Alcohol fermentation uses yeast under anaerobic conditions, that is excluding air. When yeasts ferment in the presence of air - aerobic fermentation - Little alcohol is produced. There are many useful strains of yeast , and pure cultures are normally used in controlled fermentation , with many breweries and wineries maintaining proprietary strains. A special mold , Botrytis cinerea , sometimes grows on grapes and contributes a unique flavor to certain wines . The mold helps moisture to evaporate , concentrating the sugars and leading to a sweet wine.