Mechanistic Elucidation and Establishment of Drying Kinetic Models of Differential Metabolite Regulation in Rheum palmatum During Natural Sun Drying: An Integrated Physiology, Untargeted Metabolomics, and Enzymology Study

Rhubarb, a medicinal herb in Gansu Province, China, undergoes significant quality changes during sun-drying. This study investigated color changes, drying kinetics, anthraquinone (AQ) content, metabolic profiles, and enzyme activity during the process. Results showed that drying induced enzymatic browning, with the browning index (BI) progressively increasing over extended drying periods (4–16 h) and with greater slice thickness (2–8 mm). Catalase (CAT) activity first decreased and then increased, while polyphenol oxidase (PPO) activity decreased throughout drying. Slice thickness significantly affected AQ content, with the highest in 2 mm slices and the lowest in 4 mm slices. The drying process followed a logarithmic model (R2 = 0.99418, RMSE = 0.02310, and χ2 = 0.0005). Metabolomics analysis identified 631 differential metabolites, with 8 key metabolites linked to flavonoid biosynthesis, phenylalanine biosynthesis, and tyrosine metabolism. Fifteen enzymes were involved in metabolite synthesis and decomposition, though some enzyme activity trends contradicted metabolite changes. This study provides insight into rhubarb drying mechanisms and a basis for optimizing the drying process.