It is usual to select strains of yeast for brewing from yeasts already in commercial use. For continuous fermentation using unstirred towers, it is necessary to have a yeast which is strongly sedimentary throughout the fermentation in order to maintain a yeast plug at the base of the tower. Some breweries isolate, select and maintain their yeast strains but others engage specialist laboratories to provide this service. Again, some breweries choose to have two or more strains that may be employed in mixture or separate fermentation vessels. Nevertheless, a yeast of several strains may adapt more successfully than a single clone.
Sterile wort is run into a vessel that has been presterilized by steam and cooled.
Sterile air or oxygen is perfused through the wort, and the culture of yeast from the laboratory is inoculated. At approximately 18C exponential growth occurs for 48 hr when some 54 kg of pressed yeast is available in partly fermented beer of specific gravity 1.016. Propagation of brewers' yeast enables a brewery to replace the entire stock of yeast on a predetermined basis. Frequently, a batch of yeast is used only about 12 times before it is discarded. The changes in a yeast that persuade brewers to discard them relate either to infection with bacteria or wild yeast, poor settling near the end of fermentation if a bottom yeast, or partial loss of ability to grow, ferment, and produce the expected quality of beer.