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

Interfacial (rheological) properties of soy glycinin were studied at differ ent pH. At acidic and high alkaline pH glycinin (11S form, M-w similar to 3 50 kDa) dissociates into smaller subunits, the so called 3S form (M-w simil ar to 44 kDa) and 7S form (M-w similar to 175 kDa). This dissociation behav iour is expected to affect the interfacial rheological properties of glycin in. Adsorption kinetics at the air/water interface were followed with an au tomated drop tensiometer (ADT) and ellipsometer. The changes in surface con centration, surface pressure, dilational modulus and layer thickness were d etermined. At acidic pH where glycinin is in the 3S/7S form, it adsorbs muc h faster at the air/water interface giving a higher surface concentration a nd 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 g lycinin 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 v iscosity is about the same at both pH. Foaming experiments failed to give a glycinin stabilised foam at pH 6.7 whi le at pH 3 glycinin behaves as a good foaming agent. All results indicate t hat changing the pH influences the conformation of glycinin and that this h as a great impact on the interfacial rheological properties and foaming pro perties. Based on the results a model is postulated for the modes of glycin in adsorption at pH 3 and pH 6.7, respectively. (C) 2001 Elsevier Science L td. All rights reserved.