Influence of osmotic pressure induced yeast particle size on the rheological properties of beer-yeast-suspensions

The handling of beer-yeast suspensions (BYS) occurs in several brewery processes , as in pitching, propagation or beer recovery. Highly concentrated BYS do not behave like a shear thinning fluid but like a dilatant fluid. Above a solid content >18% shear stress, as generated turbulences or wall friction increases the dynamic viscosity of the fluid. It is precisely here that influencing factors that have received little attention as the effect of different cell sizes to date can make themselves felt. Even if there is only one yeast strain used in a brewery different cell sizes can occur when there are different osmotic pressures, induced by sugar and alcohol concentration. In this work, the cell size distributions of a Saccharomyces strain were artificially varied and measured. Rheological studies were then carried out on these BYS. Results show that increasing mean cell diameters (sauter diameter) lead to a change in the rheological behavior. It is assumed that cell sizes are also relevant to processing in practice, especially where high concentrations are achieved. Especially for handling these highly concentrated BYS with high shear pumps or agitators it is therefore important to know the relationship between the mean cell diameter and the either shear thinning or shear thickening rheological behavior, because even with the same yeast concentration both rheological behaviors can occur.