Diglycerol ester of mono-oleic acid (DO), a bacteriostatic emulsifier causes severe coalescence of oil droplets stabilized by milk proteins and phase separation between oil and aqueous phase under agitating conditions. In order to clarify the destabilizing mechanism of the emulsifier, physicochemical and colloidal properties of the emulsifier were compared to similar emulsifiers without destabilizing effects. DO, that is dispersible both in oil and water, adsorbed to the oil-water interface to reduce the interfacial tension, and migrated from oil phase to aqueous phase and vice versa in a plane interface system. Experiments performed in an emulsion system revealed that DO had little ability to emulsify food-grade oil, but displaced milk proteins from the oil droplet surface. These data indicate that DO with little emulsifying ability predominantly occupies the oil droplet surfaces via emulsifier-protein competitive interactions to promote severe coalescence of emulsion oil droplets probably because DO actively migrates between oil and aqueous phase and it favors to form the planar oil-water interface. (C) 2014 Elsevier Ltd. All rights reserved.