The disc stack centrifuge and its impact on yeast and beer quality

The impact of a disc stack centrifuge on brewers' yeast (Saccharomyces cerevisiae) integrity was studied. The physiological state of brewers' yeast is principally dependant on the processing conditions it encounters during wort fermentation and yeast cropping. During cropping by centrifugation, the yeast is subjected to extreme shear stress. The aim of this project was to determine whether the passage of yeast cells through a disc stack centrifuge had a negative effect on cell viability and vitality. Cell viability was determined using flow cytometry and methylene blue staining. Flow cytometeric methods permit quantitative representation of a cell population. The results were compared, and both techniques produced similar results. However, viabilities determined by methylene blue were consistently lower than those determined by flow cytometry. The viability of both the ale and lager yeast strains analyzed decreased as they were cycled through the centrifuge. The ale yeast used was more susceptible to centrifugation damage than the lager yeast strain. In the fermentation trials with the ale yeast strain, the centrifuged cells exhibited slower fermentation rates and increased proteinase A (PrA) activity at the beginning of fermentation. PrA degrades foam-positive polypeptides, resulting in decreased beer foam stability. Beer mannan concentration increased as exposure to shear stress was applied. It was concluded that passage of yeast cells through a disc stack centrifuge resulted in a decrease in cell viability, vitality, physical stability, and beer quality. © 2007 American Society of Brewing Chemists, Inc.