Regioselective C6-OH oxidation of starch by laccase-TEMPO-system: A multi-scale structure evolution and water absorption properties study

Development of oxidized starch with functional properties by regioselective oxidation technology is of interest and provides a new research direction for starch science. In this work, the green-chemistry method of the laccase-TEMPO system was chosen to regioselectively oxidize the C6-OH groups of native maize starch (NMS), and the multi-scale structure evolution and water absorption properties of oxidized maize starch (OMS) were investigated. The results showed that regioselective oxidation slightly degraded the OMS chains and destroyed part of the short-range ordered structure, aggregate structure and crystalline structure, while the crystalline type, amorphous lamellae thickness and morphology of the starch granules were largely maintained. This implied that regioselective oxidation occurred not only in the amorphous lamellae (at low carboxyl content) but also in the crystalline lamellae (at high carboxyl content), leading to a shift from ordered to disordered structures and making the molecular chain arrangement less dense. Interestingly, the disordered transformation of the starch structure and the introduction of hydrophilic carboxyl groups enhanced the water absorption properties of OMS (increased 34.08–110.07% compared with NMS), and it increased with the increase of carboxyl content of OMS. Overall, this study could provide green processing methods and fundamental data for the development of oxidized starch with high water absorption.