Acidification of Grape Musts bySaccharomyces cerevisiaeWine Yeast Strains Genetically Engineered to Produce Lactic Acid

Engineered Saccharomyces cerevisiae strains over-expressing a bacterial lacticodehydrogenase (LDH) have been described to perform mixed lactic acid-alcoholic fermentation under laboratory conditions The acidification properties of these strains might be of great value for fermented beverages and especially in enology to solve problems of insufficient acidity in hot regions. To assess the relevance of this model during alcoholic fermentation under enological conditions, eight wine strains expressing the LDH gene on a multicopy plasmid were constructed. The level of L(+) lactic acid produced and the resulting acidification were shown to be influenced by the strain and to a greater extent by the grape must composition. Wines obtained by lactic acid-alcoholic fermentation of seven grape musts contained 2.6 to 8.6 g/L of L(+) lactic acid. In comparison with classic alcoholic fermentation, lactic acid-alcoholic fermentation resulted in a decrease of 0.2 to 0.35 pH units in wines containing 5 to 8 g/L of lactic acid. Two grape musts with moderate or low acidity levels (pH 3.36 and pH 3.75) were acidified effectively (50% increase in total acidity, 0.17 and 0.27 pH unit decrease) by the production of 5 g/L of L(+) lactic acid by an engineered wine yeast strain. A reduction of 0.25 percent (vol/vol) of alcohol was also observed as a result of the diversion of carbon towards L(+) lactic acid. The volatile acidity was unchanged in the wine obtained with the engineered strain compared to the control strain. Despite a slower fermentation rate, the ability to ferment grape musts and the growth characteristics were not affected.