Rheological characterisation of cake batters generated by planetary mixing: Elastic versus viscous effects

Studies of cake batter rheology have focused on viscous behaviour. We demonstrate that elastic effects dominate at the shear rates used in commercial mixers. The development of batter structure was investigated for two flour types using two bench-scale planetary mixers with known shear rate profiles (Kenwood-KM250, maximum 100 s⁻¹; Hobart-N50, 500 s⁻¹). These wet foams (air volume fraction 0.39-0.45) showed shear-thinning behaviour at low shear rates (0.1-10 s⁻¹), with apparent viscosity dependent on air volume fraction. Simple shear thinning behaviour ceased, for foams, above 10-20 s⁻¹: for slurries (air volume fraction, 0.11-0.15) the limit approached 100 s⁻¹. Elastic effects, predominantly arising from the bubble phase, therefore dominate cake batter behaviour at the shear rates experienced in commercial mixers. Filament thinning extensional rheometry confirmed the VE behaviour of batters. These results indicate that visco-elastic analyses are likely to be the most appropriate probe of microstructure in cake batters.     

Modelling of shear rate distribution in two planetary mixtures for studying development of cake batter structure

The shear rate experienced by a fluid near the wall of a planetary mixer when agitated by a wire whisk tool has been estimated using a simple geometrical analysis. The bowl and whisk geometries were measured for a Kenwood KM250 and a Hobart N50 mixer which are in widespread use in domestic and laboratory installations. The shear rate is shown to be a maximum at the bowl wall. This value is relatively uniform over a large fraction of the wall height, except for a small volume near the base and the region above the maximum width of the mixer. The shear rate profile is sensitive to the vertical positioning of the agitator within the bowl. For standard manufacturer speed settings, the range of maximum shear rates was estimated to be 100-500 s⁻¹ in the Hobart and 20-100 s⁻¹ in the Kenwood.