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Abstract: This study shows that controlled aggregation of acidic whey protein microspheres in water-in-oil (W/O) emulsions can be used to create semi-solid lipids. An acidic aqueous phase (10 g/100 g whey protein isolate, 100 mmol/L NaCl, pH 3.5) was homogenized with an oil phase (8 g/100 g polyglycerol polyricinoleate in soybean oil) to form a W/O emulsion. These emulsions were then heated (90 °C, 30 min) to induce thermal denaturation and gelation of the globular proteins in the internal aqueous phase, which promoted irreversible gelation of the overall W/O emulsion. Rheology measurements indicated that the resulting materials had paste-like properties: they were solid-like below a critical yield stress and fluid-like (shear-thinning) above this value. The apparent shear viscosity of the W/O emulsions increased with thermal treatment, aqueous phase content, and decreasing shear rate. W/O emulsion gels containing acidic aqueous phases were considerably weaker than those containing neutral aqueous phases due to the pH dependence of whey protein gelation. Controlled aggregation of whey protein microspheres within W/O emulsions may be a useful means of producing highly viscous or semi-solid lipids with low saturated and trans-fat contents for use in the food and other industries. [Copyright &y& Elsevier]