The surface chemistry of wheat gluten II. Measurements of surface viscoelasticity

Abstract

The dependence of surface viscoelasticity of gluten films at the O W interface on the pH and the ionic strength of the substrate was studied by means of an oscillating needle surface torsion pendulum. Some experiments were carried out at the interface between oil and 10% sodium salicylate or 24 % urea. Gluten was spread from dispersion in anhydrous chloroethanol containing 0.1 M hydrogen chloride. Film behavior showed a marked dependence on the pH and the ionic strength of the substrate. The highest viscoelasticity was observed around pH 7.5, which may be regarded as the surface isoelectric point of gluten when spread from chloroethanol. The decrease in viscoelasticity away from the isoelectric point was more pronounced on the acid side. The viscoelasticity, at the same area and pH, decreased with an increase in the ionic strength. Extrapolation of the regression of the reciprocal loss angles on area gave the area at which elasticity begins to appear as about 1.0 m. 2/mg. This agrees well with the area of minimum compressibility as obtained from the π- A isotherms. The viscoelasticity of gluten films spontaneously increased with time after spreading while the interfacial pressure remained constant. This increase is believed to reflect intermolecular bond formation. Extrapolation of the regression of the rate constants K of the time increase of the surface viscosity on the area to zero K indicated that bonding begins to occur at about 1.54 m. 2/mg. This is identical with the area of close-packing found from an extrapolation of the π- A curves to zero pressure. Urea had little effect but films spread under sodium salicylate showed hardly any viscoelasticity. These findings, together with the effect produced by a change in the pH of the substrate, point to an interplay between hydrogen bonds and ionic linkages.