This study investigates the effect of pre-acidification of skim milk between pH 6.7-5.4 on the physico-chemical properties of the resulting milk protein concentrate as well as the membrane performance. The membrane filtration process comprised ultrafiltration and 3 stages of diafiltration each with a volume concentration factor of 3. With increasing number of processing stages at a given pH, both the colloidal calcium and micellar caseins were gradually dissociated, while the micellar hydration increased and the micellar size decreased. With the increase in processing stages, casein micelles generally maintained their intact structure between pH 6.7-6.0, and then partially disintegrated into loosely entangled protein aggregates at pH 5.8-5.6, while complete micellar disruption occurred after extensive diafiltration at pH 5.4. Small-angle X-ray scattering indicated that the internal structure of the non-dissociated micelles became more homogenous when the number of processing stages increased and when the pre-acidification pH decreased. (C) 2017 Elsevier Ltd. All rights reserved.