Abstract
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 °C for 10–60 min under constant shear (100 s−1), followed by storage at 4 °C. The viscosity (η) of MPI suspension treated at 50 °C and 85 °C was ∼1–10% as those treated at 25 °C. After four days of storage at 4 °C, η showed the least value in 50°C-treated samples compared to those at 25 °C and 85 °C. The η and storage modulus (G’) was decreased with prolonged treatment at 25 and 50 °C, whereas opposite trend was found in 85 °C treated samples. Differential scanning calorimetry found proteins in 50 °C treated samples had smaller enthalpy than those in the control and 25 °C treated samples. Protein surface hydrophobicity was increased slightly from 25 °C to 50 °C, but remarkably in 85 °C treated samples. Ultra-small angle x-ray scattering showed the radius of gyration (Rg) of casein micelle was ∼38 nm at 25 °C and 50 °C treated samples but increased to ∼44 nm 85 °C treated samples with reduced compactness. A new sphere-like structure with Rg of 18 nm was generated in 85 °C treated samples. These findings suggested modulating temperature during thermal-mechanical treatment is essential to alter protein structures and morphology for desirable rheological properties.
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•50 °C treated MPI suspensions had the least viscosity and elasticity.•Proteins in 50 °C treated MPI suspensions had lower enthalpy than those at 25 °C.•85 °C increased the size of casein micelle but reduced its compactness.•Duration of treatment has milder effect on viscoelasticity compared to temperature.