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.
SUMMARY AND RESEARCH ACCOMPLISHMENTS: Preliminary results indicate that although there is indeed an increase in the aroma/bouquet of the wines whose volatiles have been added back, some of the panelists agreed in that “the increased aroma and bouquet do not necessarily translate into an increase in quality of the wines.” This was particularly true for white wines fermented at 80 deg F compared to those fermented at ’55 deg F.
Adenine supplementation stimulated fermentation rate in Montrachet at low (15°C) temperature by reducing the lag time to onset of fermentation, and by shortening the time to dryness. There appeared to be little impact on maximum fermentation rate. Prise de Mousse and Pasteur champagne were unresponsive to adenine supplementation. The effect of adenine was greatest at an intermediate concentration and not affected by nitrogen supplementation. Thus, the stimulation appears to be adenine-specific, not a simple consequence of the presence of extra nitrogen. Loss of two glucose transporter proteins SNF3 and HXT2, reduced rate of fermentation at the end of fermentation. The HXT1 glucose transporter does not play a major role during vinifcation. Interestingly, fermentations conducted by mutants lacking HXT2 were always over run with bacteria. Loss of this gene affected competitiveness of this yeast strain.