Mechanical agitation and rheological considerations of ale yeast slurry

The effect of mechanical agitation on ale yeast quality and fermentation performance was investigated. An industrial yeast slurry was subjected to 6 hr of continuous mechanical agitation in a baffled vessel fitted with a dual Rushton turbine. Three rotational speeds were employed (550, 750, and 900 rpm). Increases in slurry pH and protease concentration together with a decrease in percent viability were noted. The beer fermented with the agitated yeast slurries was hazier and had less stable foam, compared with untreated beer. Agitation at the two lower speeds revealed quite similar results, while 900-rpm agitation affected yeast quality more distinctly. The rheological characteristics of the yeast slurry were also determined. We concluded that yeast slurries at that concentration (52% wet weight) display thixotropy and that their rheological behavior has two distinct regions (below and above 30 sec^-1). The rheological pattern was also employed toward an approximate empirical estimation of shear rate and shear stress.