Interfacial properties of deamidated wheat protein in relation to its ability to stabilise oil-in-water emulsions

Isolated wheat protein (IWP) is an acidic deamidated wheat protein. The deamidation process enhances the protein solubility at pHs greater than 6, and therefore its potential ability to act as a food emulsifier. The interfacial properties and the mechanism by which this protein stabilises oil-in-water emulsions were investigated by measuring the protein's absorbed layer thickness on latex particles, its interfacial rheology, and the colloidal and thermal stability of IWP stabilised emulsions. IWP forms a relatively thick interfacial layer of 18 nm upon adsorption onto latex beads, suggesting that the protein adsorbed with the long axis perpendicular to the surface, i.e. end-on, at a full protein coverage. The interfacial rheology measurement showed that IWP formed a relatively weak fluid-like interface. Similar to other protein emulsifiers, the colloidal stability of IWP emulsions is provided largely through electrostatic repulsion. Although IWP emulsions were sensitive to salt induced flocculation, the presence of excess protein in the aqueous phase (e.g. 4 wt%) was able to reduce the effect of salt screening (50 mM CaCl2) on a 25 wt% oil-in-water emulsion completely. The emulsions underwent minimal coalescence when droplets were in close contact, e.g. flocculated, because the interfacial layer of IWP provides a barrier to droplet coalescence, even in high salt environments. IWP emulsions were resistant to thermal treatment with no changes in particle size observed when the emulsions were heated (up to 90 °C for 20 min) in the absence or the presence of 150 mM NaCl. The heat stability of IWP emulsions is thought to arise from the structure of IWP at the interface. A lack of free cysteines combined with few hydrophobic regions meant that there were minimal interactions between protein molecules adsorbed onto the same droplet or on neighbouring droplets. The unique interfacial properties of IWP, e.g. its physical layer thickness and the structure provide enhanced stability for emulsions against coalescence and heating.