Adsorption of proteins at the oil/water interface--observation of protein adsorption by interfacial shear stress measurements.

The interfacial adsorption of proteins is a problem during processing and formulation. The flexibility and aggregation of the protein cause the formation of a viscoelastic multilayer upon adsorption to the oil/water interface. Protein adsorption is a complex process and therefore it is difficult to elaborate which protein characteristics are important for the interfacial protein adsorption. From our results it seems that the molecular weight influences this the most. In this study, the adsorption of three proteins, bovine serum albumin (BSA), lysozyme and insulin, to oil/water interfaces is characterized by interfacial shear stress measurements using a sensitive rheometer equipped with a Du Noüy ring geometry. The interfacial protein adsorption is concentration dependent for the three proteins investigated, where the complex viscosity of the viscoelastic multilayer increases with increased concentration. The adsorption rate of the proteins varies, BSA reaches a plateau after 2h, insulin takes 24h to reach a plateau value and lysozyme adsorbs fast in the beginning, then levels of to a slow but steady increase in the complex viscosity, BSA and insulin were shown to have similar characteristics in the evolvement of the interfacial layer despite the rate, indicating similar mechanism of adsorption, whereas lysozyme differs. This is also seen when G' and G'' are used in order to relate the rheological measurements to the three adsorption regimes. This study shows the potentials and effectiveness of the Du Noüy ring geometry for characterization of protein adsorption to oil/water interfaces.