Hydrodynamic Shear Damage of Brewer's Yeast

The objectives of this study were to investigate the effects of mechanical agitation on the quality and fermentation performance of two types of industrial yeast slurries (one ale and one lager) and to correlate these effects to fluid mechanics parameters. Increases in pH, extracellular protease activity levels, and haze in the slurry supernatant showed satisfactory correlations with shear rate (76-228 sec^-1) and shear stress (6.0-22.6 Pa) during continuous agitation at 4°C for 24 hr. The effect of shear prior to wort pitching on attenuation rate was insignificant in 12°Plato (P) and 20°P all-malt worts. However, beer quality dramatically deteriorated with regards to haze after forced aging and foam stability, which were directly affected by the intensity of agitation prior to pitching. Increased levels of mannose and glucose residues in beer haze hydolysate indicated yeast cell wall mannan and ß-glucan as the main causes of increased beer haze, while the loss of foam stability was attributed to increased levels of protease activity during fermentation.