Effect of Hydroxypropylation on the Structure and Physicochemical Properties of Millet Starch

This study systematically investigated the hydroxypropylation of 'Jingu 21' millet starch to enhance its functional properties and broaden its industrial applications. Four hydroxypropylated starch derivatives were synthesized by incorporating propylene oxide at levels of 10%, 15%, 20%, and 25% (w/w), resulting in distinct molar substitution degrees (0.016, 0.025, 0.048, and 0.040). Structural characterization revealed that modified starch granules retained polygonal morphology with minor surface protrusions, while maintaining A-type crystalline configuration through X-ray diffraction analysis. The swelling power, solubility, transmittance, freeze-thaw stability and viscosity properties of the hydroxypropylation-modified millet starch showed some degree of increase, while the pasting temperature and coagulability showed a decreasing trend. When the addition of propylene oxide was 20%, the molar substitution of 0.048, swelling power of 34.43 g/g, solubility of 10.33%, and transmittance of 8.76% were significantly higher than the other groups (P<0.05). Meanwhile, the pasting temperature was reduced to 65.33℃, and the coagulability was 22.67%, which were significantly lower than that of the other three groups (P<0.05). In terms of freeze-thaw stability, the water precipitation ratio after one freeze-thaw cycle was 6.78%, which was 51.16% less than that of natural millet starch. Therefore, hydroxypropylation at a 20% propylene oxide addition level produced modified millet starch with optimal physicochemical properties. This method offers an effective approach for modifying millet starch, which is conducive to the development of the millet starch industry.