Grape solids: new advances on the understanding of their role in enological alcoholic fermentation

Abstract

Residual grape solids (suspended particles) in white and rosé musts vary depending on the clarification pro-cess. These suspended solids contain lipids (more especially phytosterols) that are essential for yeast meta-bolism and viability during fermentation in anaerobic conditions. We present here recent advances on the understanding of the role of the solid particles during alcoholic fermentation in liquid phase. A microscopic approach (epifluorescence with fluorescent probes, confocal and electronic microscopy) al-lowed to better characterize grape solids physical-chemical structure and lipidic fraction. Then, the lipidic composition of grape solids from white, rosé and red winemaking processes was analyzed. ß-sitosterol was the main sterol, but there was considerable variability of overall sterol content of solids depending on their origin. Must turbidity was not representative of sterol content. The use of mutant strains allowed us to pro-vide a model of sterol assimilation in Saccharomyces cerevisiae cell. Finally, experimental fermentations were performed in synthetic musts supplemented with the studied grape solids. Sterol content was deter-minant for the fermentation kinetics when lipids were the limiting yeast nutritional factor. This impact can be explained by the fact that sterols i) favor yeast nitrogen assimilation and consequently cell growth and maximum fermentation rate, ii) improve cell viability at the end of fermentation reducing risks of sluggish fermentation. Thus, taking into account the sterol content of the must should allow wine-makers to improve control of white and rosé fermentations.