IMPROVEMENT OF NITROGEN ASSIMILATION AND FERMENTATION KINETICS UNDER ENOLOGICAL CONDITIONS BY DEREPRESSION OF ALTERNATIVE NITROGEN-ASSIMILATORY PATHWAYS IN AN INDUSTRIAL SACCHAROMYCES-CEREVISIAE STRAIN

Metabolism of nitrogen compounds by yeasts affects the efficiency of w ine fermentation. Ammonium ions, normally present in grape musts, redu ce catabolic enzyme levels and transport activities for nonpreferred n itrogen sources. This nitrogen catabolite repression severely impairs the utilization of proline and arginine, both common nitrogen sources in grape juice that require the proline utilization pathway for their assimilation. We attempted to improve fermentation performance by gene tic alteration of the regulation of nitrogen-assimilatory pathways in Saccharomyces cerevisiae, One mutant carrying a recessive allele of ur e2 was isolated from an industrial S. cerevisiae strain. This mutation strongly deregulated the proline utilization pathway. Fermentation ki netics of this mutant were studied under enological conditions on simu lated standard grape juices with various nitrogen levels. Mutant strai ns produced more biomass and exhibited a higher maximum CO2 production rate than the wild type. These differences wen: primarily due to the derepression of amino acid utilization pathways. When low amounts of d issolved oxygen were added, the mutants could assimilate proline. Biom ass yield and fermentation rate were consequently increased, and the d uration of the fermentation was substantially shortened. S. cerevisiae strains lacking URE2 function could improve alcoholic fermentation of natural media where proline and other poorly assimilated amino acids are the major potential nitrogen source, as is the case for most fruit juices and grape musts.