Factors Affecting Sugar Utilization and Rate of Fermentation

Many environmental and physiological factors influence fermentation performance of Saccharomyces. Nitrogen or oxygen limitation result in sluggish or stuck fermentations, albeit for different reasons. Other factors are also important and interactive. For example, a micronutrient (minerals or vitamins) limitation may exacerbate a nitrogen shortage. Often these interactive effects are not predictable from studies of the effect of the factor in isolation. Our goal is to go beyond empirical observations and studies of one variable in defined media to understanding the biochemical basis of the yeast response. Sugar transport is the site of action of factors affecting fermentation performance such as nitrogen or nutrient limitation, yeast strain, phase of growth, temperature and possibly pH. Transport of sugars in Saccharomyces is complex and a function of the activity of different transporter proteins working in concert. The timing and level of expression of these proteins can be manipulated. In this research program, we are manipulating the level of activity of individual transporters via mutation and over expression, and assessing the effect of that genetic change on ability to complete a grape juice fermentation. Our focus is on the regulation of sugar transport by nitrogen limitation so that eventually yeast strains can be engineered that will complete a fermentation regardless of the nitrogen signal.