Chapter 13

Beer Fermentation

Yeast Collection

Aside from the need to remove most of the yeast from the beer prior to conditioning, yeast recovery for reuse in subsequent fermentations is an important process in the brewery. The total amount of yeast produced is dependent upon not only the yeast strain but also the level of aeration, the fermentation temperature, the specific gravity of the wort, and the pitching rate. Increases in any of these variables will lead to greater yeast crops.

Fermentation Systems

Traditional Lager Fermentation System

The traditional method of collecting lager yeast is to decant the "green" (unconditioned) beer from the settled yeast on the bottom of the open fermenters. The yeast is collected manually from the middle layer of the sediment on the floor.

Traditional Ale Fermentation System

Traditionally, top-cropped ale yeast was harvested by skimming the head of yeast/foam that formed on top of the beer in the shallow, flat-bottomed fermentation vessel. Generally, the second crop that forms towards the end of fermentation is the one that is harvested since the yeast is pure, with very high viability.

Cylindroconical Fermenters

Today, with the advent of cylindroconical fermenters, the differentiation on the basis of bottom and top cropping has become less distinct. Cylindroconical tanks allow improved yeast separation and collection strategies for both lager and ale yeast. The angle at the bottom of the tank allows the yeast to settle into the base of the vessel at the completion of primary fermentation. This aids in yeast collection from the bottom of the cone without exposing the yeast to outside air, leaving the beer comparatively free of yeast.

Yeast Storage

Upon harvesting, the yeast can be transferred directly to another fermenter, transferred to a yeast brink and held for the next fermentation, or stored with its own beer at 0C. The yeast will remain healthy while held at refrigerated temperatures, especially if the cells are kept suspended by gentle agitation so that "hot" spots do not accelerate cell death and autolysis. Small craft breweries may use nothing more than a white food-grade tub with a snap lid for storing the yeast.

Click on the following topics for more information on beer fermentation.