Interactions of wine polyphenols with dead or living Saccharomyces cerevisiae Yeast Cells and Cell Walls: polyphenol location by microscopy

Abstract

Tannin, anthocyanins and their reaction products play a major role in the quality of red wines. They contribute to their sensory characteristics, particularly colour and astringency. Grape tannins and anthocyanins are extracted during red wine fermentation. However, their concentration and composition change over time, due to their strong chemical reactivity1. It is also well known that yeasts influence the wine phenolic content, either through the release of metabolites involved in the formation of derived pigments1, or through polyphenol adsorption2,3. Up to now, this adsorption has mainly been attributed to cell walls, though it has been supposed that small tannin dimers and trimers could enter the periplasmic space through the wall pores and interact with the plasma membrane4. Interactions between polyphenols and inactivated yeast cells or cell walls obtained from an enological commercial strain were studied first by means of adsorption isotherms in a model wine-like solution5. The framework of this study was the aging of red wines. Polyphenols were skin and seed tannins, and a pool of polyphenols purified from a red wine (Merlot). Results evidenced a high capacity of whole cells to irreversibly adsorb grape and wine tannins whereas only weak interactions were observed for cell walls. This point was quite unexpected considering literature and raised the question of the part played by cell walls in the yeast ability to fix wine polyphenols. In the present work, polyphenol location after their interactions with inactivated yeast cells or cell walls was studied by means of transmission electron microscopy, light epifluorescence and confocal microscopy. Microscopy observations evidenced that if tannins interact with cell walls, and especially cell wall mannoproteins, they mostly diffuse freely through the cell wall and plasma membrane of dead cells to interact with their cytoplasmic components. This raised the question of yeast interactions with polyphenols in the case of living cells. The study was thus extended and interactions studied during fermentation, at different stages. The impact of polyphenols on fermentation kinetic and yeast growth rate were determined. In our experimental conditions, the exponential phase of the fermentation and the yeast growth rate were affected by polyphenols. Confocal microscopy observations allowed evidencing the diffusion of polyphenols in living cells. These results demonstrate that interactions between yeast cells and polyphenols are not limited to cell walls. They also involve cytoplasmic components and may influence yeast metabolism.

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