Physicochemical properties and water interactions of milk protein concentrate with two different levels of undenatured whey protein

Two bovine milk protein concentrate (MPC85) samples containing 85% protein were produced using different heat treatments. The physicochemical and water absorption properties (i.e., the solubility, wettability, water holding capacity (WHC) and particle size on reconstitution) of these two MPC85 samples with two different levels of undenatured whey protein (WP, i.e., MPC85S1 and MPC85S2) were investigated. The undenatured WP level in MPC85S1 and MPC85S2 was 16.6 and 6.0 g/100 g total protein, respectively. The calcium ion activity of MPC85S1 and MPC85S2 was 2.93 and 2.64 mmol/L, respectively (p < 0.05). The surface elemental profile, as determined using X-ray photoelectron spectroscopy, showed that protein (84%) and lipid (16%) were the dominant macromolecules on the surface of both MPC85 powder particles. Scanning electron microscopy analysis showed that MPC85S1 powder particles were relatively spherical while MPC85S2 particles appeared agglomerated. Particle size analysis of 5% aqueous suspensions showed a broader size distribution for MPC85S1 compared to MPC85S2. The cold solubility of the samples rehydrated at 4 °C after 192 h storage at 4 °C was higher for MPC85S1 than MPC85S2, while on increasing the temperature of rehydration from 4° to 75 °C the solubility increased for both MPC85 samples. A lower contact angle, and thus higher wettability was observed for MPC85S2 in comparison to MPC85S1. While the MPC85 samples studied herein had similar gross composition, major differences in the level of heat induced WP denaturation (~2.5 times) and a 10% difference in ionic calcium content existed. Consequently differences in the ionic calcium and more importantly the differences in the interactions between casein and native/denatured WP are considered to have resulted in altered physicochemical properties and thereby different aqueous phase interactions.