Evaluation of yeast activity and its application to our industrial brewing.

Fukui, N., Kogin, A., Furukubo, S., Kondo, H., Yomo, H. and Kakimi, Y.

Abstract
A procedure for evaluating the fermentative activity and general quality of brewers' yeast is described. The intracellular pH is calculated from the chemical shift value of 31P phosphate in the cytosol (measured by NMR spectroscopy). The leakage of protease from the yeast cells, which can lead to losses of foaming proteins and the consequent impairment of foam quality, is determined by an assay, using a fluorescent substrate, of proteolytic activity in the green beer from a trial fermentation, while the "appearance ratio" of the cell vacuole is evaluated visually using a phase contrast microscope. When four yeast strains were tested using these methods, only the one which gave the poorest results was classified as inactive by the methylene blue staining test, but the three others showed significant variations in their "triple test" results which did not show up in the staining test. Ideally, a good brewers' yeast should have an intracellular pH above 6.5, a protease leakage value of less than 45 units (1 unit being equivalent to a leaked proteolytic activity of 1x (10 to the power of -9)ng/ml/cell) and a vacuole appearance ratio of less than 40%. Only one of the four strains proved satisfactory with respect to all three criteria, which have been shown to be well correlated with other measurements of yeast activity such as free amino nitrogen assimilation, the rate of vicinal diketone removal during maturation and the flavour and foam quality of the finished beer.
Keywords : beer brewers' yeast enzymic activity fermentation measurement morphology performance pH prospect proteolytic enzyme quality vacuole