The creation of high quality reduced-fat food products is challenging because the removal of fat adversely affects quality attributes, such as appearance, texture, and flavor. This study investigated the impact of pH-induced droplet aggregation on the properties of model food systems consisting of fat droplets and starch granules. Oil-in-water emulsions (2 wt.% oil) Containing whey-protein coated lipid droplets aggregated extensively when heated (90 degrees C, 5 min) at pH values around their isoelectric point (pH 5) but not at lower (pH 3.5) or higher (pH 7) values, which was attributed to changes in electrostatic repulsion. The physicochemical properties of mixed lipid droplet-starch dispersions (2 wt.% oil, 4 wt.% starch) prepared under similar conditions (pH 3.5, 5, and 7; 90 degrees C for 5 min) were also measured. At pH 5, extensive lipid droplet aggregation was observed in mixed systems, which led to a large increase in their yield stress and apparent viscosity when compared to mixed systems at pH 3.5 and 7. These results show that the rheological properties of mixed fat droplet-starch granule suspensions can be modulated by controlling the electrostatic interactions between the fat droplets so as to change their flocculation state. This study has important implications for fabricating high quality reduced-fat products with desirable sensory attributes. (C) 2012 Elsevier Ltd. All rights reserved.