Tailoring the rheological properties of high protein suspension by thermal-mechanical treatment

The viscoelasticity of concentrated protein suspensions associates closely with the mixing efficiency and cleaning frequency of facility during high-protein food development. This study investigated the effects of thermal-mechanical treatment on the viscoelasticity of milk protein isolate (MPI) suspensions and their underlying mechanisms to develop protein ingredient with low viscoelasticity. MPI suspensions (20%) were treated at 25, 50 and 85 degrees C for 10-60 min under constant shear (100 s(-1)), followed by storage at 4 degrees C. The viscosity (eta) of MPI suspension treated at 50 degrees C and 85 degrees C was similar to 1-10% as those treated at 25 degrees C. After four days of storage at 4 degrees C, eta showed the least value in 50 degrees C-treated samples compared to those at 25 degrees C and 85 degrees C. The eta and storage modulus (G') was decreased with prolonged treatment at 25 and 50 degrees C, whereas opposite trend was found in 85 degrees C treated samples. Differential scanning calorimetry found proteins in 50 degrees C treated samples had smaller enthalpy than those in the control and 25 degrees C treated samples. Protein surface hydrophobicity was increased slightly from 25 degrees C to 50 degrees C, but remarkably in 85 degrees C treated samples. Ultra-small angle x-ray scattering showed the radius of gyration (R-g) of casein micelle was similar to 38 nm at 25 degrees C and 50 degrees C treated samples but increased to similar to 44 nm 85 degrees C treated samples with reduced compactness. A new sphere-like structure with R-g of 18 nm was generated in 85 degrees C treated samples. These findings suggested modulating temperature during thermal-mechanical treatment is essential to alter protein structures and morphology for desirable rheological properties.