Methylene blue staining: use at your own risk.

O'Connor-Cox, E., Mochaba, F.M., Lodolo, E.J., Majara, M. and Axcell, B.

Abstract
Methylene blue dye is oxidized to a colourless compound by a reaction which only takes place in living cells, so in theory, when added to a cell suspension, it should stain all the dead cells and leave only the living ones unstained. However, in practice it does not always stain all the dead cells in a sample, leading to an overestimation of the number of living cells, which can lead to problems when it is used as a viability test for brewers' yeast. Although a number of other methods for evaluating the viability and physiological condition of yeast have been described in the literature, most of them are too slow and/or too complex to be suitable for routine use in breweries, and none of them can be regarded as ideal. A protease assay, originally developed in order to study the effects of proteases released from dead or damaged yeast cells on beer foam quality, has been experimentally shown to detect the presence of dead cells which were not stained by methylene blue. It has also been found that the pH of yeast suspensions rises when the number of dead cells increases, due to the release of the cell contents following autolysis. Recommendations for using the protease assay and pH measurements for evaluating the suitability of the yeast crop for reuse as pitching yeast and the effects on yeast quality of different handling methods and equipment types are presented.
Keywords : brewers' yeast enzymic activity error measurement pH proteolytic enzyme staining viability