Rheological Description of the Disorder-Order Transition of Gellan Without Added Counter-Ions

Gellan is a microbial anionic heteropolysaccharide with a repeating unit of tetrasaccharide formed by glucose, glucuronic acid and rhamnose in a 2:1:1 molar ratio, respectively. In native gellan, the repeating unit contains one O-acetyl and one O-L-glyceril in positions C6 and C2, respectively, in one of the glucose molecules. Removal of these substituents results in low acyl gellan, which together with the high acyl form are commercialized as gelling agents. However, each form produces gels with different textures (Sanderson, 1990).

Gellan forms gels with mono and divalent ions. Gelation occurs via a two-step mechanism that involves formation of a double helix from two random coils, followed by aggregation of double helices that gives rise to a three-dimensional network. The coil-helix transition defines the sol form while double helix aggregation characterizes the gel condition (Milas et al., 1990).

The sol-gel transition is thermoreversible (i.e., it occurs on cooling and heating) and takes place over a temperature range in which there is a peak temperature normally considered the transition temperature. However, when gellan concentration is low and ions are not added, only the coil-helix transition takes place upon cooling hot solutions. This transition is also thermoreversible.