QUANTIFICATION OF BREWERS-YEAST FLOCCULATION IN A STIRRED-TANK - EFFECT OF PHYSICAL PARAMETERS ON FLOCCULATION

Quantification of yeast flocculation under defined conditions will hel p to understand the physical mechanisms of the flocculation process us ed 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 flee size and number of single cells in situ was d eveloped. In this way, it was possible to quantify the actual floccula tion during fermentation, without influencing flocculation. The effect s of three physical parameters, flee strength, fluid shear, and yeast cell concentration, on flocculation during beer fermentation, were exa mined. Increasing flee strength results in larger flocs and lower numb ers of single cells. If the fluid shear is increased, the size of the flocs decreases, and the number of single cells remains constant at ap proximately 10% of the total cells present. The cell concentration als o influences flocculation, a reduction of 50% in cell concentration le ads to a decrease of about 25% in flee size. The number of single cell s 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 fl occulation. With respect to full scale fermentation the effect of cell concentration will play an important role, for flocculation and sedim entation will occur simultaneously leading to a quasi steady state bet ween these phenomena. (C) 1997 John Wiley & Sons, Inc.