There are three basic sources of the macromolecules that may be found in wine. First, they may arise from the grape itself and be present in the form of polysaccharides and proteins. Second, they may arise from Botrytis growth and be present in the form of glucans. Third, they may arise from yeast, specifically the mannoproteins of yeast, which will be the focus of this lecture. Yeast cell walls are composed of chitin (1 to 2%), β-1,3 glucan (25%), β-1,3 glucan bound.to chitin (35%), β-1,6 glucan (5%), and mannoprotein (35%). Mannoproteins found in the yeast cell wall are bound to glucans by covalent as well as noncovalent bonds. When found in wine, mannoproteins exist as polysaccharide and protein moieties. They are released from the yeast cell wall by the action of an enzyme, β-1,3 glucanase, upon the wall. This enzyme exhibits activity during yeast growth (fermentation) as well as during aging in the presence of nonmultiplying yeast cells. The amount of mannoprotein released during fermentation is dependent upon several factors, of which yeast strain and must turbidity are paramount. During aging on lees, polysaccharides can be hydrolyzed by the liberated β-1,3 glucanase. Mannoproteins are thought to encourage the growth of malolactic bacteria, contribute to protein and tartrate stability of white wines, interact with aroma and phenolic compounds of red wines, decrease the astringency and bitterness of tannins, and increase the body of wine. They may have utility in bottle fermentation of sparkling wine, barrel aging of still wines, and in the production of flor sherry-type products.
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