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Abstract: Controlled aggregation of protein microspheres in water-in-oil (W/O) emulsions was used to form semi-solid lipid materials. The aqueous phase consisted of 10wt% whey protein isolate (WPI) in buffer solution (pH 7.0, 100mM NaCl). The oil phase consisted of a lipophilic nonionic surfactant (8 wt % polyglycerol polyricinoleate, PGPR) dispersed in a liquid oil (soybean oil). Lipid phases containing protein microspheres were formed by homogenization of the oil and aqueous phases to form a W/O emulsion followed by heating (90°C for 30min) to promote gelation of the WPI in the aqueous phase. Temperature-scanning dynamic shear measurements showed that the W/O emulsions underwent an irreversible liquid-to-solid transition when heated above the thermal denaturation temperature of WPI, which was attributed to protein gelation and microsphere aggregation. Optical microscopy indicated that a three-dimensional network of aggregated protein microspheres was formed at high aqueous phase contents (>30 wt %). Shear rheology measurements (shear stress versus shear rate) indicated that these structured emulsions were non-ideal plastic-like materials. The apparent shear viscosity increased with thermal treatment, increasing aqueous phase content, and decreasing shear rate. The structured W/O emulsions developed in this study may be useful materials for the development of foods with highly viscous or gel-like lipid phases, but low saturated or trans-fat contents. [Copyright &y& Elsevier]