Modified huauzontle (Chenopodium nuttalliae Saff.) protein-octenyl succinic anhydride corn starch soluble complexes: structural features and in vitro protein and starch digestibility

Native huauzontle protein (P_native) structure was modified by thermal (P_heat), ultrasound (P_US) and pH shifting (P_pH11). Soluble complexes (SC) were formed between the huauzontle proteins and a commercial octenyl succinic anhydride modified starch (S). The secondary structure of the native and modified huauzontle proteins differed in terms of abundance of β-sheet, α-helix, and β-turn structures. The pH at which maximum presence of SC was observed varied from 5.1 for P_native-S to 5.8 for P_us-S complexes, and being the required protein: polysaccharide weight ratio of 1:1 for P_native-S and of 1:2 for the modified proteins-S. FT-IR confirmed the formation of SC, as the characteristic carboxylate group in S disappeared from its spectra. Also, the absorption peaks of the proteins in the Amide I and II bands shifted slightly from 1630 to 1635 cm^− 1 and 1520 to 1510 cm^− 1 in the SC, indicating conformational changes due to protein-starch interactions. Aggregation of the biopolymers into relatively large particles was observed by SEM in the SC made with modified proteins. SC showed significant differences in the in vitro protein digestibility values, the lowest (85.1 ± 2.3%) occurring for P_native-S and the highest (92.7 ± 0.7%) for P_US-S. P_pH11-S presented the highest resistant starch content (92.6 ± 0.4%). It is concluded that the modification of P_native through heat, ultrasound and pH shifting allowed establishing different driving conditions for the formation of SC with S, and in vitro digestibility of protein and starch could also be modified. The improved nutritional properties of SC through modification of the protein structure make them potential ingredients for the formulation of sports drinks or low-calorie dairy products.