Thermal properties of polysaccharides at low moisture .2. Molecular order and control of dissolution temperature in agar

Differential scanning calorimetry (DSC) has been used to probe ordered structures and glassing behaviour for a range of agars containing <25% w/w water. Most commercial agars are supplied in an ordered (double-helical) state, show an endothermic helix-to-coil transition above 100 degrees C at low-moisture, and require 90-100 degrees C for solubilisation in excess water. Agars dried from the coil (single-chain) state show no corresponding endothermic transitions and only re quire a minimum of 45 degrees C for aqueous dissolution. Evidence from helix-to-coil transition enthalpies, equilibrium water content as a function of relative humidity, and solid-state C-13 NMR spectroscopy suggests that water molecules are associated enthalpically with double-helical agar. Single-chain agar is apparently not obtained in a glassy state by direct drying from solution, but in common with double-helical forms, exhibits rubber/glass transition behaviour following heating (in a DSC pan) to 180 degrees C.