School of Agriculture and Food Sciences, University of Queensland, Brisbane, QLD 4072, Australia.
J Sci Food Agric. 2011 Nov;91(14):2566-75. doi: 10.1002/jsfa.4461.
MPC 80 is a high-protein (80%) milk powder commonly used in the food industry as a functional ingredient and valued for its nutritional quality. However, its rehydration properties decline during storage, causing more time to be required for rehydration of the powder by the end user. It is thought that changes at the surface of the powder particles contribute to this reduced solubility during storage.
Surface composition and structural changes in milk protein concentrate (MPC) were observed during 90 days of storage at temperatures of 25 and 40 °C and relative humidities of 44, 66 and 84%. No significant changes to the surface composition (fat, protein and lactose) of the MPC powder samples occurred during storage; however, some changes in the microstructure of the powders were observed. Scanning electron microscopy analysis of the powder particles during dissolution showed the formation of a crust, consisting of a thin layer of fused casein micelles, on the surface of the stored powders. An increase in the hydrophobicity at the surface of the particles was evident by X-ray photoelectron spectroscopy analysis of the bonding state of the elements at or near the surface and by atomic force microscopy measurements of the adherence of particles to the surface of a material.
The development of this 'crust' is thought to contribute to the decrease in the solubility of the powder particles during storage. The increase in the hydrophobicity at the surface and the casein micelle interactions resulting in the surface crust formation appear to contribute to the decrease in the solubility of MPC during storage.
MPC80 是一种高蛋白（80%）奶粉，在食品工业中常被用作功能性成分，因其营养价值而受到重视。然而，其在储存过程中的复水性能会下降，导致最终用户需要更多的时间来复水。人们认为，粉末颗粒表面的变化导致了储存过程中溶解度的降低。
在 25 和 40°C 以及相对湿度为 44、66 和 84%的条件下，对牛奶浓缩蛋白（MPC）在 90 天的储存过程中的表面组成和结构变化进行了观察。在储存过程中，MPC 粉末样品的表面组成（脂肪、蛋白质和乳糖）没有发生明显变化；然而，观察到粉末的微观结构发生了一些变化。在溶解过程中对粉末颗粒的扫描电子显微镜分析表明，在储存的粉末表面形成了一层由凝结的酪蛋白胶束组成的外壳。通过对表面或附近元素的键合状态的 X 射线光电子能谱分析和对颗粒对材料表面的粘附的原子力显微镜测量，表面颗粒的疏水性明显增加。
认为这种“外壳”的形成是导致粉末颗粒在储存过程中溶解度降低的原因之一。表面疏水性的增加以及导致表面壳形成的酪蛋白胶束相互作用，似乎导致了 MPC 在储存过程中溶解度的降低。
