<p style="text-align: justify;">In enology, lees are mainly used in the traditional practice of « sur lies » wine ageing, which consists of carrying on the contact between wine and lees (yeasts and vegetal residues) during ageing. Lees come either from first or second fermentation, and could be used for white or red wines elaboration. Such an enological practice remains yet empirical. In the present paper, the state of art was investigated in order to collect and analyze most of the scientific works realized on wine lees. It includes also technological points relevant from such a practice.</p><p style="text-align: justify;">A clear definition of wine lees from legal and technological points of view was given in the first part of the present paper. A second part described the mechanisms of autolysis and focused more precisely on each class of identified autolysis products. Many scientific works had indeed revealed the yeast autolysis phenomenon occuring during such a way of wine ageing. All these works remained mainly based on identification of yeast macromolecules released in the wine during the autolysis phenomenon. However, the experimental methodologies followed by the different authors are variable, and most of the obtained results were difficult to extrapolate to actual wine ageing on lees. Only few works dealt with the physicochemical properties of such lees during autolysis, specially towards oxygen, polyphenols and other wine compounds. A compilation of recent data obtained on these peculiar topics was then given. In a third and last part, the effect of ageing wine on lees was approached from a technical point of view.</p>

During wine aging on lees, some membrane lipids of yeast lees, in contact with dissolved oxygen at low concentration, may undergo mild oxidation explaining the capacity of yeast lees to consume oxygen. We studied the cross-reactivity of complex polyphenols and tannins from wine and yeast lees towards oxygen during simulation of wine aging. We observed a total decrease of oxygen consumption capacity of mixed yeast lees and wine polyphenol by comparison with the reactivity of each component studied alone. A strong loss of reactivity of yeast lees towards oxygen was observed when separated from soluble polyphenols, although only a fraction of the total polyphenols remained adsorbed on lees.

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Under anaerobic conditions, yeast growth normally requires oxygen in order to favour the synthesis of sterols and unsaturated fatty acids. However, in such conditions, superfluous oxygen consumption by yeast cells is observed. The superfluous oxygen consumed by the yeast cells appears to be not related to classical respiration, but mainly to the operation of several alternative oxygen consumption pathways. In this study, the potential relationship between this superfluous oxygen consumption and the yeast sterol synthesis pathway was investigated during enological fermentation. Additions of small (7 mg l(-1)) and excess (37 mg l(-1)) amounts of oxygen at the end of cell growth phase were used as a method of comparing oxygen consumption by normal synthetic pathways with that by alternative respiration pathways. The superfluous oxygen consumption by yeast cells during fermentation seemed not to alter and strongly favoured fermentation kinetics and cell biomass formation. However, a marked decrease of the orderliness of the membrane phospholipids is observed, which is not related to the drop of cell viability. After oxygen additions, squalene contents of the cells decreased, while the relative proportions of ergosterol or its precursors in the total sterol fraction did not correlatively increase. It was further found that an oxygen-dependent sterol degradation occurred when oxygen was added in excess amounts with respect to the cellular requirements for sterol synthesis. At present, this modification of the sterol contents of yeast membranes has not been related to any physiological parameters.