Modification of Jiuzao glutelin with pullulan through Maillard reaction: Stability effect in nano-emulsion, in vitro antioxidant properties, and interaction with curcumin

Pullulan-Jiuzao glutelin (JG) conjugates (PJCs) were prepared via Maillard reaction in this study. PJCs were prepared by optimizing pullulan to JG ratios (0.5:1, 1:1, 2:1, and 4:1, expressed as PJC-0.5, PJC-1, PJC-2, and PJC-4, respectively) and reaction times (0-180 min) at pH 7 and 11. The secondary structure changes of PJC compared to JG, potential conjugation sites between pullulan and JG, PJC emulsifying properties, in vitro antioxidant activities, and interaction with curcumin (CUR) were investigated. Among the four PJCs, PJC-2 obtained after 180 min reaction at pH 11 and 90 °C exhibited the best ability in nano-emulsion stabilization with the lowest particle size (180-200 nm and 290-450 nm against NaCl during storage), PDI (0.2-0.4 and 0.4-0.7 against NaCl during storage), highest zeta-potential (-20.10 mV), and lowest backscattering intensity. The spontaneous conjugation binding sites between pullulan and JG were Arg-39, Arg-54, and Asp-168. In contrast to native JG, PJC-2 exhibited better antioxidant capacities, low toxicity for CCD 841 CON and Caco-2 cells, and enhancement of antioxidant enzyme content (i.e., SOD, GPX, and CAT) after AAPH-induced oxidative stress. In addition, there exists an interaction between CUR and PJC-2 by residues Ala-74, Asp-376, Arg-368 and -374, Val-45, and Ala-44 of JG. The results above exhibit important implications for the fabrication of PJC-stabilize nano-emulsion and even for developing PJC product as a potential carrier of CUR in the functional food industry.