section: Wine
question: Why are pure yeast cultures added to the must?
article: Yeast
candidates: Yeast is used in winemaking, where it converts the sugars present in grape juice (must) into ethanol. Yeast is normally already present on grape skins. Fermentation can be done with this endogenous "wild yeast," but this procedure gives unpredictable results, which depend upon the exact types of yeast species present. For this reason, a pure yeast culture is usually added to the must; this yeast quickly dominates the fermentation. The wild yeasts are repressed, which ensures a reliable and predictable fermentation. Most added wine yeasts are strains of "S. cerevisiae", though not all strains of the species are suitable. Different "S. cerevisiae" yeast strains have differing physiological and fermentative properties, therefore the actual strain of yeast selected can have a direct impact on the finished wine. Significant research has been undertaken into the development of novel wine yeast strains that produce atypical flavour profiles or increased complexity in wines. The growth of some yeasts, such as "Zygosaccharomyces" and "Brettanomyces", in wine can result in wine faults and subsequent spoilage. "Brettanomyces" produces an array of metabolites when growing in wine, some of which are volatile phenolic compounds. Together, these compounds are often referred to as ""Brettanomyces" character", and are often described as "antiseptic" or "barnyard" type aromas. "Brettanomyces" is a significant contributor to wine faults within the wine industry. Researchers from University of British Columbia, Canada, have found a new strain of yeast that has reduced amines. The amines in red wine and Chardonnay produce off-flavors and cause headaches and hypertension in some people. About 30% of people are sensitive to biogenic amines, such as histamines.