Making Yeast Make More of Nitrogen

Slow and incomplete fermentations which arise out of a deficiency of assimilable nitrogen are considered far from rare. When they develop, they reduce production efficiency and usually require winemaker intervention. Each year around the globe the quality of many millions of litres of wine is jeopardised. Strategies for the avoidance or correction of such fermentation problems typically involve treatments that increase the available assimilable nitrogen, either through supplementation (in the vineyard or winery), avoidance of nitrogen ‘losses’ or a release of assimilable nitrogen from more complex non-assimilable forms. As an alternative, the practice of using ‘nitrogen efficient’ yeasts so as to achieve greater sugar attenuation for a given amount of available assimilable nitrogen is becoming more widespread. Apart from greater fermentation reliability, ideally without the need for supplementation with nitrogen, such strains may also allow fermentation completion with reduced formation of undesirable odour compounds. Surveys of commercial strains for their relative requirement for assimilable nitrogen can therefore facilitate a more informed choice of strains by the winemaker. In addition, we have sought to determine the basis for such strain differences. To this end, mutant libraries have been screened to identify at least two genes whose deletion results in increased nitrogen efficiency. The precise role of these genes in nitrogen metabolism is now being determined. It is hoped that by exploiting the insights gained through these approaches, it will be possible to reduce the incidence of fermentation problems, achieve greater predictability of fermentation outcome and an increased, or at least better preserved, wine quality.http://cast.csufresno.edu/ve/veevents/Wine%20Micro%202006/Wine%20Micro%202006%20index.ht