Protein/polysaccharide complexes at air/water interfaces

KEYWORDS:protein, polysaccharide,b‑lactoglobulin, pectin, electrostatic interaction, complex coacervation, adsorption, air/water interface, oil/water interface, surface pressure, surface rheology, spectroscopy

Proteins are often used to create and stabilise foams and emulsions and therefore their adsorption behaviour to air/water and oil/water interfaces is extensively studied. Interaction of protein and polysaccharides in bulk solution can lead to the formation of soluble or insoluble complexes. The aim of this thesis was to understand the influence of (attractive and non-covalent) protein/polysaccharide interaction on adsorption behaviour at air/water interfaces (and oil/water interfaces) in terms of adsorption kinetics, and rheological and spectroscopic characterisation of the adsorbed layers. The approach was to first identify the relevant parameters (like charge density, charge distribution or molecular weight of the ingredients) in the mixed protein/polysaccharide adsorption process. Subsequently, for each parameter a range of ingredients was selected/prepared allowing variation of only this single parameter. After investigation of the phase behaviour in bulk solution of the different protein/polysaccharide mixtures to be used, the role of each parameter in mixed protein/polysaccharide adsorption was studied. The parameters most thoroughly assessed were: protein/polysaccharide mixing ratio, polysaccharide charge density and molecular weight and the sequence of adsorption. The majority of the measurements were performed withb‑lactoglobulin (in combination with various polysaccharides e.g. pectin or carboxylated pullulan) at air/water interfaces, at standard conditions of pH 4.5 and low ionic strength (< 10 mM). In addition, experiments were performed at higher ionic strengths, different pH's, with different proteins or at an oil/water interface, to extend the insight in mixed protein/polysaccharide adsorption. This results obtained lead to a generic mechanistic model of mixed protein/polysaccharide adsorption.

In conclusion, protein/polysaccharide interaction can be exploited to control protein adsorption at air/water interfaces. Any parameter affecting protein/polysaccharide interaction (e.g. ingredient parameters like polysaccharide molecular weight, charge density and distribution or system parameters like charge ratio, pH and ionic strength) may be varied to obtain the desired adsorption kinetics, surface rheological behaviour, or net charge of the surface layer. The choice of simultaneous protein/polysaccharide adsorption (in the form of complexes) versus sequential adsorption (first the protein, than the polysaccharide) provides an extra control parameter regarding the functionality of mixed adsorbed layers.