MECHANISMS OF YEAST FLOCCULATION - COMPARISON OF TOP-FERMENTING AND BOTTOM-FERMENTING STRAINS

The flocculation of two brewing yeast strains, top-fermenting strain S accharomyces cerevisiae MUCL 38485 and bottom-fermenting strain Saccha romyces carlsbergensis MUCL 28285, has been investigated by means of a turbidimetric test. The two strains showed different electrical prope rties, a different hydrophobicity, and a different surface chemical co mposition. They flocculated according to completely different mechanis ms; however, no correlation between the cell physicochemical propertie s and the onset of flocculation was found for either strain. Flocculat ion of the bottom strain was governed by a lectin-mediated mechanism. It was inhibited by mannose and some other sugars, required calcium sp ecifically, occurred in a narrow pH range different from the isoelectr ic point, and was not influenced by ethanol. The onset of flocculation at the end of the exponential phase was controlled both by the appear ance of ''active'' lectins at the cell surface and by the decrease in sugar concentration in the solution. Flocculation of the top strain wa s not inhibited by mannose, did not require the addition of calcium, a nd took place at the cell isoelectric point. Low concentrations of eth anol broadened the pH range in which the cells flocculated, and floccu lation was favored by an increase of ionic strength. Adsorbed ethanol may induce flocculation by reducing the electrostatic repulsion betwee n cells, by decreasing steric stabilization, and/or by allowing the pr otrusion of polymer chains into the liquid phase. The onset of floccul ation was controlled by both a change of the cell surface and an incre ase in ethanol concentration. The only evidence for an adhesin-mediate d mechanism was the specific requirement for a small amount of calcium .