Experimental investigation on cross-flow and pressure ultrafiltration (UF) fermentation broth of lactic acid during the whey fermentation using a bioreactor equipped with zirconium oxide stainless steel (ZOSS) membrane was carried out. Different filtration resistance (R-f and R-i) components were calculated to understand better the interactions between the membrane and fermentation feed. Various raw materials (deproteinated synthetic whey S1, model/synthetic whey S2 and ultrafiltration dairy whey permeate S3) were used as a feed for UF tests under constant conditions. It was found that the highest concentration of lactic acid in permeate (0.69 +/- 0.95%wt.) as well as permeate flux (0.5 +/- 5.5 . 10(-6) m(3) m(-2) s(-1)) was obtained with the transmembrane pressure of 2 . 10(6) Pa and the cross-flow velocity of 1 m s(-1). Under these conditions the fouling resistance of 6.87 . 10(11) m(-1) was minimal. The integration of bioreactor with ZOSS membranes seems to be promising concept in the production and filtration of lactic acid in the dairy industry. (C) 2016 Elsevier B.V. All rights reserved.