Rehydration of high-protein-containing dairy powder: Slow- and fast-dissolving components and storage effects

As dissolution of powdered milk proteins is necessary for the expression of their functional properties, it is regarded as a critical property by the dairy industry. However, milk proteins exhibit a progressive loss of solubility during storage in the dry form, especially in the case of high-protein-containing powders such as milk protein concentrate (MPC). To further understand the mechanisms responsible for this loss of solubility, the rehydration process of MPC powder before and after storage at 23% equilibrium relative humidity and 24 °C for up to two months was studied. This work clearly showed that the storage-induced loss of milk protein powder solubility was due to changed rehydration kinetics and not to the formation of insoluble material in the course of storage. The concentrations of the various constituents of MPC (caseins, whey proteins, lactose, calcium, magnesium, phosphorus, sodium and potassium) were determined in the dispersed phase during the powder dissolution process. The results suggest that the release of micelles from powder particles is the rate-limiting step of the MPC rehydration process and is inhibited upon storage. In contrast, water penetration into the powder particles is shown not to be a rate-limiting factor as molecules larger than water (whey proteins and lactose) were freely released out of the powder structure in both fresh and aged MPCs