Drying-driven quality differentiation of rhubarb rhizomes: a multi-level metabolomics analysis of microstructural...

Pithiness is one of the most common quality defects occurring during the drying process of Rhubarb (Rheum palmatum L.), yet its effects on quality characteristics and metabolic profiles remain poorly understood. In this study, fresh Rhubarb (X), normally dried Rhubarb (NK), and pithy Rhubarb (K) were systematically compared using sensory evaluation, microstructural observation, physicochemical analysis, and untargeted metabolomics. Compared with NK, K exhibited lower bitterness, darker color, more pronounced tissue collapse, and extensive cavity formation. X showed the highest total phenolic (TPC) and total flavonoid (TFC) contents. TPC in X was 26.21% and 48.03% higher than in NK and K, respectively, while TFC was 24.5% and 24.2% higher. K exhibited lower DPPH and FRAP antioxidant activities, whereas ABTS activity did not differ significantly among groups. Compared with NK, K exhibited markedly lower levels of major anthraquinones, including emodin, chrysophanol, aloe-emodin, physcion, and rhein. Untargeted metabolomics identified 1607 metabolites, with 199, 158, and 261 differentially accumulated metabolites in NK_vs_X, NK_vs_K, and K_vs_X, respectively. KEGG analysis revealed marked alterations in glycerophospholipid metabolism and phenylpropanoid-flavonoid biosynthesis. NK showed higher levels of several aromatic precursor- and polyketide-related metabolites and anthraquinones, whereas K exhibited distinct lipid-related and secondary metabolic profiles. These findings provide a basis for understanding post-drying quality deterioration and optimizing Rhubarb processing.