Factors affecting the outcome of the acidification power test of yeast quality: critical reappraisal.

Brewery bottom yeast strain 95 from the Pilsner Urquell propagation unit was used to reappraise the efficiency of the acidification power (AP) test consisting in determining the spontaneous (oxygen-induced) and glucose-induced medium acidification caused by yeast and lactic acid bacteria under standard conditions, and used widely for assessing and predicting the vitality of industrial strains. AP was evaluated in yeast stored for different periods of time (0-28 d) at 4 degrees C, at different temperatures before and during the test (0-55 degrees C), and at different concentrations of cells and glucose and different cells-to-glucose ratios. All these factors had a strong effect on acidification kinetics and the AP value. By contrast, the duration of the lag period between yeast collection and the test (0-6 h) had no perceptible effect on the AP value. The best results were achieved at saturation concentrations of cells (> 10 g pressed yeast or approximately 14 g yeast slurry per 100 mL) and glucose (approximately 3 %) and at 25 degrees C. Since an exact evaluation of acidification characteristics depends strongly on the kinetics of the process, the AP test should include monitoring the time course of the acidification.