Comparative Study on the Formation, Structure, and Properties of Granular Cold-Water-Swelling Oat Starch by Subcritical Ethanol–Water Treatment at High and Low Temperatures

Granular cold-water-swelling oat starch was prepared under both low-temperature (90–130 °C) and high-temperature (150–190 °C) conditions using subcritical ethanol–water treatment, and the resulting structural and property changes were systematically investigated. The low-temperature treatment largely retained the native orthorhombic crystal structure and produced granules with finer morphology, whereas the high-temperature treatment induced crystallization of a new orthorhombic amylose phase and generated a swollen granular state with superior cold-water swelling capacity (11.60 g/g vs. 1.99 g/g for native starch). X-ray diffraction analysis showed that the proportion of nanocrystals with the native orthorhombic crystal structure decreased, while the proportion of the new orthorhombic phase increased with increasing temperature. The gelatinization endotherm of modified oat starch obtained by the high-temperature method disappeared. The molecular weight of modified oat starch exhibited a broad distribution, with weight-average molecular weight (Mw) values ranging from 86,633.57 to 140,842.31 kDa and a polydispersity index (PDI) between 5.80 and 9.70. These findings demonstrate that elevated temperatures primarily drive the crystallization of the new orthorhombic amylose phase, providing a foundation for designing functional starch-based materials with tailored properties.