Fermentation trials are described in which two lager yeast strains, one highly flocculent, the other nonflocculent (but otherwise very similar) were compared. The optimum conditions for the flocculation of the flocculent strain (under which flocculation begins as soon as yeast growth ceases) were found to be as follows: the presence of calcium at a concentration over 30 mg/litre, pH 4.5, a temperature under 27 degrees C, the absence of inhibitors, conditions of cell density and mixing such that cells readily come into contact with one another, and a low level of turbulence. A highly hydrophobic coating (with a distinctive "greasy" appearance) was observed on the surface of the flocculating cells. This nonspecific hydrophobic effect combines with a specific mechanism (a lectinlike cell wall protein and its receptor) to bring about floc formation. A series of fermentations comparing the two pure strains with variously proportioned mixtures of them showed that the intensity of both hydrophobicity and flocculence was proportional to the percentage of flocculent yeast in the total yeast population. Further trials, in which the first fermentation used a 50:50 mixture of the two strains, subsequent batches being pitched with the yeast crop from the preceding fermentation in the series, showed a progressive shift in the balance towards the flocculent strain, which after six cropping generations made up 80% of the yeast population. This was attributed to the cropping procedure, in which the yeast is drawn off from the bottom of the tank where the most flocculent yeast settles first. The respective advantages and disadvantages of highly flocculent and nonflocculent/weakly flocculent yeasts are compared. Highly flocculent yeasts settle out well, producing a large yeast crop and a well clarified beer of highly consistent quality, but not very well attenuated and with high levels of diacetyl and acetaldehyde; in addition, maturation may be impaired by the lack of sufficient yeast cells remaining in the beer to carry out secondary fermentation. Nonflocculent and weakly flocculent yeasts, on the other hand, are less consistent and difficult to crop but produce a well attenuated beer with sufficient yeast remaining in it to ensure a problem free maturation, although the high yeast count in the matured beer can cause difficulties during filtration.
Keywords : brewers' yeast comparative test fermentation flocculation performance properties yeast strain