Dyed Compounds in the Water Extract of Liquidambar formosana Hance Leaves and Their Antioxidant Effect on Glutinous Rice

Abstract

ABSTRACT Background and Objectives The water extract of Liquidambar formosana Hance leaves (LFHL) can be used to dye glutinous rice, a traditional festival food in China. However, research on the dyeing compounds in LFHL remains limited, and the dyeing process has not been standardized. In this study, the dyeing mechanism was investigated by comparing changes in compounds in the LFHL water extract before and after dyeing, in combination with experimental results from single‐compound dyeing. Findings A total of 27 compounds were identified in the LFHL water extract, and nine compounds, mainly flavonoids, disappeared after dyeing. A new absorption peak at 580 nm appeared after dyeing with the LFHL water extract, which was associated with the formation of purplish‐black pigments derived from iridoid structures and reactions with amino compound side chains. Combined with these results, phenolics and iridoid glycosides, particularly monotropein, were found to collectively contribute to the formation of the purplish‐black color in dyed rice. Dyeing experiments using catechin and monotropein demonstrated that polyphenol oxidase (PPO) plays a significant role in the color‐formation process. Process optimization revealed that low temperature (below 60°C) and acidic conditions are crucial for effective dyeing. After dyeing, the antioxidant activity of glutinous rice was significantly increased by more than 13.5%. Conclusion Dyeing glutinous rice with LFHL water extract is a complex process catalyzed by PPO and involves the transformation of phenolics, mainly flavonoids, and monotropein. Under appropriate process conditions, glutinous rice is dyed purple‐black, and its antioxidant activity is enhanced. This dyeing process not only imparts a distinctive color to glutinous rice but also improves its nutritional properties. Significance and Novelty Dyeing rice with LFHL water extract represents a beneficial approach for enhancing the health‐promoting properties of glutinous rice. This study provides new insight into the role of plant‐derived pigments in food dyeing and their functional contributions.