Les interactions entre levures et bactéries lactiques sont déterminées par la nature des souches en présence, leur population et la constitution chimique du moût et du vin. Normalement, pendant la fermentation, les bactéries lactiques sont inhibées par les produits du métabolisme levurien. Mais dans certaines conditions, la croissance précoce des bactéries provoque une chute de la viabilité des levures et, par conséquent, un ralentissement ou un arrêt de la dégradation des sucres. Tous les mécanismes mis en jeu ne sont pas connus.
ln wine-making, lactic acid bacteria succeed to yeasts to promote the malolactic fermentation (MLF), after the completion of the alcoholic fermentation. Yeasts are better adapted than bacteria to growth in grape must. So their development easily starts and the alcoholic fermentation triggers soon after the harvest. In the same time, the natural bacterial inoculum decreases, due to an antagonism between these two microorganisms. ln addition to ethanol, fatty acids synthesized by actively growing yeasts exert their toxic effect against lactic acid bacteria. At the end of the alcoholic fermentation, the remaining bacterial population (104 to 103 cell.p.ml) begins growing. It is mainly constitued by Leuconostoc oenos more resistant than lactobacilli. At that time, autolysis products from decline phase yeasts act as bacterial growth factors. Then, when the bacterial population is large enough (about 106 cell.p.ml) MLF happens. So, normally, MLF follows alcoholic fermentation within a few days. But in some conditions, the interactions between yeasts and bacteria do not work in this way. If the weather is particularly hot and dry during the last weeks of the maturation, the pH of the must may be high and generally sulfiting is lowered as there are no rotten grapes. Growing conditions are better than usually for bacteria that, in such a case, compete with yeasts. The bacterial population reach a sufficient level that can actually increase the yeast decline rate. Hydrolase activities of bacteria against yeast cell walls may occur. That leads to a fermentation stuck. These antagonism effects between yeasts and lactic acid bacteria in fermenting must control the wine-making process. Their extent vary with must composition even with both yeast and bacteria strains. Further research is needed to clear up involved machanisms.
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