Dairy applications of a vibratory shear-enhanced (VSEP) filtration system were investigated using a 500 cm(2) membrane area pilot. Commercial UHT skim milk was ultrafiltered with a 50 kDa PES membrane for concentrating total proteins. The effect of mean membrane shear rate (γ) over bar (w) was investigated by lowering the vibration frequency. The permeate flux was found to vary as ((γ) over bar (w))(0.533) at initial concentration and as ((γ) over bar (w))(0.567) at a volume reduction ratio (VRR) of 1.8. The permeate flux decayed with the logarithm of concentration and the maximum VRR calculated by extrapolation to zero flux was 8.66. The critical flux for stable filtration was found to be 76 1 h(-1) m(-2) at the maximum frequency of 60.75 Hz, corresponding to a mean shear stress on the membrane of 50 Pa. Microfiltration tests were carried out with a 0.1 mum pores Teflon membrane for separating casein micelles. The permeate flux increased with (γ) over bar (w), but with a lower exponent than in UF, especially at high frequency. The critical flux at 60.75 Hz and initial concentration was 64 1 h(-1) m(-2), thus lower than in U F. A comparison with critical fluxes reported in the literature for UF and MF of skim milk indicates a larger domain of filtration stability for the VSEP than for cross-flow filtration with tubular membranes. (C) 2002 Elsevier Science B.V. All rights reserved.