Development of composite structures in the gellan polysaccharide/sugar system

A sensitive in-house rheometer and two measuring configurations were employed to examine the structural properties of gellan/sugar preparations at stoichiometric levels of added calcium ions. Perforated cylinders allowed recording of the gelation process in slippery gellan gels (from 0 to 40% sucrose) whereas systems with greater amounts of co-solute could also be analysed with a flat cone-and-plate geometry. The sharp sigmoidal transition of the highly enthalpic aggregated helices in aqueous or low sucrose (up to 30%) gellan gels was observed at temperatures below 60 degrees C upon cooling (1 degrees C/min). This gave way at 60% sucrose and beyond (high solids regime of sucrose plus corn syrup) to a thermally stable structure with a low modulus monotonic temperature profile (from 90 to 5 degrees C). At the intermediate range of co-solute (30 to 60%) a bimodal cooling profile was recorded which was rationalised on the basis of a composite network comprising lightly cross-linked and therefore flexible gellan chains, and rigid aggregates. In the absence of external calcium, at the intermediate level of solids, only the aggregated type of network was formed at the low temperature end, apparently supported by the batch salts. Therefore, the added calcium facilitated stabilisation at the top temperature end of limited intermolecular associations between the thermally vibrant gellan chains in a sugar (>40%) environment. (C) 1997 Elsevier Science Ltd.