Gel-sol transition in gellan gum solutions. I. Rheological studies on the effects of salts

Abstract

The mechanical spectra of sodium gellan gum solutions with and without sodium chloride, potassium chloride, calcium chloride and magnesium chloride were observed at the frequency range from 10 −2 to 10 rad/sec and at the temperature range from 30 to 0°C. At any temperature from 30 to 0°C, the viscoelastic behaviour of a 1% gellan gum solution without salt was typical of a dilute solution. However, that of a 2% gellan gum solution varied from a dilute solution to a weak gel with decreasing temperature. The behaviour of a 3% gellan gum solution tended towards that of a weak gel, even at the relatively high temperature of 25°C. Storage shear modulus G′ for 1% gellan gum solutions increased on addition of salts at lower temperatures where gellan molecules take helical conformations whilst it decreased on addition of salts at higher temperatures where gellan molecules take coil conformations. Salts promote aggregation of helices at lower temperatures whilst they reduce the coil dimension at higher temperatures. The increase in G′ for gellan gum solutions by the addition of cations at lower temperatures was attributed to the electrostatic shield of the repulsions between carboxyl groups, and more remarkable for potassium ions than sodium ions, and for calcium ions than magnesium ions. On addition of fairly lower concentrations of NaCl, CaCl 2 or MgCl 2 (except for KCl), G′ became slightly smaller than that without salt. This appears to be related to the fact that these ions deprive the hydrated water molecules surrounding carboxyl side groups in gellan gum molecules, so that the repulsion of these ‘naked’ carboxyl groups becomes stronger by the presence of these ions. However, G′ at a constant temperature and at a constant frequency for gellan gum solutions as a function of added salt concentration showed a complicated behaviour. G′ for a 1% gellan gum solution in the presence of 75 mmol/dm 3 NaCl as a function of temperature increased abruptly around 55°C on cooling and showed a maximum around 35°C. On heating G′ showed a maximum around 45°C, and levelled off, and then decreased rapidly about 70°C. On the other hand, G′ for a 1% gellan gum solution in the presence of 5.1 mmol/dm 3 CaCl 2 began to rise rapidly at 35°C on cooling, and decreased with decreasing temperature. On heating G′ did not change remarkably up to 90°C in contrast to the situation in the presence of NaCl. This shows that the mechanism of gel formation in gellan gum with divalent cations is markedly different from that with monovalent cations.