Light scattering, sound velocity and viscoelastic behavior of aqueous gellan solutions

Abstract

The gelation of gellan gum solutions was investigated by means of sound velocity and viscoelastic measurements. The molecular weights determined by light-scattering measurements were 21.7 × 10⁴ at 25.0 °C (the ordered state) and 9.47 × 10⁴ at 40.0 °C (the disordered state), and the ratio is 2.29, supporting the scheme of conformational transition of gellan gum between a double helix and a single coil. Sound velocity measurements were performed in the concentration range from 0.05 to 2.4% at 25 and 35 °C. The decrease in the sound velocity difference between the solutions and pure solvents, ΔV, at 25 °C was observed in a fairly narrow concentration range of 0.70–1.0%, and this decrease indicates a dehydration of gellan molecules followed by the conformational change of random coil to helix. The effects of gellan concentration on gelation were examined viscoelastically in the concentration range from 0.85 to 4.85% at 25 °C. The crossover concentration of gelation where the equivalent power-law behaviors of the storage modulus, G′, and the loss modulus, G″, appeared against the angular frequency was about 3.1%. Gelation curves of the complex moduli and ΔV curves were examined as a function of time by quenching from 35 to 25 °C. Two distinct processes were observed in the gelation curve of G′ and G″. On the other hand, only a single process, which corresponds to the first process in the gelation curve, was observed in the ΔV curve. The results were explained as indicating that the first process was attributable to the conformational change from coils to helices of gellan molecules, and the second process to the development of aggregates of helices or the formation of bundles of helices resulting in gelation.