Autolysis. Brewing yeasts are remarkably robust organisms armed with strong cell walls and protective abilities to survive in acid conditions and alcoholic solutions. However, they are not immortal and will eventually succumb to a variety of stresses during fermentation, conditioning, and prolonged storage. Autolysis is the final result of these stresses, where the vacuolar membranes inside the yeast disintegrate; releasing hydrolytic enzymes that cause the cells to burst open, releasing the contents of the cell into the beer. The word “autolysis” essentially means “self-destruction.”
This autolysis has a number of consequences, the two most important of which are flavor changes and enzymatic digestion.
Flavor changes are readily noted as the general characteristic sometimes termed “yeast bite.” This is generally a sharp, bitter taste with a meaty and sulphury edge caused by some of the amino acids and nucleotides present in yeast. If yeast is in high concentrations, these compounds can increase the pH of the beer and alter its acidity, also changing flavor. Finally, lipid release may increase the chance of rancidity. The meaty aroma of autolyzed yeast is so powerful that it is an important flavor additive in the food industry, adding “meat” flavor to everything from soups to “barbecue flavor” potato chips.
Enzymatic digestion is particularly related to proteases which leak from the cell and then digest the proteins in beer. A major consequence of this is a reduction in head retention, resulting in turn in a rapid collapse of foam and the impression of a flat beer. Hazes may be accelerated. In bottle- conditioned beers, enzymatic digestion of complex sugars into simpler sugars can actually re-start fermentation by still-living yeast cells in the bottle, causing over-carbonation and other problems.
Many causes may be cited for autolysis, not least simple old age. However, poor handling of yeast and beer will accelerate autolysis. Common examples are high temperatures, particularly above 25°C, or sudden changes in temperature at pitching or at chilling, and osmotic shock where yeast is pitched into high gravity worts. Some yeast strains may be inherently sensitive to conditions such as high alcohol, high carbonation, and high acidity and autolyse faster than others. In other conditions extensive re-pitching of yeast from batch to batch may create stress, as can the presence of contaminants including lactic acid bacteria and other yeast species.
While autolysis flavors are generally considered negative in beer, they are often considered positive in wine, especially vintage Champagne, where they comprise a large part the famed “sur lie” (on sediment) flavor and aroma. This aroma is often described as “toasty” or “hazelnuts” in the wine context, and the flavors are considered to be consistent with umami characteristics. Similarly, when bottle-conditioned beers are aged on yeast, similar flavors can eventually arise from autolysis, and in balance with other flavors and aromas, they can be very pleasant.