Studies on Stuck Fermentations and on Factors that Control the Rates of

We have found that the factor(s) that determine whether or not a fermentation will stick are vineyard-determined. If grapes from a given vineyard are crushed and the resultant must is placed in more than one fermentation tank, if one tank sticks all tanks from the same must also will stick. Stuck fermentations occur for all types of must and also occur for both inoculated and natural fermentations. We have found that viabilities of Saccharomyces cerevisiae cells in stuck fermentations are very low and this appears to be the reason the fermentations have stopped. We have also found that dilution of a stuck wine with water or even with a dry wine followed by addition of live yeast results in a reinitiation of fermentation which continues to dryness. We have considered six models that have been proposed to explain stuck fermentations. These models are: 1) nutrient depletion, 2) excess temperature during fermentation, 3) killer yeast, 4) toxin introduced with the grapes, 5) toxin produced by another microorganism and 6) toxin produced by the wine yeast. The dilution experiments eliminate model 1 and occurrence of stuck fermentations in temperature-controlled fermenters eliminates model 2. Experiments done by others eliminate model 3. The dilution experiments and the low viabilities of the wine yeast cells point to the presence of a toxin in the stuck wine. This toxin could be a fungicide brought in with grapes (model 4) or a toxin produced by another microorganism (model 5) or by the wine yeast (model 6). We believe model 5 may occur but is exceptional. Some fermentations become infected with acetobacter and the acetic acid kills or inhibits the wine yeast. We saw no other microorganisms in the 14 fermentations we studied and volatile acidity levels were near the normal range. We considered model 4 to be unlikely because one would expect the fungicide to act from the beginning of fermentation and this does not appear to be the case. Fermentation kinetics are normal until the time of sticking. This leaves model 6, a toxin produced by the wine yeast. This model was proposed several years ago by a group in Bordeaux and the toxin was proposed to be medium chain fatty acids, octanoic, decanoic and dodecanoic acid. These compounds are extremely toxic and are produced during fermentation. Several experiments done by others seem to lend strong support to this model. The goal of the study supported by AVF was to test this specific model. We studied 4 stuck fermentations and did gas chromatographic analyses of the stuck wine. We found that the concentrations of octanoic and decanoic acid were at or below normal levels in these stuck wines. This eliminates this specific model. We now believe that there may be another toxic compound produced by wine yeast and that this kills the cells and stops the fermentation. We are also now reconsidering model 4. If the fungicide brought in with the grapes needs to be modified during the fermentation process to be toxic to wine yeast this would explain the lag in onset of the stuck state. Glenn Andrade of Sutter Home Winery (personal communication) has found a correlation between spraying of the grapes with a fungicide and the occurrence of stuck fermentations.