Crystallinity and pasting properties of freeze-thawed high amylose maize starch

摘要

Native and defatted high amylose (about 70%) maize starch gels were freeze-dried or repeatedly freeze-thawed, and the effects of the treatments on the crystallinity, pasting viscosity, and resistance to digestive enzymes of the dried starch were examined. Both native and defatted starches showed a B-type crystal structure in the X-ray diffractogram, but the crystallinity was decreased by repeating the freeze-thawing cycle. In the DSC thermogram, the freeze-thawed starches exhibited two endothermic transitions in the temperature ranges of 90-110 degreesC and 130-160 degreesC, representing amylose-lipid complexes and amylose-amylose double helix crystals, respectively. By defatting, the melting enthalpy for the amylose double helices was increased, indicating that the residual lipids inhibited the amylose crystal formation. Ice crystals in the starch gel matrix became smaller and the ice cell membrane became thinner as freeze-thawing was repeated. The freeze-dried or freeze-thawed starch powders swelled to a paste by heating in water as did typical granular starch, but the setback by cooling was significantly high due to the rapid retrogradation of leached amylose. By the treatments, the resistance of the starch to digestive enzymes was also raised. The defatted starches displayed greater paste viscosity and resistance to digestive enzymes than the native starches. But the overall viscosity was decreased as the number of freeze-thawing cycles increased.