Interfacial properties of protein nanofibrils with different morphology prepared using aqueous solvent with ethanol: Part II. Effect of oil phase hydrophobicity

Self assembled β-lactoglobulin nanofibrils of different morphologies (long semi-flexible or short worm-like) were prepared at pH 2 and 85 °C in the aqueous solution containing different amount of ethanol. The effects of fibril morphology and polarity of the hydrophobic sub-phase on the interfacial adsorption kinetics, viscoelasticity, and structuration of the protein fibrils were investigated using dilatational rheology. It was found that the fibrils have three-stage adsorption at all the oil-water interfaces: diffusion-controlled adsorption in early stage, penetration adsorption near the interface and rearrangement within the interface. Diffusion adsorption rate (Kdif) decreases while penetration rate (kads) and rearrangement (kr) increase with increasing oil polarity. The strength and structuration of the interfacical fibril networks decrease with increasing oil polarity. The forces maintaining the stability of the protein fibril interface were mainly hydrophobic interaction. Kdif, kads and kr of the short fibrils are higher than those of the long fibrils due to their smaller size and higher surface hydrophobicity. The interfacial adsorption behaviors of short worm-like fibrils and long fibrils resemble those of flexible protein and surfactant molecules respectively. The long fibril interface displayed strain softening response during both extension and compression, but form stronger interface than short fibrils, and thus demonstrate better foaming and emulsification ability. The short worm-like fibril interface displays strain softening during extension but strain hardening during compression, which is to some degree similar to the behavior of native flexible protein molecules.