The flocculation of two brewing yeast strains, top-fermenting strain Saccharomyces cerevisiae MUCL 38485 and bottom-fermenting strain Saccharomyces carlsbergensis MUCL 28285, has been investigated by means of a turbidimetric test. The two strains showed different electrical properties, a different hydrophobicity, and a different surface chemical composition. They flocculated according to completely different mechanisms; however, no correlation between the cell physicochemical properties and the onset of flocculation was found for either strain. Flocculation of the bottom strain was governed by a lectin-mediated mechanism. It was inhibited by mannose and some other sugars, required calcium specifically, occurred in a narrow pH range different from the isoelectric point, and was not influenced by ethanol. The onset of flocculation at the end of the exponential phase was controlled both by the appearance of "active" lectins at the cell surface and by the decrease in sugar concentration in the solution. Flocculation of the top strain was not inhibited by mannose, did not require the addition of calcium, and took place at the cell isoelectric point. Low concentrations of ethanol broadened the pH range in which the cells flocculated, and flocculation was favored by an increase of ionic strength. Adsorbed ethanol may induce flocculation by reducing the electrostatic repulsion between cells, by decreasing steric stabilization, and/or by allowing the protrusion of polymer chains into the liquid phase. The onset of flocculation was controlled by both a change of the cell surface and an increase in ethanol concentration. The only evidence for an adhesin-mediated mechanism was the specific requirement for a small amount of calcium.

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The surface physico‐chemical properties (hydrophobicity, electrophoretic mobility, chemical composition) of a large set of top‐ and bottom‐fermenting brewing yeasts, harvested in the exponential and stationary growth phases, have been investigated. Bottom‐ and top‐fermenting strains showed different surface properties. Top strains were generally more hydrophobic than bottom strains, due to higher surface protein concentrations. Bottom strains possessed higher surface phosphate concentrations. The different profiles of electrophoretic mobility versus pH for top and bottom strains could be explained by modelling the surface charge according to the surface chemical composition as given by X‐ray photoelectron spectroscopy. For bottom strains, the electrical properties were mainly controlled by phosphate, resulting in a low isoelectric point (pH 2 or below) and an electrophoretic mobility that did not become much more negative above pH 4. For the top strains, they were mainly determined by the balance of protonated amino‐ and carboxylate groups in proteins, which gave a high isoelectric point (pH 4) and an electrophoretic mobility changing greatly with pH in the range of 2 to 7. No difference in surface properties was found between flocculating and non‐flocculating strains, or between cells from the exponential and stationary growth phases, even for strains where flocculation occurred during the transition from one growth phase to the other. © 1997 John Wiley & Sons, Ltd.

The flocculation behaviour of a large set of top and bottom fermenting brewing yeasts was investi gated. Bottom and top fermenting strains flocculated according to different mechanisms. Bottom strains flocculated in the stationary growth phase in the presence of sufficiently high Ca2+ and suffi ciently low sugar concentrations; these strains possessed a lectin-mediated flocculation mechanism.Top strains flocculated in the stationary growth phase without addition of Ca2+, only in the presence of sufficiently high concentrations of ethanol. Some of the top strains were inhibited with mannose, but not with sucrose or galactose, while others were not inhibited by any of these sugars. The different sensitivity of flocculation of top and bottom strains with respect to ethanol may be related to the hydrophobicity of the cell surface. Flocculation models for bottom and top fermenting yeasts were proposed. It is suggested that, besides the sugar inhibition pattern, the sensitivity of flocculation with respect to ethanol should be included as an additional parameter for classification of brewing yeasts.

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