Influence of alkaline concentration on molecular association structure and viscoelastic properties of curdlan aqueous systems

Curdlan is an extracellular polysaccharide produced from soil microorganism Alcaligens faecalis var. 10C3K, and the linear structure consists of β-1,3-glycoside linkages. Curdlan is not soluble in water but it is soluble in alkaline aqueous solution, and we can obtain the gel when curdlan alkaline solution is heated above 60°C or neutralized by acids. In the present study, the gelation mechanism and dispersing structure of curdlan in the alkaline solutions are studied in terms of correlation between the molecular association structure and viscoelastic properties, using static light scattering and rheological measurements. The degree of association for the curdlan molecules in dilute solution increases with decreasing alkaline concentration. The viscoelastic properties also depend strongly on the alkaline concentration. The concentrated curdlan solution shows almost Newtonian flow at high alkaline concentrations and shows a gel-like behavior at low alkaline concentrations. It was elucidated that the molecular association in the dilute solution reflects on the viscoelastic properties of the concentrated solution and that the gelation mechanism is related to the association structure of curdlan molecules. In the case of lower NaOH concentration systems, the molecular association is likely to consist of a hydrophobic core and hydrophilic surface. The gelation mechanism above 60°C is considered to include the dissociation process of the molecular association and reformation of the network structure. © 1997 John Wiley & Sons, Inc. Biopoly 42: 479–487, 1997