Expanded corn extrudates with Andean amaranth. Part II: thermal and molecular properties

This study investigates the thermal, pasting, and molecular transformations of starch in expanded corn extrudates supplemented with Andean amaranth flour at two substitution levels (20% and 50%) and two target moisture contents (TMCs; 14% and 18%). Selected samples were analyzed using Differential Scanning Calorimetry (DSC), Rapid Visco Analysis (RVA), and Asymmetric Flow Field-Flow Fractionation coupled with multiangle light scattering and refractive index detection (AF4–MALS–dRI). The starch was pre-gelatinized during the extrusion process, resulting in a cold-swelling pasting behavior which was found to be related both to the TMC and the high amylopectin content (waxy) of the amaranth starch. Molecular analysis revealed a substantial reduction in molar mass (M w ) with increasing specific mechanical energy (SME) input, reflecting glycosidic bond cleavage. In contrast, the apparent density (ρ app ) of the polymers increased with higher TMC, indicating moisture-induced compaction of the molecular structure. Notably, extrusion may promote resistant starch (RS) formation, with RS content positively correlated with TMC and the presence of amylose in the feed material. These findings demonstrate that the interplay between raw material composition and processing conditions critically determines the characteristics of extruded products, highlighting opportunities for designing nutritionally enhanced snacks using Andean grains.