Effects of sugar on the sol-gel transition in gellan gum aqueous solutions

Abstract

The effects of glucose, fructose, sucrose and trehalose on the sol-gel transition in gellan gum aqueous solutions have been studied using rheological measurements and differential scanning calorimetry (DSC). Thermal scanning rheological results suggested that a 1 % gellan gum solution without sugar could not form a gel even on cooling to 0 °C; however, on addition of any sugar at a very low concentration, gellan gum solutions could form gels at temperatures lower than the helix-coil transition temperature. In the presence of disaccharide (sucrose or trehalose) or glucose, both transition temperatures of coil—helix, T ch, and sol-gel, T sg in gellan gum solutions shifted to higher temperatures with increasing concentration of added sugar; however, by the addition of fructose, T ch did not change so much. In DSC measurements, in the presence of disaccharide or glucose, the main exothermic and endothermic peaks corresponding to the helix-coil transition gradually shifted to higher temperatures with increasing concentration of added sugar; however, in the presence of any sugar, no exothermic or ndothermic peak was detected in DSC measurements at the sol—gel transition temperature determined as the crossover temperature of G′ and G″, which was essentially different from that for gellan gum alone or in the presence of monovalent cations. Disaccharides could promote both the formation of helices and gelation in gellan gum systems, this promotion being related to the volume of hydration of sugar in gellan gum solutions. In conclusion, sugar indirectly influences the stabilizing of gellan gum gels due to the hydration and structural changes of water surrounding the sugar, and so the stabilities of gellan gum solutions gradually increase with increasing concentration of added sugar. Judging from both rheological and DSC results, only fructose inhibited the formation and aggregation of helices in gellan gum, and so the possibility of direct hydrogen bonding between hydroxyl groups in polymers and in sugars seems to be more plausible, especially by the addition of sufficient fructose.