Quantification of brewers' yeast flocculation in a stirred tank: Effect of physical parameters on flocculation

Abstract

Quantification of yeast flocculation under defined conditions will help to understand the physical mechanisms of the flocculation process used in beer fermentation. Flocculation was quantified by measuring the size of yeast flocs and the number of single cells. For this purpose, a method to measure floc size and number of single cells in situ was developed. In this way, it was possible to quantify the actual flocculation during fermentation, without influencing flocculation. The effects of three physical parameters, floc strength, fluid shear, and yeast cell concentration, on flocculation during beer fermentation, were examined. Increasing floc strength results in larger flocs and lower numbers of single cells. If the fluid shear is increased, the size of the flocs decreases, and the number of single cells remains constant at approximately 10% of the total cells present. The cell concentration also influences flocculation, a reduction of 50% in cell concentration leads to a decrease of about 25% in floc size. The number of single cells decreases in linear proportion to the cell concentration. This means that, during yeast settling at full scale, the number of single cells decreases. The results of this study are used in a model for yeast flocculation. With respect to full scale fermentation the effect of cell concentration will play an important role, for flocculation and sedimentation will occur simultaneously leading to a quasi steady state between these phenomena.

125th Anniversary Review: Yeast Flocculation and Sedimentation in Brewing

“…As settling out proceeds, the floc size decreases because the concentration of yeast cells still in suspension decreases. In turn, the smaller flocs settle out more slowly 96 .…”

“…The hydrodynamic conditions must be favourable and promote a sufficient collision rate between cells, but agitation must not be violent enough to break up cell clusters. The concentration of yeast cells in suspension must be sufficient to cause the number of collisions necessary to form flocs 96 . Factors that increase the hydrophobic character of the yeast cell walls (cell-surface hydrophobicity) and factors that decrease the repulsive negative electrostatic charges on the cell walls (cell surface charge) cause stronger flocculation, presumably because they facilitate cell-cell contact 32,114 .…”

“…At the end of fermentation, conditions are favourable for sedimentation: carbon dioxide production is low, flocculation ability is high and yeast concentration is maximal 96 . These conditions favour the formation and settling out of large flocks.…”

“…These conditions favour the formation and settling out of large flocks. Although floc formation takes only a few seconds, the settling-out time in a large industrial brewery fermentation is measured in hours 96 . Several factors influence the rate at which flocs sediment.…”

Carrier-free, continuous primary beer fermentation

“…Hydrodynamic conditions in the reactor have a direct impact on flocculation and floc size, as the liquid agitation increases the chances of cell collisions, but strong movement breaks up cell clusters (24). Moreover, the higher the concentration of yeast cells that are in suspension, the greater the number of collisions, and consequently the faster the formation of flocs (25). Additionally, factors that increase cell-surface hydrophobicity and/or that decrease the repulsive negative electrostatic charges on the cell wall cause stronger flocculation, as they increase the probability of cell-to-cell contact (26).…”