Abstract
The morphological variations of starch with maleic acid during heating and cooling processes are observed by differential scanning calorimetry, small/wide angle x-ray scattering (SAXS /WAXS) and rheometer. The WAXS fitting profile shows the mixed of A- and B-type crystal in the tapioca starch with a ratio of A and B is about 1:4, and the SAXS fitting results suggest that the semicrystalline layers are composed of blocklets. Blocklets are lamellar-structure particles, which are alternately composed of polydisperse crystalline layers and amorphous layers, and their thickness are tc ≈ 7.5 nm and ta ≈ 1.1 nm with a lamellar long period L ≈ 8.6 nm of 20 % polydispesity. The results from viscosity profile and SAXS fitting profile in heating process indicate that amylose chains are co-crystallized with amylopectin in crystalline region. When the crystal size decreases significantly, corresponding to the melting of crystalline region after 60 °C, which amylose would be unleashed from the crystalline layer and increases its viscosity. After the gelatinization temperature at about 66 °C, starch granules collapsed and large amount of amylose chains released into water solution. During the cooling process from 85 to 30 °C, it gels at about 70 °C and the fractal dimension increased from about 2 to 2.3. Viscosity is also continuously increasing during cooling. It shows some amylopectin and amylose nanocluster aggregate as a fractal structure in the system. Differential scanning calorimetry and viscosity profiles indicate that maleic acid does not affect much its transition temperatures during melting, which are consistent with the variation of lamellar Q invariant from SAXS and relative crystallinity from WAXS; crystalline region start to melt after 60 °C and gelatinize after 66 °C. Maleic acid may enhance the effect of releasing amylose chains and increase its viscosity in higher maleic acid content case after gelatinization. In cooling process, Q invariant of ellipsoidal fractal structure from SAXS would increase, which can describe the behavior of free particles aggregate into fractal structure. Higher maleic acid content will increase its Q invariant of fractal structure. This result suggests that maleic acid would enhance the cross-link reaction and the formation of fractal structure. According to the results above, it demonstrate that amylose chains would be unleashed out of the crystalline region increase the viscosity of solution after melting; moreover, maleic acid would enhance the cross-link reaction slightly in heating and significantly in the cooling process below a retrogradation temperature ca. 46 °C.