Ageing-induced solubility loss in milk protein concentrate powder: Effect of protein conformational modifications and interactions with water

Background: Protein conformational modifications and water-protein interactions are two major factors believed to induce instability of protein and eventually affect the solubility of milk protein concentrate (MPC) powder. To test these hypotheses, MPC was stored at different water activities (aw 0.0-0.85) and temperatures (25 and 45 °C) for up to 12 weeks. Samples were examined periodically to determine solubility, change in protein conformation by Fourier transform infrared (FTIR) spectroscopy and water status (interaction of water with the protein molecule/surface) by measuring the transverse relaxation time (T2) with proton nuclear magnetic resonance (1H NMR). Results: The solubility of MPC decreased significantly with ageing and this process was enhanced by increasing water activity (aw) and temperature. Minor changes in protein secondary structure were observed with FTIR which indicated some degree of unfolding of protein molecules. The NMR T2 results indicated the presence of three distinct populations of water molecules and the proton signal intensity and T2 values of proton fractions varied with storage condition (humidity) and ageing. Conclusion: Results suggest that protein/protein interactions may be initiated by unfolding of protein molecules that eventually affects solubility.