Milk powders ageing: effect on physical and functional properties.

Milk powders are now considered as food ingredients, mainly because of the functional properties of milk proteins. During the storage of milk powders, many physicochemical damages, mainly dependent on lactose glass transition occur. They have important consequences on physical (flowability) and functional properties (solubility, emulsifying, and foaming properties) of milk powders. First, lactose crystallization modifies the microstructure and chemical composition of the surface of powder particles. Thus, milk powders flowability is decreased. Since the structure of milk proteins is destabilized, its solubility is damaged. Moreover, particle collapse and caking occur and mainly decrease the physical properties of milk powders (density and flowability). The mechanical stresses involved may also enhance proteins unfolding, which is detrimental to solubility. Finally, molecular mobility is favored upon ageing, and both chemical (Maillard reaction) and enzymatic reactions occur. Maillard reaction and oxidation enhance protein interactions and aggregations, which mainly lessen milk powders solubility. Maillard reaction also decreases emulsifying and foaming properties. Storage temperature and relative humidity have been considered as the predominant factors involved, but time, milk components, and their physical state also have been implied.