| Summary: | 碩士 === 文化大學 === 生理學系 === 85 === The effects of starch compositions on the thermal behavior, rheological properties and microscopic structure during the gelatinization and retrogradation of the mixed sweet potato/TCN1 rice starches and sweet potato starch/TCN1 rice flour systems were investigated by Brabender viscoamylography, differential scanning calorimetry (DSC), dynamic rheometry, X-ray diffractometry and scanning electron microscopy.
The results of Brabender viscoamylography indicated that, the pasting temperatures of mixed sweet potato/TCN1 starch systems (20:80∼80:20) are form 69.0 to 69.5℃ withing the range of those pure rice (80.0℃) and sweet potato (68.5℃) starches. The peak and hot-paste viscosities of these mixed systems are lower than those of the pure sweet potato starch, but higher than those of rice starch. While the setback viscosities are lower for these mixed starch systems than for the pure sweet potato starch. The DSC results showed that the gelatinization temperatures and enthalpy changes of these mixed systems increase with increasing the concentration of sweet potato starch, in agreement with the tendency in the mixed sweet potato starch/rice flour systems. The increase in storage modulus (G'') during gelatinization of these mixed systems rises in proportion to the ratio increment of rice starch or flour. The system of sweet potato/rice starches=50:50 gives the lowest G'' among the mixed starch systems measured, but not in flour systems. The SEM suggest that different degrees of molecular entanglement and aggregation are observed in the mixed systems gelatinized at different temperatures, depending on the interactions between starch granules and granules, granules and solubles as well as solubles and solubles. These interactions together with the rigidity of starch granules may be responsible for these changes in rheological properties of the mixed systems during heating.
On retrogradation of the mixed sweet potato/rice starch systems, the changes in retrogradation enthalpy and the G'' increase during the early storage are mainly governed by the ratio of sweet potato starch. In the late storage, the G'' increment primarily depends on the ratio of rice starch for the 80:20 and 60:40 systems, but depend on the ratio of sweet potato starch for 50:50、 40:60 and 20:80 systems. The G'' values increase and retrogradation rate of these mixed systems are higher than those of corresponding rice flour systems. The SEM micrograhs further indicate that the structures of mixed starch gels become finer during storage, regardless of the starch ratio.
Since the changes in the physical and rhological properties of sweet potato starch/rice flour are resembling to those of sweet potato/rice starches, it may conchnde that the interactions between starch and starch plays the principal factor for these changes.
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