Optimised acidification power test of yeast vitality and its use in brewing practice

Abstract

The optimised acidification power test (APT) of brewer's yeast quality includes storing the yeast slurry at 2°C under beer (AP remains constant for up to 6 days), a 15 min sample equilibration to room temperature, decantation, and washing by triple centrifugation in deionised water. The final yeast pellet keeps its AP for up to 6 h at room temperature under water and thus the APT does not need to be performed immediately after yeast collection. The correct AP value (maximum acidification produced by given yeast) is determined at 25 ± 0.1°C in a 15 mL sample containing >5% glucose and >1.5 g yeast wet weight. The cell concentration is conveniently measured as absorbance (A660). Cell flocculation and/or sedimentation that can distort APT results can be prevented by stirring the sample at ≥200 rpm. The AP of yeast of different generations used to pitch brewery fermentations in cylindroconical tanks had a very low correlation with the wort half-attenuation time (T 1/2) due to large scatter, while each yeast generation separately showed a clear T1/2-AP relationship. The lowest AP of yeast cropped from cylindroconical tanks was displayed by the first cropped fraction. Post-cropping cooling had no effect on AP. Variations in pitching yeast vitality and their effect on the outcome of a brewery fermentation can be masked by variations in pitching rate, wort composition, ambient conditions in the cylindroconical tanks and other factors. © 2008 The Institute of Brewing & Distilling.