Drying kinetics and metabolite dynamics of rhubarb during shade drying

Rhubarb, valued in traditional Chinese medicine for both medicinal and edible uses, requires effective postharvest drying. Shade drying’s impact on its metabolic profile remains insufficiently studied. Fresh three-year-old Rheum palmatum L. roots were shade-dried. Drying indices, color, kinetics, metabolites, and enzyme activities were measured. A mathematical model for shade drying kinetics was established, and comprehensive metabolomic profiling alongside key enzyme activity analyses were performed. As slice thickness increased, color difference reduced, while drying time prolonged and moisture loss accelerated. Catalase activity gradually declined throughout the drying process, whereas polyphenol oxidase activity first decreased and subsequently increased. The shade drying kinetics fitted the Wang and Singh model best, with anthraquinone variation following zero-order kinetics. In total, 1,966 metabolites were identified, and 1,397 were classified. Flavonoids (15.12%) constituted the most abundant category of differential metabolites, followed by organooxygen compounds (12.50%), fatty acyls (9.59%), and benzene/substituted derivatives (8.14%). Changes in key flavonoid biosynthesis enzyme activities with relative content variations of pinocembrine, pelargonidin, epicatechin, delphinidin, and quercetin 3-beta-D-glucoside. This study demonstrates that Shade drying significantly alters the metabolic profile and key enzyme activities of rhubarb, advancing the scientific understanding of this process and offering practical guidance for optimizing its processing methods.