Synthesis and characterization of a novel antioxidant RS4 by esterifying carboxymethyl sweetpotato starch with quercetin

• Carboxymethyl sweetpotato starch (CMSS) is esterified by quercetin.
• Esterification with quercetin improves the thermal stability of CMSS.
• Antioxidant capacity conferred on CMSS by quercetin esterification increases with DS.
• Significant but limited changes occur in CMSS digestibility by quercetin modification.

Quercetin is grafted to carboxymethyl sweetpotato starch (CMSS) by esterification. Upon esterification, the water solubility of CMSS decreases and the CMSS-quercetin conjugates (CMSS-Q) are yellowish. FT-IR and 1H NMR indicated the covalent attachment of quercetin to CMSS. Thermogravimetry revealed the superior thermal stability of CMSS-Q over CMSS and native sweetpotato starch (NSS). The in vitro digestibility assays showed that CMSS is highly resistant to digestion while the quercetin graft with degree of substitution (DS) above 0.074 slightly increased its digestibility. The quercetin graft imparted CMSS with strong antioxidant activity and enhanced its thermal stability, which increased with quercetin DS. In vitro cyotoxicity assessment revealed that CMSS-Q is as safe as CMSS and NSS. This study showed that CMSS-Q is a novel antioxidant-resistant starch in RS4 form.