Rheological study of the effect of shearing process and κ-carrageenan concentration on the formation of whey protein microgels at pH 7

Gelation under shear during phase separation is evaluated as potential process to prepare microstructured functional ingredients with tunable rheological properties. The physical and chemical reactions leading to gelation have been extensively studied under quiescent conditions. However, a clear understanding of the correlation between microstructure and bulk rheological properties under shear is still lacking. Our work reports a number of interesting new findings, regarding WPI–κ-car gel behaviour. Steady shear flow behaviour and dynamic viscoelastic properties of WPI–κ-car (pH 7.0) were evaluated at 80 °C. The steady viscosity was determined at shear rates from 0.02 to 200 s−1. Frequency sweep measurements of the dynamic viscosity were performed in the angular frequency range of 0.015–15 Hz at a strain of 0.5%. Confocal microscopy was used to look at the gels microstructure after gelation and demixing process. The effect of shear treatment applied during heating was studied on 5% WPI mixes with varying κ-car concentrations (0%, 0.2%, 0.3%, 0.5% and 0.6%). A steep increase in viscosity was observed at κ-car concentration more than 0.3% and was related to segregating conditions. It was concluded that the low shear Newtonian viscosity for κ-car in isolation changed linearly with κ-car concentration. However, in mixtures, the low shear Newtonian viscosity changed non-linearly with κ-car concentration for non-sheared as well as for sheared systems. The effect of varying shear rates (3.6, 14.4, 43.2 and 86.4 s−1) was also studied on a 5% WPI–0.5% κ-car mixture. A competition arises between the gelation and the phase separation depending on whether shear is applied during or after gelation. Applying shear during gelation modifies the microstructure and the viscoelastic properties of the microgels. This work brings further understanding on the segregating interactions taking place between whey proteins and κ-car when used as functional ingredients in dairy products.