Extrusion-Assisted Formation of Rice Starch-Propyl Gallate Complexes: Structural Characteristics, Antioxidant Activity, and In Vitro Digestibility.

Propyl gallate (PG) is an effective food antioxidant, but its performance in food systems may be limited by poor water compatibility and processing instability. In this study, rice starch was used as a carrier matrix to prepare starch-PG complexes by extrusion cooking, and the effects of PG incorporation on starch structure, antioxidant activity, and in vitro digestibility were evaluated. Starch was blended with PG at 0, 25, 50, and 100 mg/g and processed by extrusion, and the resulting samples were characterized by complex index analysis, small-angle X-ray scattering, Fourier-transform infrared spectroscopy, solid-state carbon-13 nuclear magnetic resonance, X-ray diffraction, pasting and rheological measurements, 1-diphenyl-2-picrylhydrazyl (DPPH) radical-scavenging assay, in vitro digestibility, and density functional theory calculation. Extrusion disrupted the native semi-crystalline structure of starch, while PG incorporation promoted complex formation, with the highest complex index (88.28%) observed at 50 mg/g PG. Structural analyses indicated increased short-range order, higher single-helical content, and the development of V-type crystalline features in the PG-containing extruded starches. These starches also retained DPPH radical-scavenging activity and showed slower in vitro starch hydrolysis, with resistant starch increasing to 25.78%. Overall, extrusion cooking appears to be a feasible approach for preparing starch-PG complexes that preserve antioxidant functionality and reduce in vitro digestibility.