Mankind has harnessed the abilities of yeast in brewing and bread making since the beginning of recorded history, and yeasts are probably one of the earliest organisms to be domesticated. However, it was only clearly established in the mid nineteenth century that conversion of glucose and fructose to ethanol and carbon dioxide (fermentation) is a microbial process. Pasteur, in around 1876, showed that fermentation did not arise spontaneously and isolated the yeasts responsible for it (Madigan et al., 2003). He also demonstrated the effect of oxygen on the assimilation of sugar as well as the production of secondary products by fermentation, i.e. glycerol and carbon dioxide. As noted in Chapter 2.1, the most significant species involved in winemaking and brewing are Saccharomyces cerevisiae. According to Ribereau-Gayon and coworkers (2000a), after investigating the delimitation of winemaking species, all the strains of ‘bayanus’ isolated in their study were found to belong to the species S. cerevisiae. On the other hand, yeast called ‘uvarum’ principally belong to the species S. bayanus. In short, yeast species involved in commercial fermentations for wine and beer comprise a very large number of genetically similar strains, with varied technological properties. Yeast strain can affect the rate of fermentation, the efficiency and success of conversion of sugar to ethanol, and the nature and quantity of by-products.