STUCK FERMENTATION

During mead fermentation, several problems are generally encountered. For example, the anticipated alcohol content may not be achieved within the time desired. There may also be a lack of uniformity in the final product, due to differences in water content of the honey used. In some situations, such as worts with high sugar contents, successive addition of honey is needed to avoid premature termination of fermentation. This likelihood of stuck fermentation is increased as most mead is made empirically, without adjustments. This can lead to subsequent yeast refermentation and secondary fermentations by lactic and acetic acid bacteria. These can undesirably increase acidity and the production of volatile esters (Casellas, 2005). The presence of these compounds alters... 

The late Edmund Henri Twight, who worked for the California Wine Association in the late 1890s, told the writer (about 1936) thathe recalled huge amounts of sweet-sour wine in California at that time—perhaps 20 percent, he estimated. There is considerable other evidence that incomplete (stuck) fermentations and unsound faulty wines were common in the early 1900s. This... 

Sablayrolles, J.M., Dubois, C., Manginot, C., Roustan, J.L., Barre, P. (1996). Efectiveness of combined ammoniacal nitrogen and oxygen additions for completion of sluggish and stuck fermentation. J. Fermen. Bioeng., 82, 377-3S1... 

Ivorra, C., Perez-Ortin, J.E., and del Olmo, M. 1999. An inverse correlation between stress resistance and stuck fermentations in wine yeasts. A molecular study. Biotech. Bioeng. 64, 698-708. 

The Ce, Cg and Cio fatty acids are formed by yeast. As they are fermentation inhibitors at concentrations of only a few mg/1, they may be responsible for stuck fermentations (Volume 1, Section 3.6.2). 

The same lactic bacteria, however, are likely to break down sugars. The consequences may be serious, especially if the wine has a high sugar concentration. The most common situation occurs when alcoholic fermentation stops, leaving the sweet medium open to lactic bacteria. For this reason, winemakers take great care to avoid stuck fermentations, although the bacteria may also take over just before the end of fermentation if it has slowed down. The initial result of this bacterial contamination is malolactic fermentation, but lactic spoilage may follow. This situation must be avoided, especially as the development of these bacteria may prevent the completion of alcoholic fermentation. 

Practical applications of alginates concern mainly secondary fermentation inside the bottle in sparkling wine production for easy clarification and removal of cells Other applications are the treatment of sluggish and stuck fermentations ... 

Although acetic acid is known to be inhibitory to some strains of fermentatively growing Saccharomyces (Rasmussen et al., 1995 Fugelsang et al., 1993), it is generally believed that other inhibitory compounds are produced as well, and the final stuck fermentation represents the contribution of these multiple insults (Edwards, 1996). However, the nature of these additional inhibitors has yet to be determined. 

Fusel oil formation varies with yeast strain, temperature of fermentation, pH, nutritional status, suspended solids level, and oxygen concentration of juice/must. Under oxidative conditions as would occur before the onset of alcoholic fermentation, or in cases of stuck fermentation, Pichia, Hansenula, and Candida may produce substantial quantities of fusel alcohols from fermentable sugars. The fusel alcohol, 2-phenylethanol (arising from 2-phenylalanine), has the unmistakable odor of roses and is also... 

The causes of sluggish and stuck fermentations include fermentation at temperature extremes, nutritional deficiencies, osmoregulation, ethanol toxicity, and in low-temperature fermentations, long-term anaerobiosis. To these classically ascribed sources can be added failures in adequately preparing yeast starters as well as the presence of inhibitory compounds which may include pesticides and those produced by microorganisms. 

During harvest (and particularly in cool growing regions), it may not be feasible to deal with stuck fermentations immediately. In these cases, the wine should be stabilized against further biological deterioration by draw-... 

Where stuck fermentations have been allowed to set over winter, few, if any, viable yeast remain. In these cases, the inoculum must be increased from normally recommended 2-4 x 10 CFU/mL upward to 16-20 x 10 CFU/mL, corresponding to 8-10 lbs/1000 gal (Cone, 1996 personal communication). Additionally, dietary supplements at 2-4 lbs/1000 gal are also recommended. 

It should be noted that Saccharomyces is glucophilic that is, the yeast prefers glucose over fructose. As such, glucose present in a grape must will be exhausted before complete utilization of fructose. In that fructose is sensorially sweeter than glucose, this may be of importance in the case of stuck fermentations (Section 8.5.1). 

Juice and wine pH play a mjgor role in the development of yeast and bacterial populations. Species respond differently in terms of growth rate as well as production of sensorially important metabohtes. In the case of Saccharomyces, growth and fermentation rates slow as pH decreases to near 3.0, concomitantly increasing the risk of sluggish or stuck fermentations. Kudo et al. (1998) found that the relative concentration of and H in a grape must plays an important role in successful completion of alcoholic fermentation, with a minimum of 25 1 being necessary. Whereas... 

Causes of sluggish (slow) and stuck (stopped) fermentations include nutritional deficiency, inhibitory substances, processing difficulties, as well as bacterial antagonism (Section 6.6.2). Although various factors can contribute to a sluggish or stuck fermentation, the exact cause (s) of a particular occurrence cannot always be identified. 

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