Conventionally, filtration and centrifugation are used to separate yeast from suspensions. Mini hydrocyclones (10 mm diameter) have previously been shown to achieve limited dewatering of non-flocculating Bakers' yeast. This indicated the potential of the application of hydrocyclones to the concentration of naturally flocculating Bakers' or Brewers' yeast suspensions, where the effective particle size is larger. In this study, the effect of degree of flocculation on the concentration of flocculating Brewers' yeast suspensions was investigated. The separation performance was determined when varying the floe size and feed concentration. Variation in yeast flocculation was achieved by reducing the pH with acetic acid, which produced narrow floe size distributions, with median sizes between 200 mu m and 20 mu m. The feed yeast concentration range investigated was between 0.6 and 14g/L. Stable yeast floes do not break on passage through the hydrocyclone. Separation improved significantly with increasing floe size. Flocculation allowed enhanced separation, as compared to results obtained for unicellular, non-flocculating Saccharomyces cerevisiae. Increasing the feed concentration decreased the recovery of yeast to the concentrated product and the concentration ratio between feed and product. This is in agreement with previous findings for single cellular yeast. An interaction between floe size and suspension concentration on the separation was found with separation performance maximised for concomitant increase in floe size and decrease in yeast concentration. Typical results indicate that the concentration can be doubled and some 70-85% of the yeast recovered in a single pass through the system. Furthermore, yeast quality and fermentation performance were not compromised by passage through the hydrocyclone.