Low-temperature electron microscopy for the study of polysaccharide ultrastructures in hydrogels. II. Effect of temperature on the structure of Ca2+-alginate beads

Calcium alginate beads were thermally treated at temps. ranging from 25 DegC to 130 DegC for periods of up to 30 min. Important modifications to the structure of the alginate beads were shown to be a function of the temp. and period of incubation at each temp. Modifications to the alginate beads included redn. in size, mech. resistance, and mol. wt. cut-off with increasing temp. and incubation period. Thus, heating 700 mm calcium alginate beads for 20 min at 130 DegC resulted in a 23% redn. in diam., 70% increase in mech. resistance, and 67% redn. in mol. wt. cut-off. Incubation of calcium alginate beads contg. 2 * 106 kDa blue dextran for 20 min at 130 DegC resulted in no detectable loss of either dye or alginate. This indicates the shrinkage of the beads was due to rearrangement of the alginate chains within the beads, coupled with loss of water. This hypothesis was verified by direct visual observation of calcium alginate beads before and after thermal treatment using cryo-SEM. Unlike other microscopy methods cryo-SEM offers the advantage of extremely rapid freezing which preserves the original structure of the alginate network. As a result cryo-SEM is a powerful tool for studies of hydrogel and capsule structure and formation. Differential scanning calorimetry (DSC) showed that the water entrapped in 2% alginate beads was present in a single state, irresp. of the thermal treatment. This result is attributed to the low alginate concn. used to form the beads. [on SciFinder (R)]