School of Land, Crop and Food Sciences, University of Queensland, QLD 4072 Australia.
J Agric Food Chem. 2010 Jul 14;58(13):7748-55. doi: 10.1021/jf1007055.
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.
蛋白质构象的改变和水-蛋白质相互作用是导致蛋白质不稳定并最终影响浓缩乳蛋白（MPC）粉末溶解度的两个主要因素。为了验证这些假设，将 MPC 在不同水活度（a(w)0.0-0.85）和温度（25 和 45°C）下储存长达 12 周。定期检查样品以确定溶解度、傅里叶变换红外（FTIR）光谱和主成分分析（PCA）下蛋白质构象的变化，以及质子核磁共振（1H NMR）测量横向弛豫时间（T2）来确定水的状态（水与蛋白质分子/表面的相互作用）。MPC 的溶解度随老化而显著降低，并且该过程通过增加水活度（a(w)）和储存温度而增强。FTIR 观察到蛋白质二级结构的微小变化，表明蛋白质分子有一定程度的展开。FTIR 数据的 PCA 能够根据水分含量和储存期对样品进行区分。偏最小二乘（PLS）分析表明 FTIR 光谱特征与溶解度之间存在一定的相关性。NMR T2 结果表明存在三种不同的水分子群体，质子信号强度和质子分数的 T2 值随储存条件（湿度、温度）和老化而变化。结果表明，蛋白质/蛋白质相互作用可能是由蛋白质分子的展开引发的，最终影响溶解度。
