The aim of this work was to demonstrate the direct interaction between membrane sterols of yeast lees and some polymerized phenolic compounds resulting from wine model solution browning. For this purpose, we first demonstrated by measurement of steady-state fluorescence anisotropy of the cationic fluorescent TMA-DPH probe the effect of polymerized compounds from the model reactions of (+)-catechin/acetaldehyde and (+)-catechin/glyoxylic acid on the plasma membrane order of Saccharomyces cerevisiae yeast lees enriched with different sterols. In a second set of experiments, we used S. cerevisiae plasma membrane vesicles spiked with different sources of sterol (ergosterol, cholesterol or a mix of grape phytosterols) to assess the effect of the same polymerized compounds on both vesicle integrity and membrane leakiness to protons by ACMA fluorescence. All the obtained results prove that yeast membrane sterols are able to strongly interact with some polymerized compounds resulting from the browning of model solutions, likely explaining the yeast ability to adsorb polyphenolic compounds and mainly the colorless intermediate compounds of the browning reactions.

Wine model solutions were used to study the ability of dehydrated yeasts to retain the brown products formed in the reaction between (+)-catechin and acetaldehyde. Saccharomyces cerevisiae races capensis and bayanus, two typical flor yeasts involved in the biological aging of sherry wines, had a higher capacity to retain coloured compounds than S. cerevisiae fermentative yeast. Of the flor yeasts, capensis exhibited a higher colour reduction capacity than bayanus. Such differences may account for the different rate at which browning compounds are removed at different times of year during the biological aging of wines.

The purpose of this work was to examine the possible involvement of yeast membrane components in the adsorption of browning compounds from oxidized white wine. For this purpose, different yeast strains and growth conditions (aerobiosis and anaerobiosis) were tested for their ability to prevent browning of two model solutions consisting of (+)-catechin/acetaldehyde and (+)-catechin/glyoxylic acid. The obtained results showed that the effects of yeast lees are different according to the type of the studied model solution and the growth conditions that affect both the quantity and the quality of membrane sterols of the yeasts. Moreover, in vitro experiments proved that yeast membrane sterols could be likely involved in the yeast's ability to adsorb polyphenolic compounds and mainly the colorless intermediate compounds of the browning reactions.

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