The yeast, Saccharomyces cerevisiae, is a fungus, one of the group of eukaryotes (organisms with membrane- enclosed organelles and nuclei in their cells) that lie on that branch of the tree of life that, as shown in Figure 18.1, includes plants and animals. Many years of debate preceded their notation as a separate branch on the tree while advocates forcing them into either plant or animal families battled. Thus, although the cell walls of yeast are strikingly similar to plants (save that yeasts utilize N-acetylglucosamine and related nitrogenous carbohydrate polymers [chitin-like] in place of polyphenols [lignin] for cross linking), it is clear that chloroplasts, common to plants, are missing. Similarly, while their organization and food disposition is similar to animals, the very presence of a cell wall, rather than a simple membrane, forces their exclusion from the family of animals. Of course, all life utilizes the same set of purine and pyrimidine bases bonded to a ribose or deoxyribose carbohydrate and amino acids. So while classifications are necessary, they may also be specious. A generic eukaryotic cell and a plant cell (seen before in Figure 7.1) are shown in Figure 18.2. Hundreds of yeasts and strains of those yeasts are available for use in the wine industry for fermenting the must obtained on crushing the grapes. Some of the yeasts are referred to as “wild” and are brought in with the grapes from the vineyard. Others, originally “wild,” have been isolated and maintained because it is held that their use adds value to the vintage. Indeed, it is here that a great deal of experience is required. Generally, the vintner has a good idea of the amount of sugar (measured as glucose) in the grapes harvested. However, different strains of yeast (some 1500 yeast species, including S. cerevisiae are a subgroup of 700,000 or so fungi), while probably processing glucose in the same way, will also process other sugars too and, in that vein, there are other issues to be faced.
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