Optimised Acidification Power Test of Yeast Vitality and its Use in Brewing Practice

P. Gabriel¹, M. Dienstbier¹, D. Matoulková², K. Kosař² and K. Sigler^3,4
¹ Charles University, Faculty of Mathematics and Physics, 121 16 Prague 2, Czech Republic.
² Research Institute of Brewing and Malting, 120 44 Prague 2, Czech Republic.
³ Institute of Microbiology, Academy of Sciences of the Czech Republic, 142 20 Prague 4, Czech Republic.
⁴Corresponding author: sigler@biomed.cas.cz
Parts of this paper were presented at the 22nd Brewing and Malting Days, Prague, Czech Republic, 1-2 November 2007.

J. Inst. Brew. 114(3), 270–276, 2008 | VIEW ARTICLE

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 (T1/2) 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.

Key words:
Acidification power, brewery practice, optimised AP test, wort attenuation, yeast cropping, yeast quality