Two types of microparticulated whey protein ingredients with different particle characteristics were added to a non-fat milk system and allowed to hydrate for different periods of time prior to heat treatment. Differences during acidification of the milk systems were investigated by rheology and H-1 nuclear magnetic resonance spin -spin relaxation. The amounts of more or less mobilised water were shown to differ depending on the type of microparticulated whey protein. Large protein microparticles did not actively participate in gel network formation and showed an increased fraction of more mobilised water, and thus higher syneresis upon storage. An enhanced surface reactivity was shown in small microparticles which may also be related to the increased interaction potential of the native whey proteins present (C) 2016 Elsevier Ltd. All rights reserved.