake did not impact the synthesis of ethyl esters, which depended on the concentration of exogenous lipids.