Practical procedures to measure yeast viability and vitality prior to pitching

It is common practice for brewers to calculate yeast inoculation rates based on some measure of cell mass or count combined with a simple viability stain, normally methylene blue. The limitations of this procedure are well known, as yeast performance cannot be predicted and methylene blue staining is notoriously inaccurate if the actual cell viabiity drops below 95%. In this research, we describe a series of steps recommended to overcome the problems associated with the normal pitching procedures. The wet weight of a slurry or some measure of its biomass is first determined. The pH of the slurry is then measured and, if found to be significantly higher than the end beer pH, the slurry has undergone extensive autolysis and is rejected for reuse. If the slurry passes the pH test, it is then subjected to a protease test. If the value generated from this assay is above the set specification, the slurry is discarded due to a loss in cell integrity and an increase in cell permeability, both indicative of poor yeast quality. If the slurry passes the protease test, it is subjected to a vitality assay, the Magnesium Release Test. Should the slurry also pass the set specification for this test, it is reused for subsequent fermentations with a high degree of confidence in the performance of the yeast. This procedure was proven at scales ranging from laboratory (2 L) to plant (3,000 hL) volumes, and can lead to proactive fermentation control, predictive fermentor residency times, and better quality end beers. The methods are all simple, rapid, and easily implemented in commercial brewery laboratories.