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).

Due to the recent increase in ecological consciousness, research has turned toward finding edible materials. Viable edible films and coatings have been produced using milk proteins. These films and coatings may retard moisture loss, are good oxygen barriers, show good tensile strength and moderate elongation, are flexible, and generally have no flavor or taste. Incorporation of lipids in protein films, either in an emulsion or as a coating, improve their properties as barriers to moisture vapor. Interactions between chemical, structural properties, as well as film-forming conditions and functional properties of edible milk films are elucidated. Some potential uses of milk protein packaging, which are hinged on film properties, are described with examples.

pH-Induced structural changes during complex coacervation between beta-lactoglobulin (BLG) and Acacia gum (AG) in aqueous solutions were determined by coupling slow in situ acidification of BLG/AG mixed dispersions and different experimental methods. The combined signal evolution of dynamic light scattering at 90 degrees scattering angle (I(90)), electrophoretic mobility, turbidimetry (tau), circular dichroism, and phase contrast microscopy allowed the distinction of critical structural transitions and the definition of their corresponding pH. The formation of soluble BLG/AG complexes was initiated at pH(sc) (4.90), since I(90) and tau significantly increased from the baseline. In parallel or just following complexation, a conformational change of BLG was detected at pH(pct) (4.8). An increase in positive charge density of BLG induced complex aggregation at pH(ca) (4.7). More efficient charge neutralization of aggregated complexes, especially through the lowering of the number of AG negative charges, promoted initiation of phase separation at pH(psi) (4.4). Mixed dispersions became unstable and phase separation occurred at pH(ps) (4.2). The phase separation of mixed dispersions was suggested by the maximum value of scattered light, by an important acceleration of the dispersion turbidity, by a strong increase of hydrodynamic radii, and by the first appearance of light fluctuations as observed by phase contrast microscopy. At the microscopic level, the first coacervates were observed at pH(coa) (4.0), near the pH of the maximum of turbidity. It was also noticed that, from the onset of interactions between biopolymers, the pH decrease led to (i) a gradual homogenization of particle size in the mixed dispersion as suggested by the decrease of dispersion polydispersity and (ii) conformational transitions of the protein (a loss of alpha-helix structure at pH(pct) and a gain in protein secondary structure near pH(coa), probably involving beta-sheet components).

No abstract

No abstract