Component Distribution, Shear-Flow Behavior, and Sol-Gel Transition in Mixed Dispersions of Casein Micelles and Serum Proteins.

The shear flow and solid-liquid transition of mixed milk protein dispersions with varying concentrations of casein micelles (CMs) and serum proteins (SPs) are integral to key dairy processing operations, including microfiltration, ultrafiltration, diafiltration, and concentration-evaporation. However, the rheological behavior of these dispersions has not been sufficiently studied. In the present work, dispersions of CMs and SPs with total protein weight fractions (ω_PR) of 0.021-0.28 and SP to total protein weight ratios (R_SP) of 0.066-0.214 and 1 were prepared by dispersing the respective protein isolates in the permeate from skim milk ultrafiltration and then further concentrated via osmotic compression. The partition of SPs between the CMs and the dispersion medium was assessed by measuring the dry matter content and viscosity of the dispersion medium after separating it from the CMs via ultracentrifugation. The rheological properties were studied at 20 °C via shear rheometry, and the sol-gel transition was characterized via oscillatory measurements. No absorption of SPs by CMs was observed in dispersions with ω_PR = 0.083-0.126, regardless of the R_SP. For dispersions of SPs with ω_PR ≤ 0.21, as well as the dispersion medium of mixed dispersions with ω_PR = 0.083-0.126, the high shear- rate-limiting viscosity was described using Lee's equation with an SP voluminosity (v_SP) of 2.09 mL·g^-1. For the mixed dispersions with a CM volume fraction of φ_CM ≤ 0.37, the relative high shear-rate-limiting viscosity was described using Lee's equation with a CM voluminosity (v_CM) of 4.15 mL·g^-1 and a v_SP of 2.09 mL·g^-1, regardless of the R_SP. For the mixed dispersions with φ_CM > 0.55, the relative viscosity increased significantly with an increasing R_SP (this was explained by an increase in repulsion between CMs). However, the sol-gel transition was independent of the R_SP and was observed at φ_CM ≈ 0.65.