Flocculation mechanisms of top and bottom fermenting brewing yeast

The flocculation behaviour of a large set of top and bottom fermenting brewing yeasts was investigated. 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 sufficiently 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.