This week I take a look at yeast autolysis which is the decomposition of yeast, and how it affects beer flavor as your beer ages.
Yeast Autolysis
The word autolysis literally means self-destruction, and yeast autolysis is the final stage for a yeast cell life where the yeast cell wall literally bursts open and releases a variety of enzymes and chemicals contained in the cell.
Yeast autolysis is not a problem during fermentation or even early aging, as the yeast cell is remarkably robust and yeast cells to fall out of the beer in the first few weeks and sediment at the bottom of the fermenter. It is typically caused by either very long periods of aging on the yeast. However it can also be caused by other handling issues I will mention below.
Yeast Autolysis in Beer
Fermenting “on the lees” (“Sur lie”) or on the yeast is a technique widely used in wine making, including some of the most popular white wine styles including Chardonnay, Champagne and many French styles. In white wine it adds a bit of creamy, rich depth including some sweetness, caramel, clove and umami flavors. In sparkling wines it adds some bread-like, cheese and buttermilk aromas.
Unfortunately the flavor changes from yeast autolysis in beer are not usually favorable. Decomposition of the yeast cells often results in a “yeast bite” which is a bitter, sharp flavor with a sulfur overtone to it. The fatty lipids released from the cell that give wine a creamy finish instead result in a “meaty” or barbecue aroma and finish in beer.
Yeast autolysis also increases the pH of the finished beer which can result in significant flavor profile changes and can also cause problems with the long term stability of the beer.
Avoiding Yeast Autolysis
Because of all the reasons listed above, yeast is typically separated from the beer within a month or so of the end of fermentation. Typically this is a concern mainly for beers with extended aging, as other beers are often packaged or kegged before autolysis can really settle in.
Professional brewers, and many modern homebrewers use conical fermenters which makes it easy to draw off yeast as the yeast begins to sediment at the bottom of the fermenter. The same can be accomplished by simply transferring your beer to a secondary fermenter after the sediment has formed to separate the beer from the yeast. Since it typically takes several months for autolysis to occur, simply separating the yeast from the beer is a great way to prevent it.
I will mention that there are a few other rarer causes of yeast autolysis. Fermenting at high temperature (above 25 C or 78 F) can also cause autolysis with many strains. Suddenly changing the temperature when pitching your yeast can also cause autolysis. Finally there is something called osmotic shock which can occur when you pitch yeast directly into a very high gravity wort without first acclimatizing it that can also cause autolysis. These causes are less common, but can occur and leave you with a yeast bite.
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