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A word or two on yeast viability

Viability is the key measure of yeast health in brewing and it is used to correct pitching rate, assess fermentation performance and the impact ofin-pro- cess handling. As noted above, the ‘gold standard' is the vital stain methylene blue, which is invariably used to benchmark and calibrate sophisticated methods such as radiofrequency permittivity and other up-and-coming methods. However, despite its position and longevity, methylene blue has its criticisms. Most notably, vital staining infers the cell is a metabolically viable cell but throws no light on whether the cell can divide and replicate. Accordingly, cell division is routinely tested by a functional test involving the plating of samples on nutrient agar. This approach is not ‘real time' and takes two or more days incubation to enable a viable cell count. Additionally, plate counts do not necessarily support the growth of all microorganisms in the sample as the media is (intentionally) selective but also the organisms themselves may be physiologically non-culturable for a host of reasons.

Given the debate about measuring yeast viability, it is (with hindsight) somewhat bizarre that an additional concept - vitality - has been added to the mix as a measure of yeast physiological state. Many diverse vitality tests have been proposed to ‘assess fitness of individual batches of yeast to pitch, either in a go, no-go approach, or as predictors of subsequent fermentation performance, which preferably allows selection of optimum pitching rates and/or wort oxygenation' (Boulton, 2012). Tellingly, the same author concludes that ‘there is little evidence that they provide more information than a simple viability test such as the usual counting of unstained and stained cells treated with the vital dye methylene blue.

Practically, rightly or wrongly the methylene blue method for yeast viability is firmly embedded in the global brewing industry. Perhaps we should simply pick up on Johnny Mercer's lyrics from 1945 and ‘accentuate the positive, eliminate the negative and latch on to the affirmative, do not mess with Mister In-between.

 
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