Adsorption of soy protein isolate at air–water and oil–water interfaces

The adsorption behaviour of soy protein isolate (SPI) is studied at the air–water and an oil–water interface. The experimental results are interpreted taking into account a prior structural and electrokinetic characterisation of the SPI in solution. Hence, the SPI forms a suspension of monodisperse aggregates in a steady state and hence, the size and the electrokinetic charge, which is numerically quantified for the first time herein, appear determinant in the comprehension of the adsorption process undergone by the protein at both interfaces. At the air–water interface, the behaviour of the SPI is compared to literature data of its major fractions, glycinin and β-conglycinin. The differences between adsorption of SPI and its fractions appear strongly dependant on the concentration and correlate with the structural and electrokinetic properties of the suspension. Namely, the compact structure, large size and large electrokinetic charge explain the low rate of diffusion onto the surface found and the low surface pressure attained by the SPI as compared to its fractions at low bulk concentrations. Finally, these experimental observations are used to fit the adsorption isotherms of SPI at the air–water and the oil–water interface with a model. The theoretical predictions prove the larger affinity of SPI for the tetradecane–water interface and the absence of interfacial denaturation, i.e. compact structure, of the protein at both interfaces.