Colloidal properties of flocculent and nonflocculent brewing yeast suspensions

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Abstract
The orthokinetic flocculation rates of four suspensions of Saccharomyces cerevisiae and Saccharomyces uvarum (calsbergensis) were measured using steady shear rheometry and light microscopy. These results, along with cell ζ potential values previously reported by the authors, allowed us to test the applicability of DLVO (Derjaguin, Landau, Verwey, and Overbeek) mediated orthokinetic flocculation models. The findings also permitted examination of the validity of the elastic floe model and allowed estimation of the force required to separate cell doublets. Cellular suspensions of brewing yeast were subjected to a high shear treatment to reduce any floes to single cells. Periodic samples were then taken while shearing the suspension at 200 s−1, and the extent of flocculation was determined via light microscopy. The logarithm of floe concentration was linearly dependent on shearing time. It was possible to estimate orthokinetic capture coefficients (αo) for the four strains, with values ranging from 0.00025 to 0.0010. By using measured and “typical” colloidal parameters, a theoretical αo value was derived which was much lower than the experimental values (i. e., αot ≪ 0.0001). Thus, it was concluded that the DLVO flocculation model could not be used to explain brewing yeast flocculation. Flow behavior of the yeast suspensions followed the Bingham model above a critical shear rate. Values of this critical shear rate (which predicts the point at which cell doublets separate) were used to calculate the force required to separate doublet cells. A calculated force of 2.2 × 10−11 N was similar to that of lectin‐like bonds that are believed to be involved in yeast flocculation. As well, the elastic floe model could be employed to predict Bingham yield stress values.

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