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Preventing sluggish or stuck fermentations which may be caused by sugar uptake deficiency in saccharomyces cerevisiae wine yeast strains by using glucose isomerase

Ünal, Nahide Seray
The objective of this study was to prevent stuck fermentation, which may be caused by sugar uptake deficiency in yeast by using glucose isomerase, whose activity was tested in different media. The unfermented grape juice contains almost equal amounts of glucose and fructose. After fermentation, amount of residual fructose is higher than that of glucose and the glucose/fructose discrepancy (GFD) occurs. One of the main reasons of GFD is that affinity of hexose transporters of yeast towards fructose decreases as the accumulation of ethanol increases. Due to formation of ethanol, fermentation slows or stops even if there is residual sugar in wine medium. This is called stuck fermentation. This is an undesirable situation since residual sugar causes unwanted sweetness and increases the risk of microbial spoilage. To prevent GFD, using glucose isomerase during or after wine fermentation may be a solution. The activity of this enzyme was tested in synthetic and original wine media to find out whether it can be a solution for stuck fermentation or not. Glucose formation, 0.5% w/v, from 1% w/v fructose occurred in synthetic medium containing 13% v/v ethanol and 1% v/v glycerol at pH 3.3 and at temperatures of 60ºC or 30ºC in approximately 48 hours. However, the glucose formation did not occur in synthetic medium if there was 0.3% w/v tartaric acid at pH 3.55 whereas glucose was formed at pH 6.33. In original wine medium, the glucose formation did not occur except that wine media with dilution factor of 20% v/v, 10% v/v and 5% v/v and at pH values equal or higher than 6 whether there was tartaric acid or not. Since dilution and increasing the pH of wine cannot be applicable, other ways to employ this enzyme should be tried.