Physico-chemical properties of skim milk retentates from microfiltration

Physico-chemical properties of retentates obtained from selective concentration of skim milk up to 8 times its original weight using a microfiltration system were studied. The effects of process variables, namely concentration (8.6 to 27 wt.%), temperature (20 to 50 degrees C) and pH (6.0, 6.3, and 6.5) on density (rho), apparent viscosity (mu(a)), consistency coefficient (K), flow behavior index (n), and activation energy (Ea) of the retentates were examined. Depending on pH, retentates showed significant increase in apparent viscosity, deviated from classical Newtonian behavior and exhibited shear-thinning between 11 to 17% solids concentration. No yield stress was detected in the range of concentration studied. The power law parameters (n and K) followed a similar trend. An Arrhenius-type equation described well the effect of temperature on apparent viscosity. Although activation energy increased 120 to 130% for a threefold increase in solids concentration, it was not significantly different from that of other types of concentrated milk at approximately the same concentration. Increasing solids were responsible for change in flow properties with concentration, while the effect of pH was attributed to differential protein (primarily casein) retention and the change in solvation properties and voluminosity of casein micelles. Models relating concentration, temperature, and pH to retentate apparent viscosity and consistency coefficient were identified. Skim milk microfiltration with in-process pH adjustment produces retentates depleted in whey proteins and calcium with significantly altered properties.