Understanding the air-water interfacial behavior of suspensions of wheat gliadin nanoparticles

The low solubility of many plant proteins (such as those of cereals) is a main obstacle preventing their use for stabilizing food foams and emulsions. Protein based nanoparticle suspensions hold promise for stabilizing such systems. Here, we shed light on how wheat gliadin based nanoparticles (WGNPs) behave at air-water interfaces, which at present remains largely unknown. At pH 4.0 and pH 6.0, WGNPs display very poor and excellent foam stability and result in interfacial films with low and high visco-elasticity, respectively. Fourier Transform InfraRed and fluorescence spectroscopy revealed substantial differences neither in structural nor in surface properties of WGNPs, nor in WGNP morphology at varying pH values ranging between 4.0 and 6.0, implying that the differences in interfacial behavior originate at the interface during or after adsorption of WGNPs. Cryo scanning electron microscopy imaging of foams stabilized by WGNPs showed that at pH 4.0 and pH 6.0 NP-like structures and a more coherent film are present at the interface, respectively. This is consistent with the higher visco-elasticity of adsorbed interfacial films at pH 6.0 than that at pH 4.0. Foam fractionation revealed that proteins in foams produced at pH 6.0 contain a substantial amount of intermolecular disulfide bonds. Thus, the excellent foam stability of WGNPs at pH 6.0 may at least to some extent be ascribed to formation of a covalently cross-linked protein network at the air-water interface.