In the Cognac appellation, the production of white wines is almost exclusively dedicated to elaborate Charentaise eaux-de-vie. In this sense, the quality of Cognac eaux-de-vie intrinsically depends on the quality of the base wines subjected to the distillation stage. In this context, the production of these base wines differs from those of classic white wines to release particular organoleptic properties during the distillation stage. Thus, the settling stage is one of the stages that most illustrates the identity of Cognac wines. The freshly pressed white grape juice is placed in a settling tank but without the presence of pectolytic enzymes, without sulfiting and for a relatively short period of time, contrary to conventional oenological practices. Under these operating conditions, Cognac musts reach very high turbidities in the order of 500 to 2000 NTU against 150 to 200 NTU in conventional oenology. These Charentais musts, rich in solid particles and therefore in lipids , allow to guarantee an organoleptic quality that is both delicate and full of character for future eaux-de-vie. Associated with lipids, nitrogen is a nutrient with a major role in alcoholic fermentation  that will also influence the aromatic profile of wines  intended for distillation. To understand the impact of these main nutrients on the desired organoleptic quality of Cognac, we studied their influence under natural fermentation conditions with three strains of S. cerevisiae commonly used for the Cognac appellation. To understand the influence of each nutrient and their interaction, an experimental plan called “Central Composite Design” (CCD) was developed. The CCD allows to model the aroma productions from the fermentation conditions. Fermentations were carried out with natural ugni blanc must at 23°C. Assimilable nitrogen concentrations ranged from 115 to 285 mg/L and turbidity from 500 to 2700 NTU. Finally, a statistical analysis of covariance (ANCOVA) was also performed to evaluate the strain effect. The main results showed that lipids and assimilable nitrogen have a significant impact on the aromatic quality of Cognac wines. Indeed, high lipids concentrations favor the production of organic acids but inhibit the synthesis of esters. The metabolism of the 3 yeast strains reacts in the same way to changes in nitrogen and lipid nutrition. However, each strain keeps its own aromatic profile whatever the fermentation conditions. This study made it possible to study and model the impact and interaction of two essential nutrients for alcoholic fermentation on the metabolism of yeast in natural conditions with excess lipids. In addition, it should be noted that, even if each strain of the Cognac appellation has its aroma properties, all strains respond in the same way to the variations of nitrogen and lipid nutrition.
Authors: Charlie Guittin – UMR SPO, INRAE of Montpellier,Faïza, MACNA, UMR SPO, INRAE Montpellier Isabelle, SANCHEZ, UMR MISTEA, INRAE Montpellier Jean-Marie, SABLAYROLLES, UMR SPO, INRAE Montpellier Xavier, POITOU, Hennessy, Cognac Jean-Roch, MOURET, UMR SPO, INRAE Montpellier Vincent, FARINES, UMR SPO, INRAE Montpellier
Keywords: cognac, nitrogen, lipids, centered composite design, alcoholic fermentation, Saccharomyces cerevisiae, metabolism, aromas