Effects of freeze–thawing treatment on mechanical and structural properties of konjac glucomannan gels containing additives

Abstract

The mechanical, structural, and water-holding properties of freeze-thawed konjac glucomannan (GM) gels prepared using sodium carbonate as a solidifying agent were examined using various techniques. For GM gels prepared at pH = 11, the freezing-thawing (FT) treatment considerably enhanced the compressive stress because treatment promotes the crystallization and aggregation of GM. Additionally, we examined the effect of three additives, such as curdlan (CUD), antifreeze glycoprotein (AFGP), and tremella fuciformis polysaccharide (TFP), on the various properties of freeze-thawed GM gels prepared at pH = 8.5. The compressive stress of the CUD-added GM gels was almost unchanged after the FT treatment but it lowered the water-holding capacity (WHC) of the gel. The compressive stress of the freeze-thawed GM gels containing AFGP and TFP moderately increased after the FT treatment, whereas both gels possessed high WHC even after the FT treatment. Synchrotron simultaneous small-angle X-ray scattering and wide-angle X-ray scattering experiments were performed to examine the structures of these GM gels and their structural changes during the FT process. The GM gels containing additives formed a network with a fractal dimension of 2.4–2.5 that was constructed by the aggregates with partly crystallized GM. The aggregated phase contains a large amount of water, which causes concentration fluctuations within the phase. During freezing, water molecules transitioned to a solid state and formed ice crystals, whereas during thawing, the water molecules returned to their original phase, but the gel network did not revert to its original state. Among freeze-thawed GM/additive gels, the GM/CUD gel was restored to the original structure most closely before FT treatment.