Yeast Viability and Replacement
Viability is a measure of yeast's ability to ferment-a property not possessed by dead cells. Yeast viability is determined by selective staining, by the standard-slide culture method, or by more advanced methods such as the Slide Viability Method, and fermentation tests.
A more objective result of yeast viability is obtained by selective staining using buffered methylene blue or methylene violet. These stains show dead cells as blue or pink, respectively, on microscopic examination. Various other dyes have been recommended from time to time, such as methyl green, acridine orange, neutral red, and erythrosine.
Standard-Slide Culture Method
A more accurate method in determining the viability of yeast is the standard slide-culture method that consists of 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 (22).
Slide Viability Method
In the Slide Viability Method, cells are suspended in a growth medium containing 6% gelatin, and the suspension is placed on a hemacytometer slide (22).
Although the above methods can assess whether a cell is alive or viable, they cannot readily assess the vigor of the yeast and whether they can produce a normal fermentation.
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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