In the fermentation process of ethanol production, the yeast Saccharomyces cerevisiae is recovered and recycled to a new stage of fermentation. This separation process is a challenge because the yeast is small and low density. Thus, the hydrocyclone is an interesting alternative because it uses the same principle of centrifugal separation, requires low maintenance and can be readily sterilized. This work aimed to study the separation process of yeast from fermentation using hydrocyclones. A factorial experimental design (3(4)) was applied, and the results were used for the determination of empirical mathematical models that describe capacity, total separation efficiency and flow ratio as functions of different geometric variables and pressure drops. Total separation efficiencies were obtained between 36.54% and 92.02%, capacities from 0.1183 to 0.4579 m(3)/h and flow ratios ranging from 31.74% to 84.30%. The centrifugal forces generated within these devices were also investigated, which ranged from 291 to 4153g. The effect of the separation process on cell viability was also evaluated for the largest pressure drop conditions, verifying the reduction of viability below 7%. (C) 2011 Elsevier B.V. All rights reserved.