Quality deterioration and metabolic regulation during hot-air drying of Rheum palmatum L.: an integrated analysis of physiology, drying kinetics, non-targeted metabolomics, and enzyme activity dynamics

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Abstract
Drying is a critical post-harvest step in the processing of Chinese medicinal materials; however, the mechanistic links between key processing parameters and final product quality remain incompletely understood. To address this, we conducted a multidimensional investigation into the mechanisms of quality deterioration and metabolic regulation in Rheum palmatum L. during hot-air drying at 45 °C, with a particular focus on the role of slice thickness (2–8 mm). We systematically examined the synergistic effects of thickness and drying time on color evolution, key enzyme activities, and phytochemical composition. The results indicate that color deterioration occurs in two consecutive stages: an initial phase dominated by polyphenol oxidase (PPO)-mediated enzymatic browning, followed by a later stage of non-enzymatic browning. Slice thickness strongly regulated moisture migration, which in turn governed the dynamic retention of bioactive compounds, with 4 mm slices exhibiting optimal preservation of total anthraquinones. The drying kinetics of these optimal slices were most accurately described by the Wang and Singh model (R2 > 0.999). Non-targeted metabolomics further revealed extensive metabolic reprogramming, identifying 652 differentially accumulated metabolites. Pathway enrichment analysis highlighted flavonoid and tyrosine biosynthesis as the most significantly altered pathways. From these data, we delineated a regulatory network involving 7 key metabolites and 10 associated enzymes, providing a mechanistic scaffold for quality formation. This study proposes an optimized drying strategy (4 mm slice thickness with endpoint moisture control) and establishes an integrated “processing–structure–metabolism” framework that links physical drying conditions to biochemical responses. These findings offer a theoretical basis for the precision drying of rhubarb and provide a methodological reference for the processing of other medicinal plants.
Funding Information
  • National Natural Science Foundation of China (No. 81660581)
  • Key research and development plan of Gansu Province (No. 21YF5NA081)

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