Factors Affecting Sugar Utilization and Fermentation

The quantity as well as quality of available nitrogen during fermentation of grape juice appears to affect the rate of fermentation. If too high, rapid fermentation occurs generating excessive heat that may pose a problem or require temperature control. If too low, nitrogen limitation will result in a sluggish or even stuck fermentation. The site of control of fermentation rate is uptake of the sugar into the cell. Sugar uptake is mediated by specific transport proteins located in the plasma membrane at the cell surface. Five glucose transporter genes have been identified in my laboratory. The role of each of these genes in anaerobic grape juice fermentation is being analyzed. Simultaneous loss of two of these genes (SNF3 and HXT1) resulted in a yeast strain completing grape juice fermentation under conditions where the wild type parental strain yielded a stuck fermentation. This finding has important implications for the genetic engineering of a yeast strain for wine production that will be less likely to stick or fail to complete fermentation. In addition, it was found that adenine concentration can be stimulatory to fermentation increasing the maximum fermentation rate and decreasing overall time to dryness. The adenine effect was not a simple consequence of extra nitrogen, as supplementation with the same or higher levels of non-adenine nitrogen compounds did not result in the same stimulation. However, the adenine effect was both strain and temperature of fermentation specific. There was a greater effect with Montrachet than with Prise de Mousse or Pasteur Champagne, and a greater effect at warmer temperatures (20 versus 15°C). The effect of timing of addition of adenine on fermentation performance was complex, requiring additional studies to evaluate properly.