Effects of substituting native beta-lactoglobulin B (beta-lactoglobulin) with heat-treated beta-lactoglobulin as emulsifier in oil in water emulsions were investigated. The emulsions were prepared with a dispersed phase volume fraction of Phi = 0.6, and accordingly, oil droplets rather closely packed. Native beta-lactoglobulin and beta-lactoglobulin heated at 69 degrees C for 30 and 45 min, respectively, in aqueous solution at pH 7.0 were compared, Molar mass determination of the species formed upon heating as well as measurements of surface hydrophobicity and adsorption to a planar air/water interface were made. The microstructure of the emulsions was characterized using confocal laser scanning microscopy, light scattering measurements of oil droplet sizes, and assessment of the amount of protein adsorbed to surfaces of oil droplets. Furthermore, oil droplet interactions in the emulsions were quantified theologically by steady shear and small and large amplitude oscillatory shear measurements. Adsorption of heated and native P-lactoglobulin to oil droplet surfaces was found to be rather similar while the theological properties of the emulsions stabilized by heated beta-lactoglobulin and the emulsions stabilized by native beta-lactoglobulin were remarkably different. A 200-fold increase in the zero-shear viscosity and elastic modulus and a 10-fold increase in yield stress were observed when emulsions were stabilized by heat-modified beta-lactoglobulin instead of native P-lactoglobulin. Aggregates with a radius of gyration in the range from 25 to 40 run, formed by heating of beta-lactoglobulin, seem to increase oil droplet interactions. Small quantities of emulsifier substituted with aggregates have a major impact on the rheology of oil in water emulsions that consist of rather closely packed oil droplets.