Crossflow microfiltration of skimmed milk to fractionate casein micelles and whey protein was investigated regarding length dependency of flux and whey protein permeation using a 1.2 m long, 0.1 mu m tubular ceramic membrane. A special module consisting of four sections was constructed allowing to assess the effects of membrane length online by measuring flux and permeation of the whey protein P-lactoglobulin as a function of local processing conditions. It was found that under the applied filtration parameters (mean transmembrane pressure Delta p(TM,m) = 0.5 bar; temperature nu = 55 degrees C; wall shear stress tau(w) = 115 Pa) main parts of the membrane were controlled by a deposit layer. In consequence, the transmission of the whey protein P-lactoglobulin increases from 38% to 87% from membrane inlet to membrane outlet. Results show that a local optimum for protein fractionation exists regarding membrane resistance and process conditions. (C) 2008 Elsevier B.V. All rights reserved.