Food proteins and polysaccharides are the two key structural entities in food materials. Generally, interactions between proteins and polysaccharides in aqueous media can lead to one- or two-phase systems, the latter being generally observed. In some cases of protein-polysaccharide net attraction, mainly mediated through electrostatic interactions, complex coacervation or associative phase separation occurs, giving rise to the formation of protein-polysaccharide complexes. Physicochemical factors such as pH, ionic strength, ratio of protein to polysaccharide, polysaccharide and protein charge, and molecular weight affect the formation and stability of such complexes. Additionally, the temperature and mechanical factors (pressure, shearing rate, and time) have an influence on phase separation and time stability of the system. The protein-polysaccharide complexes exhibit better functional properties than that of the proteins and polysaccharides alone. This improvement could be attributed to the simultaneous presence of the two biopolymers, as well as the structure of the complexes. Consequently, the interesting hydration (solubility, viscosity), structuration (aggregation, gelation) and surface (foaming, emulsifying) properties of these complexes can be used in a number of domains. Among others, these could be macromolecular purification, microencapsulation, food formulation (fat replacers, texturing agents), and synthesis of biomaterials (edible films, artificial grafts).

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.

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