Effects of Natural Storage on Polyphenol Composition and Biosynthesis in Rice

Polyphenols constitute the major antioxidant constituents of rice and substantially influence its nutritional value and storage quality. Herein, a marked increase in total polyphenol content was observed in rice grain after 12 months of natural storage, which conferred enhanced antioxidant capacity. Polyphenol metabolomic profiling identified 565 polyphenol metabolites in both fresh and stored rice grains. Flavonoids and phenolic acids represented the major differential polyphenol metabolites between fresh and stored rice, of which most phenolic acid metabolites were up-regulated in stored rice. Kaempferide, dihydrobisdemethoxycurcumin, methyl acetoxy-[6]-gingerol, and methyl-6-paradol contents significantly increased in all stored rice samples, indicating their potential utility as polyphenol biomarkers for monitoring rice quality deterioration. The polyphenol biosynthesis pathway was upregulated during rice natural storage, as evidenced by the significantly increased activities of phenylalanine ammonia-lyase, cinnamate 4-hydroxylase, 4-coumarate CoA ligase, and flavanone-3-hydroxylase in stored rice. This study provides a comprehensive characterization of polyphenol composition and biosynthesis during rice natural storage and offers new perspectives for quality assessment and value-added utilization of stored rice.