Synthesis, characterization, and antioxidant activity of caffeic‐acid‐grafted corn starch

The antioxidant molecule of caffeic acid was grafted onto corn starch by an N , N ′‐carbonyl diimidazole‐mediated reaction. The structural, crystalline, thermal, and antioxidant properties of caffeic‐acid‐grafted starch (caffeic acid‐g‐starch) were investigated. The grafting ratio of caffeic acid‐g‐starch was determined as 66.1 mg/g by the Folin–Ciocalteu method. The Fourier‐transform infrared spectrum of caffeic acid‐g‐starch exhibited a band at 1720 cm −1 attributed to the vibration of the esterified carboxyl group, and bands from 1440 to 1633 cm −1 attributed to the stretching of the aromatic ring. The 1 H nuclear magnetic resonance spectrum of caffeic acid‐g‐starch showed all the characteristic signals of starch at δ 3.30–5.47 ppm, and methine protons of caffeic acid moieties at δ 6.77–7.33 ppm. Caffeic acid‐g‐starch was piece or virgate in shape with a rough surface as observed by scanning electron microscopy. The X‐ray diffraction pattern of caffeic acid‐g‐starch showed an amorphous character. The small‐angle X‐ray scattering pattern further indicated the semi‐crystalline growth rings in native starch were disrupted during the synthetic processes. Thermogravimetric analysis showed caffeic acid‐g‐starch had a higher thermal stability than native starch. Moreover, the reducing power and free‐radical scavenging activity of native starch were greatly enhanced after grafting with caffeic acid. These results suggest the potential of caffeic acid‐g‐starch as a novel antioxidant agent.