When a yeast cell becomes exhausted and dies, it slowly sinks to the bottom of the fermenting vessel where a very complex series of chemical reactions collectively known as autolysis begins. Proteolytic enzymes secreted within the cell itself become active rupturing the cell and gradually degrade the cell into its basic constituents. The products of decompositions are released into the beer, wine, mead or cider. Although many of these substances are utilized by the living cells which are still active, certain nitrogenous substances liberated at the same time are less desirable.
When there is a large mass of yeast on the bottom of the fermenter, the potential for off flavors due to yeast autolysis is high. A lightly autolyzed product will have yeasty or brothy aroma and flavor. Moderate autolysis will be perceived as a more meaty aroma and flavor. A heavily autolyzed product will have a rubbery aroma/flavor and will be virtually undrinkable. Lard-like, fatty or soapy characteristics are also contributions of yeast autolysis as the dormant yeast cells explode their fatty acids and lipids into the beer, wine, mead or cider.
Autolyzed yeast characteristics are not desirable in beer. Soapy flavors will result from the break down of fatty acids in the trub when leaving the beer on the yeast too long after primary fermentation is complete. Long depends on the style and other fermentation factors If yeast is unhealthy and begins to autolyze compound described as yeasty, beef bouillon, or a jar of Vitamin B will be released. As a rule of thumb, ales should be racked off flocculated yeast in about two weeks. Due to the cooler fermentation temperature and yeast strain, lagers will take a bit longer.
In general a limited degree of yeast autolysis is beneficial to a wine. This process must not be overdone otherwise the flavor of the wine will be seriously impaired. Rack at regular intervals, that's three weeks after racking into the carboy and every three months there after, to avoid this later danger by removing accumulated dead yeast cells before autolysis has progressed too far.
During the final step in champagne production yeast converts sugar added to the cuvee into alcohol and carbon dioxide. The carbon dioxide which is unable to escape gradually builds up in the bottle until the supply of added sugar is exhausted. Simultaneously, a small deposit of yeast is formed. During this time yeast autolysis occurs and decomposition products, notably amino acids, are released into the wine with beneficial effects on bouquet and flavor. Most still wines would be spoiled by such prolonged contact with autolysing yeast. But, the special properties for which the yeast is cultured and abnormal conditions under which the autolysis occurs ensures that the opposite is true in champagne production. It can be concluded that it is the presence of carbon dioxide under pressure and the use of a special champagne yeast that enhances the character of a sparkling wine made by the methode champenoise.
Sherry is another type of with which benefits by prolonged contact with autolysing yeast. Here again, a special yeast is used and a rather unusual system of maturation is practiced.