Fabrication and Characterization of Curcumin-Loaded Complex Acid-Induced Cold Gels of Soybean and Red Bean Protein Isolates Incorporated with Common and Waxy Corn Starches

Soybean protein isolate (SPI) and red bean protein isolate (RBPI) were mixed with common starch (CS) and waxy starch (WS) from corn respectively, and then subjected to glucono-δ-lactone (GDL)-induced crosslinking to form the protein–starch complex cold gels, which was used as a delivery carrier for curcumin. The complex gel with addition of CS had denser network structure and higher gel strength, resistance to deformation and viscoelasticity, while the complex gel with WS had the opposite effect. The protein interacted more strongly with CS than WS, enhancing the structural and thermal stability of the complex gel, which resulted in a higher curcumin encapsulation ratio in the gel and its chemical stability during light and heat treatment. The intensity and location of many characteristic absorption peaks in the infrared spectra changed during simulated gastrointestinal digestion, which indicated the destruction of gel structure, leading to the transformation of α-helix and β-sheet into β-turn and random coil in the secondary structure. The addition of CS and WS reduced the release of curcumin in gastric phase, but promoted its directional release in intestinal phase, due to the changes in the digestion pattern of the complex gel, especially the complex gel with addition of WS had a better controlled release effect, which was also related to the more disordered and flexible secondary structure of its gastric digests. The protection of curcumin by gel network and the targeted release behavior of curcumin during intestinal digestion improved its bioaccessibility. In addition, the curcumin in RBPI-based gels had higher encapsulation ratio, chemical stability and bioaccessibility than that in SPI-based gels. Therefore, protein–starch complex cold gels can be used as controlled release carriers for the protection and delivery of sensitive bioactive compounds or nutrients.