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Methylene blue stains the dead yeast cells which then appear as dark blue cells.
This could look like shown in Fig 1. The dark blue cells are dead, the other ones still alive.
Fig 1: Methylene blue stain of yeast sample
To perform a viability count, mix equal parts of your yeast solution with a 0.1% (w/v) methylene blue solution.
Mix 0.1 g of methylene blue and dissolve it in 100 mL of distilled water.
Mix well and let it react for one minute then count the cells by use of a counting chamber.
Viability [%] = (Total counted cells – total counted dead cells) / total counted cells x 100