Bourbon - Yeast Management
Each distillery in Kentucky and Tennessee has their own yeast strains, which have survived prohibition from 1919 to 1933 in cooled rooms. The yeast strains are so much kept as a secret that the companies have filed patents for their isolated yeasts, which is possible in the United States, unlike in Germany, for example.
Historically, selecting the yeast was a simple process. Containers with a nutrient solution were placed under an apple or pear tree. After some time, natural yeasts on the fruits also accepted the nutrient solution. Small samples of the nutrient solution were then put on carriers, and the individually appearing yeasts were bred in ovens at 35° to 40°C (95° to 104°F). From a single yeast cell you can breed the whole yeast for the alcoholic fermentation.
From a test tube about a pencil tip of yeast is extracted and then mixed with a nutrient solution (malt extract) in a bulb. The pH of the solution must be between 5.4 and 5.8 for yeast reproduction, depending on the yeast.
All objects used for yeast reproduction must first be sterilised in an autoclave so no vinegar bacteria or foreign yeasts contaminate the desired yeast strain.
After approximately half a litre of pure yeast has been produced in this bulb, the yeast is put in a larger container, the so-called 'dona tub'.
A large amount of yeast is produced in the dona, so that later a large container can be filled, which serves as storage tank for all the fermenters.
The distilleries usually produce different bourbons with different yeasts. Therefore each whiskey has its own yeast tank. Yeast reproduction in the dona is only carried out occasionally, when the yeast stock has been contaminated by vinegar bacteria or foreign yeasts.