Despite the apparent simplicity of the wine fermentation, as evidenced by so many successful outcomes, there are occasions when the fermentation fails. Such wines typically contain a significant amount of residual, unfermented sugar and an insufficient concentration of ethanol. The fermentation may actually still be occurring, just more slowly, or it may be at a complete standstill. These slow or halted fermentations are referred to as sluggish or stuck fermentations, respectively.Although they occur infrequently, they represent a significant problem and source of economic loss for the wine producer, since wine quality will inevitably be poor. Stuck wine is especially susceptible to spoilage, since the low ethanol concentration and the availability of fermentable sugar may promote growth of undesirable bacteria and yeasts. Under more severe conditions, the wine may simply have to be discarded.Therefore, despite their rare occurrence, it is important to understand the causes and to know how to prevent sluggish or stuck wine fermentations.
Among the possible causes of a stuck fermentation are those that are due to the must composition, the handling of the must and wine, or the presence of wild yeasts that inhibit desirable wine yeast. The must may contain, for example, an insufficient level of nitrogen or other nutrients necessary to support adequate yeast growth. The sugar concentration in the grapes or must may be too high, resulting in osmotic pressures that inhibit the yeasts. Some yeasts also are inhibited by high ethanol con-centration.As noted previously, the ethanol fermentation is exothermic, and if the temperature is not controlled or cooling is inadequate, the resulting high temperature (>30°C) may cause the fermentation to come to an abrupt halt. In contrast, too cool an incubation temperature (e.g., <10°C),as might occur during white wine production, can also result in a stuck fermentation. Most of these situations, however, are easily corrected, either by supplementing the juice with appropriate yeast nutrients, using osmotolerant or ethanol-tolerant yeast strains,or by proper temperature control.
Finally, some wild yeast strains secrete proteins called killer toxins that inhibit or kill other indigenous or starter culture yeasts. There are more that five types of killer toxins, but the most common are K1 and K2. These proteinaceous toxins first attach to cell wall receptors, then integrate and form pores within the cytoplasmic membranes of sensitive cells, thereby disrupting ion gradients and interfering with energy-transducing reactions. Producer strains resist the toxin they produce, but are sensitive to those toxins produced by other strains. Some yeast strains do not produce killer toxins, but are nonetheless resistant. Several yeast genera, including species of Hansenula, Pichia, and Saccharomyces, are able to produce killer toxins. Importantly, so-
called killer yeasts are found throughout the wine environment, and even many of the Kloeckera and S. cerevisiae strains isolated from natural wine fermentations have the "killer" property. In fact, since yeast strains with killer toxin activity can potentially inactivate competing strains, this trait may be desirable for yeast starter cultures (Box 10-6). Certainly, starter culture strains that are immune to these toxins would not be affected by other killer strains and would not be the cause of a stuck fermentation.
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