Although alcohol is a major component of wine, there are many by-products of fermentation which will have a major impact on quality. Glycerol is not only produced by yeasts, but can also serve as carbon source in aerobically grown cultures. During wine fermentations, the main role of glycerol synthesis is to supply the yeast cell with an osmotic stress-responsive solute and to equilibrate the intracellular redox balance by converting the excess NADH generated during biomass formation to NAD+ (Pretorius, 2000). Glycerol-3-phosphate, the precursor of glycerol, is an essential intermediate in the biosynthesis of membrane lipids. 86 Fermented Beverages: Beers, Ciders, Wines and Related Drinks Glycerol is viscous and has a slightly sweet taste (threshold 5.2 g/l in wine), giving smoothness, consistency and overall body to the product, so if it is produced abundantly during fermentation, it is usually seen as a positive contribution. The amount of glycerol produced by yeast during fermentation will vary according to nitrogen composition, sugar levels, sulfur levels and pH of the grape must, as well as fermentation temperature, aeration, choice of starter yeast strain and inoculation level. Typically, under controlled conditions, glycerol concentrations are higher in red wines than in white wines, varying from 1 to 15 g/l. About 4–10% of the carbon source is usually converted to glycerol, resulting in glycerol levels of 7–10% of that of ethanol. The overproduction of glycerol by yeast at the expense of ethanol could fulfil a growing need for table wine with lower levels of ethanol. Conversely, wine yeasts in which the glycerol pathway has been minimized would yield more alcohol, which would be of great value for the production of brandy and other distilled products (Pretorius, 2000) (Chapter 3.6).