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Brewers Yeast
Yeast Viability and Replacement
Yeast Viability
Viability is a measure of yeast’s ability to ferment. Yeast viability is determined by the standard-culture method, by selective staining, or by more advanced methods such as the Slide Viability Method, flocculation tests, and fermentation tests.
The standard slide-culture method of determining viability of yeasts has three steps: perform a hemacytometer count on a suspension of cells, plate a measured quantity on a wort gelatin medium, and then incubate and count the resultant colonies (19). However, this method is inaccurate due to cell clumping and the death of cells during preparation.
Yeast Replacement
Most brewers discard yeast after a successive number of fermentations because it may be intermixed with other yeasts, contaminated with wild yeast and bacteria, or mutated to less desirable strains. Depending on their yeast-handling facilities and procedures, some brewers use their yeast in production for less than three generations, while others only discard their yeast after 5 to 10 successive fermentations. However, there are exceptions to the rule, with some brewers routinely discarding yeast after 30 brewery fermentations (46). Those brewers who employ high pitching rates will probably have to replace their yeast more often because continuous use and high pitching rates tend to increase the average age of the yeast, thus reducing their vigor (22).
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