Fabrication of chitosan-protocatechuic acid conjugates to inhibit lipid oxidation and improve the stability of β-carotene in Pickering emulsions: Effect of molecular weight of chitosan

In this study, chitosan (CS) with different molecular weights was functionalized with protocatechuic acid (PA) by free-radical grafting reaction, and used for the inhibition of lipid oxidation and the enhancement of stability of β-carotene in Pickering emulsions. The order of grafting ratio of PA in CS-PA conjugates was CS400 (400 kDa CS) > CS200 (200 kDa CS) > CS100 (100 kDa CS). UV-vis, FT-IR and 1H NMR spectra proved that PA was covalently bonded to CS through amino and ester linkages. Compared with native CS, three CS-PA conjugates exhibited reduced crystallinity and thermal stability and improved antioxidant activity, with a molecular weight-dependent relationship. Besides, CS-PA-conjugate particles formed by ionic gelling procedure were spherically shaped and homogeneously dispersed, which substantially improved the stability of β-carotene in Pickering emulsions than CS particles under ultraviolet irradiation, natural light exposure and heat treatment, and the retention rates of β-carotene were in the following order: CS200-PA- > CS400-PA- > CS100-PA-conjugate particles. Furthermore, the oxidation stability of Pickering emulsions fabricated by CS-PA-conjugate particles was also higher than that of CS particles. These results will provide valuable information for the application of CS-PA conjugates to protect bioactive components and inhibit lipid oxidation in emulsion systems.