ASBC 2001 meeting abstract - Orthokinetic doublet formation and flocculation of Saccharomyces cerevisiae.

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Orthokinetic doublet formation and flocculation of Saccharomyces cerevisiae.
R. ALEX SPEERS, Allan T. Paulson, and Jean W-C. Hsu, Department of Food Science and Technology, Dalhousie University.

The objective of this study was to examine the flocculation behaviour of two Saccharomyces cerevisiae strains expressing either the Flo1 (LCC1209) genotype or NewFlo (LCC125) phenotype. A model to predict the flocculation rates of yeast during laminar shear was developed and tested. The orthokinetic capture coefficient (a flocculation parameter which reflects the ratio of attractive and repulsive interactions between colliding cells) was measured under a number of different environmental conditions. Specifically, the effects of shear (5.95-223 1/s), temperature (5-45C) and pronase treatment on the capture coefficients of these strains were examined. Further measurements of (1) flocculation using the ASBC standard assay, (2) cell surface hydrophobicity (by hydrophobic interaction chromatography and a fluorescent probe technique), (3) cell surface zymolectin density, (4) carbohydrate ligand density and (5) an Alcian Blue cell surface charge density, were also undertaken. The capture coefficients of the yeast suspensions were directly proportional to the inverse of shear rate. The capture coefficients also increased as the temperature was raised to 45C suggesting the importance of hydrophobicity in the flocculation of the strains examined. The pronase treatment caused a drastic decline in flocculation as reflected by the ASBC flocculation assay. The hydrophobicity assays showed no specific trends and were difficult to interpret given our present state of knowledge of yeast cell surface architecture. The Alcian Blue cell surface charge assay indicated that the Flo1 cells had a more negative surface charge than NewFlo cells. A new theory, which predicts capture coefficient values due to zymolectin interactions, was developed based on the model of Long et al. (1999. Biophys. J. 76:1112). This new modified theory, along with estimates of: (1) cell wall densities of zymolectins and carbohydrate ligands; (2) cell wall collision contact area; (3) collision time; and (4) the forward rate coefficient of binding was used to predict theoretical capture coefficients. These theoretical values agreed with measured capture coefficient values at shear rates up to about 100 1/s.

R. Alex Speers A native of British Columbia, Dr. Speers received his graduate education in Food Science at the University of British Columbia. He is presently a Professor of Food Science and Technology at Dalhousie University, Halifax. In the past, Alex was employed in the Quality Assurance departments of both Labatt and Molson Breweries. His research interests include various aspects of the brewing process including yeast flocculation and the physical properties of wort and beer. He instructs graduate students in Industrial Biotechnology and Rheology and participates in brewing and food science workshops on occasion. Alex is a member of several professional societies including ASBC, MBAA and IOB. He is presently a member of the editorial board of the Journal of the ASBC and Food Research International.