Mechanical properties of casein-whey protein (CN-WP) mixtures were studied during cooking in a torque rheometer. Factors studied were CN:WP mass ratio (80:20, 90:10 and 100:0) and mixing speed (20200 rpm, shear rate: similar to 15-230 s(-1)). Samples containing 25.0% rennet casein (RCN) and 6.3%, 2.8% or 0% whey protein isolate (WPI) were prepared to obtain CN:WP ratios of 80:20, 90:10 and 100:0, respectively. Disodium phosphate (DSP, 2.5%) was used as emulsifying salt, and the total sample mass was set at 80 g. pH before cooking was 7.3 for all samples. Immediately after the premixing cycle in the torque rheometer (3 min at 150 rpm followed by 1 min of relaxation at 0 rpm), samples were cooked at 80 degrees C for 20 min while mixing at 20, 100 or 200 rpm, and the torque exerted was monitored. Both CN:WP ratio and mixing speed significantly affected the torque response of the mixtures studied (p<0.01). Higher shear rates resulted in higher peaks and lower times to reach peak, indicating an effect of shear rate on speed of protein hydration. Step changes in mixing speed suggested that structural changes occur in CN-WP mixtures after mixing at higher constant speeds for 20 min at either 20, 100 or 200 rpm. The relationship between torque and shear rate correlated well with a power model, resembling the power-law model for shear-thinning materials. The results suggest that high shear rates during cooking at 80 degrees C can promote CN-WP interactions expressed as an increase in torque measurements, but further experiments and analyses are needed to confirm this finding. (C) 2009 Elsevier Ltd. All rights reserved.