Stuck Fermentation

A stuck fermentation occurs in brewing beer or winemaking when the yeast become dormant before the fermentation has completed. Unlike an "arrested fermentation" where the winemaker intentionally stops fermentation (such as in the production of fortified wines), a stuck fermentation is an unintentional and unwanted occurrence that can lead to the wine being spoiled by bacteria and oxidation. There are several potential causes of a stuck fermentation-the most common are excessive temperatures killing off the yeast or a must deficient in the nitrogen food source needed for the yeast to the thrive. Once the fermentation is stuck, it is very difficult to restart due to a chemical compound released by dying yeast cells that inhibit the future growth of yeast cells in the batch. At the winery winemakers take several steps to limit the possibility of a stuck fermentation occurring, such as adding nitrogen to the must in the form diammonium phosphate or using cultured yeast with a high temperature and alcohol tolerance. These steps that winemakers may take to prevent a stuck fermentation will each have their own subtle or dramatic effect on the resulting flavors and quality of the wine.