Molecular structures of gellan gum imaged with atomic force microscopy in relation to the rheological behavior in aqueous systems. 1. Gellan gum with various acyl contents in the presence and absence of potassium

Abstract

Four samples of gellan gum in sodium form, with equivalent average molar mass but with different acyl contents, were investigated using atomic force microscopy (AFM) and dynamic viscoelasticity measurements. The investigation was carried out at concentrations which form local molecular assemblies, including associations, networks, and gels. AFM showed that continuous network structures developed mainly through end-to-end type inter-helical associations rather than side-by-side type ones in the presence of potassium without significant increase in the vertical height on the image. Although end-to-end type inter-helical associations certainly occurred, continuous network structures did not develop in the absence of potassium. In the presence of the cation, the formation of continuous network structures could relate to the rheological thermal hysteresis between the sol-to-gel and the gel-to-sol transitions and to the dynamic storage modulus at 20 °C for gellan gum with lower acyl contents. During gelation, acyl groups increase the flexibility of the molecular bundles, inhibiting associations between the backbones when the added potassium minimizes the electrostatic repulsion. Also, acyl groups lower the charge density of the molecular bundles, which would promote associations in the absence of the cation or the stabilization of the double helix (especially via glycerate groups). This would increase the elasticity of the gelled system in the absence of the cation. Our results point to the fibrous model of gelation rather than the conventional model that assumes distinct junction zones with disordered flexible polymer chains connecting adjacent junction zones.