Factors Affecting Sugar Utilization and Rate of Fermentation During Vinification

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.

Investigation of pattern of utilization of nitrogen compounds and urea

A field experiment, fertilized at seven different nitrogen levels was tested. All the samples responded in a similar manner with no significant effect on rates of fermentation or amounts of residual urea. This was the first year of the treatments and for all practical purposes was base line information. No differences were noted between on skin or off skin fermentations due to treatments. The results of ten dessert grape fermentations made from Davis grapes showed that two of the varieties ended up with extremely high urea values of 98 and 134 mg/L. This is due to high nitrogen values in the grape and failure to use all the assimilable amino acids.

Volatile Sulfur Compounds: Incidence and factors affecting their formation in Californian Wines.

Research Accomplishments: As previously suggested, nitrogen-deficient grape juices produced higher concentration of hydrogen sulfide. However, despite the examination of musts reported to have produced serious sulfur spoilage problems and which were shown to be nitrogen deficient, no wines were produced which contained serious volatile sulfur spoilage problems as shown in these two studies by the absence of mercaptans. The role of nitrogen deficiency in musts or of different yeast strains in the production of sulfur compounds, despite very rigorous analytical analyses of precisely controlled fermentations is still unclear. As demonstrated by the results of the last two years, no simple relationship between must composition and production of sulfur off-flavors exists. Complex factors which require painstaking and systematic study appear to regulate the production of these compounds.

Winemaking Without Sulfur Dioxide

Spoilage organisms: Zygosaccharomvces and Kloeckera, whose presence in wines seems to be increasing as use of sulfur dioxide is decreasing, were found to be reticent to control at levels which controlled all other spoilage organisms. Studies carried out during this year show that both of these organisms can be efficiently controlled by carbon monoxide but at much higher levels than those employed to control Brettanomyces, Dekkera, and Hansenula.