FLOCCULENCE OF BREWERY YEASTS AND THEIR SURFACE PROPERTIES: CHEMICAL COMPOSITION, ELECTROSTATIC CHARGE AND HYDROPHOBICITY

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Abstract
The flocculence of a top and a bottom fermentation brewery yeast was studied through the cell surface properties: surface composition (X-ray photoelectron spectroscopy), surface charge (microelectrophoresis) and hydrophobicity (adhesion tests). Aerobic Cultures and fermentation simulating industrial conditions were performed. The flocculence (tendency to flocculate in favourable conditions) has been determined by a test which separated the occurrence of flocculation and the measurement of its manifestation (suspension clearing); the flocculating agents were ethanol and calcium. The top fermentation strain does not flocculate with calcium alone but is very sensitive to ethanol (0.01% (v/v)). The lower flocculence observed for cells cultured on malt extract as compared with cells cultured on yeast extract is accompanied by a more negative zeta potential. The bottom fermentation strain which has a more negative surface flocculates only in the presence of calcium but ethanol enhances the process. The variation of flocculence according to the culture medium and harvesting time is correlated to the variation of the phosphate surface concentration. This is not paralleled by a variation of the zeta potential which does not decrease below a value of about-35 mV as the phosphate surface concentration increases. The increase of flocculence as a function of culture time correlates also with an increase of the cell hydrophobicity; this is more obvious for the cultures than for the fermentations.