A study on freeze–thaw characteristics and microstructure of Chinese water chestnut starch gels

The influence of the repeatedly freeze-thawed (FT) treatment on the microstructure, crystallinity, thermal properties, textural properties and resistant starch content of Chinese water chestnut starch (CWCS) gels were investigated, using scanning electron microscopy (SEM), X-ray diffractometry, differential scanning calorimetry (DSC) and textural analysis (TA). The microstructure of the native starch gel was a compact and random phase. Freeze-thawed starch gels formed a honeycomb-like network structure, that was almost broken at the 7th FT cycle. The freeze-thawed starch gels showed a typical B-type crystal structure in the X-ray diffractogram, significantly different from native starch (A-type crystal structure). The crystallinity was increased by repeating the FT cycles. The thermodynamic parameter values (To, Tp, Tc and ΔH) of the freeze-thawed starch gels were also increased with the number of freeze–thaw cycles, and the retrogradation ratio reached 30% at the 7th FT cycle. Textural properties (hardness and springiness) of freeze-thawed starch gels were significantly influenced by repeating the FT cycles. The change of hardness corresponded with micro-structural transformation. By repeated freeze-thawing, the resistance to digestive enzyme of the starch gels increased, which was due to the rapid retrogradation of leached amylose.