The Nature of the Interactions between Flocculent Cells in the Flocculation of Saccharomyces cerevisiae

Abstract

SUMMARY The flocculation of a strain of brewers' yeast was absolutely dependent upon the presence of calcium; a concentration of 200 mM-CaC1, was sufficient to ensure almost complete flocculation. No other metal could replace calcium ; several metals aggregated potentially flocculent cells but also aggregated non-flocculent cells. Sodium ions antagonized the action of the calcium. The effects of pH value and esterification suggested that carboxyl groups were involved. The flocs had a ' melting temperature ' of 50-60" and were dispersed by urea, suggesting that hydrogen bonds were also present. Non-flocculent yeast was aggregated when the dielectric constant of the medium was decreased by the addition of organic solvents, but this aggregation was also dependent on the presence of traces of calcium. Conversely, increase of the dielectric constant of the medium, by adding formamide, dispersed flocculent yeast. Certain specific sugars also dispersed flocculent yeast. It is suggested that flocculent yeast cells are linked by salt bridges formed by calcium atoms joined with two carboxyl groups in the surfaces of different cells and that this structure is stabilized by hydrogen bonds formed between complementary patterns of carbohydrate hydrogens and hydroxyls in the cell surfaces.