Interfacial rheological properties and conformational aspects of soy glycinin at the air/water interface

Interfacial (rheological) properties of soy glycinin were studied at different pH. At acidic and high alkaline pH glycinin (11S form, Mw∼350 kDa) dissociates into smaller subunits, the so called 3S form (Mw∼44 kDa) and 7S form (Mw∼175 kDa). This dissociation behaviour is expected to affect the interfacial rheological properties of glycinin. Adsorption kinetics at the air/water interface were followed with an automated drop tensiometer (ADT) and ellipsometer. The changes in surface concentration, surface pressure, dilational modulus and layer thickness were determined. At acidic pH where glycinin is in the 3S/7S form, it adsorbs much faster at the air/water interface giving a higher surface concentration and a higher dilational modulus than at pH 6.7 (where glycinin is in the 11S form). Surface shear viscosity measurements showed that after short ageing times glycinin gives a protein network that is much more resistant to deformation in shear at pH 3 than at pH 6.7. After ageing for 24 h, the surface shear viscosity is about the same at both pH. Foaming experiments failed to give a glycinin stabilised foam at pH 6.7 while at pH 3 glycinin behaves as a good foaming agent. All results indicate that changing the pH influences the conformation of glycinin and that this has a great impact on the interfacial rheological properties and foaming properties. Based on the results a model is postulated for the modes of glycinin adsorption at pH 3 and pH 6.7, respectively.