Surface properties of top- and bottom-fermenting yeast.

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.