Protein Conformational Modifications and Kinetics of Water−Protein Interactions in Milk Protein Concentrate Powder upon Aging: Effect on Solubility

Abstract

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 (a(w) 0.0-0.85) and temperatures (25 and 45 degrees C) for up to 12 weeks. Samples were examined periodically to determine solubility, change in protein conformation by Fourier transform infrared (FTIR) spectroscopy and principal component analysis (PCA), and water status (interaction of water with the protein molecule/surface) by measuring the transverse relaxation time (T(2)) with proton nuclear magnetic resonance ((1)H NMR). The solubility of MPC decreased significantly with aging, and this process was enhanced by increasing water activity (a(w)) and storage temperature. Minor changes in protein secondary structure were observed with FTIR, which indicated some degree of unfolding of protein molecules. PCA of the FTIR data was able to discriminate samples according to moisture content and storage period. Partial least-squares (PLS) analysis showed some correlation between FTIR spectral feature and solubility. The NMR T(2) results indicated the presence of three distinct populations of water molecules, and the proton signal intensity and T(2) values of proton fractions varied with storage conditions (humidity, temperature) and aging. Results suggest that protein/protein interactions may be initiated by unfolding of protein molecules that eventually affects solubility.

Comparison of nanofiltration and evaporation technologies on the storage stability of milk protein concentrates

“…The insoluble material dissolves very slowly and requires much longer rehydration time for complete dissolution (Anema et al 2006;Mimouni et al 2009). Haque et al (2010) observed that proteinprotein interactions may be initiated by unfolding of protein molecules that eventually affects solubility of MPC upon aging. They also suggested a link between minor protein unfolding/refolding and gradual loss of solubility upon ageing of MPC.…”

“…It has been reported that the solubility of MPC decreases with time and temperature of storage (Anema et al 2006;Udabage et al 2012;Haque et al 2010). The insoluble material of MPC formed during storage, mainly consists of casein micelles (McKenna 2000; Anema et al 2006;Havea 2006).…”

Effects of Protein Conformational Modifications, Enthalpy Relaxation, and Interaction with Water on the Solubility of Milk Protein Concentrate Powder

“…4, with increasing a w, T 2 value for water population close to the protein surface decrease up to a w 0.45 (mc 8.2 %) and then increase for higher a w samples, indicating higher interaction with protein surface for lower a w samples. This study also show that with longer storage period water population close to the protein surface (short T 2 fraction) increased with time, indicating enhanced water-protein interactions (Haque et al 2010).…”

“…The NMR-T 2 results indicated the presence of three distinct populations of water molecules (differing in their interaction with and distance from protein surfaces) and that the proton signal intensity and T 2 values of proton fractions varied with storage condition (humidity, temperature) and ageing (Fig. 4) (Haque et al 2010). As shown in Fig.…”

“…PCA of the FTIR data was able to discriminate samples according to moisture content, storage period. Partial least squares (PLS) analysis shows some correlation between FTIR spectral feature and solubility (Haque et al 2010). …”