Differential scanning calorimetry and structural studies of the sol-gel transition of gellan gum in water

Abstract

The sol-gel transition of gellan gum aqueous solution in the absence of added salts has been studied by differential scanning calorimetry (DSC), mechanical methods, wide-angle X-ray diffraction (WAXD), and small-angle X-ray scattering using synchrotron radiation (SR-SAXS). DSC data show that the endothermic enthalpies have a maximum at about 4% gellan gum concentration and almost disappear at a concentration beyond about 8%, whereas the exothermic enthalpies increase with the concentration of gellan gum. The endothermic peak temperatures are almost coincident with the exothermic temperatures at gellan gum concentrations below about 4%. In a concentration range between about 5 and 7%, two endothermic peaks were observed.

At gellan gum concentrations above about 8%, no endothermic peak was observed in the DSC heating curves, suggesting a splitting into many endothermic peaks. Two transition curves determined by DSC and test-tube tilting methods cross over at about 5 wt% gellan gum concentration. The solution was transparent below about 5% gellan gum concentration, while it was cloudy at higher concentration and the cloudiness increased on storing overnight at around 30 °C. WAXD and SR-SAXS data indicated that some peaks indicating an ordered structure were observed in the cloudy gels. The data on the cloudy gels were characterized by peaks at three scattering vectors which are in a simple ratio of 1:2:3, suggesting that the junction zones in these gels are formed by lamellar structures. These peaks shifted to a lower scattering vector as the content of added ions in the gellan gum sample was increased, indicating larger lamellae.

The molecular origin of the multiple endothermic peaks observed in the DSC heating curves for gellan gum solutions at higher concentration is the melting of these lamellae.