Impacts of freezing and molecular size on structure, mechanical properties and recrystallization of freeze-thawed polysaccharide gels

Abstract

Abstract Freezing modifies microstructures and affects physico-chemical changes during thawing of foods. This study investigated impact of freezing temperature (−20 °C, −50 °C and −90 °C) and controlled freezing process, and dextrose equivalent (DE5, DE15 and DE18) of maltodextrin (MD) on microstructure, mechanical and structural changes (shrinkage and recrystallization) of freeze-thawed MD-agar gels. Shrinkage and turbidity of matrices were dependent on freezing conditions and microstructures of solids. X-ray tomography revealed that low cooling rate formed large ice crystals contributed to thicker solid networks. However, a slow cooling to a supercooled region (∼0 °C to −1 °C) followed by a quench cooling caused rapid ice nucleation and inhibited ice growths resulting in only few clusters of large ice with a high number of fine ice crystals. Raman spectra indicated amorphous–crystalline transition (recrystallization) of gel components particularly maltodextrin after thawing in low DE systems which increased gel turbidity and subsequently accelerated shrinkage. Lower temperature freezing gave thinner but higher connectivity of solid networks providing higher mechanical strength and delayed recrystallization of maltodextrin which primary increased firmness values. The results indicated that the manipulation of freezing process in achieving small ice crystals effectively reduced structural changes attributing to recrystallization of high DE maltodextrin components in gel systems.