Effects of Different Gelatinization Pretreatments on the Structure and Physicochemical Properties of Resistant Starch Type Ⅲ from Sweet Potato

In this study, under identical gelatinization conditions, different pretreatment methods (hydrothermal, microwave, and autoclaving) combined with an ultrasound-assisted enzymatic approach were used to prepare sweet potato resistant starch type Ⅲ (RS3), and their structural and physicochemical properties were compared and analyzed. The results indicated that native sweet potato starch primarily exhibited spherical granules, showing Maltese crosses under polarized light microscopy. However, the Maltese cross disappeared after all three gelatinization pretreatments. The surface of RS3 prepared using hydrothermal and autoclaving pretreatments exhibited porous structures, whereas the surface of RS3 prepared by microwave showed partial depressions with a scaly structure. Compared with native sweet potato starch, the particle size of sweet potato RS3 increased, and the crystal structure changed from type A to type B. Additionally, the gelatinization enthalpy (ΔH) increased from 6.28 to 13.14–24.69 J/g, the amylose content from 15.54% to 19.19%–23.54%, and the resistant starch content from 21.69% to 46.25%–54.72%. Among the three pretreatment methods, sweet potato RS3 pretreated by microwave showed the highest degree of short-range order and crystallinity; the greatest amylose content, gelatinization enthalpy, and resistant starch content; and the lowest solubility, swelling power, and starch hydrolysis rate. In conclusion, under identical gelatinization conditions, microwave pretreatment more effectively promoted the formation of compact granular structures in sweet potato RS3 compared with hydrothermal and autoclaving pretreatments. Moreover, microwave pretreatment enhanced the double helix content and thermal stability of starch, reduced the solubility and swelling capacity, and increased the amylose content to a greater extent. Consequently, sweet potato RS3 obtained via microwave pretreatment exhibited the highest resistant starch content and lowest starch hydrolysis rate, indicating superior digestive resistance.