Characterization of flocculation for cell removal from fermentation broth via polyelectrolyte addition is commonly based on qualitative methods such as physical appearance of the floc. The use of zeta potential as a quantitative measure of floc character was evaluated as an indicator of optimal polymer addition. Zeta potential was found to increase with increasing cationic polyelectrolyte dosage, but never reached zero regardless of the total amount of polymer added, indicating flocculation occurs at least partially through a bridging type mechanism. Experiments were conducted using various polymer concentrations (25 - 75 g/L) and dosing methods (batch, incremental and continuous addition) that resulted in variable overall polymer requirements to achieve optimum flocculation. Zeta potential was found to be constant at optimal floc character regardless of the total amount of polymer added, polymer concentration, or method of polymer addition. Experiments with two additional types of fermentation broth also showed characteristic zeta potentials at optimal flocculation. Polymer requirements to achieve a particular floc character can vary greatly, depending on polymer dosing conditions and fermentation batch. The effect of polymer dosing conditions on the polymer requirement to obtain optimal floc character was evaluated. Polymer dosing method and calcium concentration were both found to have a significant effect (P < 0.0001) with continuous polymer addition and high calcium concentration requiring less polymer than did batch polymer addition and low calcium concentration, respectively. Polymer dosing concentration did not significantly affect polymer requirement for optimal flocculation. (C) 2004 Wiley Periodicals, Inc.