Profiling dynamic alterations of bioactive metabolites and quinones in response to moisture loss stress during the drying process of rhubarb

Dried roots and rhizomes of rhubarb are widely used both as a vegetable and a medicinal product around the world. To date, most studies on the drying process of rhubarb have focused primarily on comparing different drying techniques and assessing whether the content of functional compounds meets standard requirements. However, few studies have explored the dynamic changes and underlying mechanisms of these functional ingredients during drying process. In this study, HPLC-DAD-based dynamic monitoring of anthraquinones, combined with widely targeted metabolomics, was employed to investigate the impact of moisture loss on the quality of rhubarb and to elucidate potential molecular mechanisms during the drying process. The results indicated that the contents of total and free anthraquinones reached their highest levels when the moisture content was 40 %, whereas other moisture levels generally led to a decrease. In contrast to anthraquinones, flavonoids, phenolic acids, and stilbenoids were identified as the most significantly changed functional components throughout the drying process. Further analysis revealed that the key metabolic pathways influencing these functional ingredients were glycolysis, TCA cycle, and pentose phosphate pathway, which are associated with respiratory metabolism. Phosphoenolpyruvate and D-erythrose-4-phosphate were the key precursors of shikimate pathway for the biosynthesis and interconversion between phenolic acids, flavonoids, stilbenoids and anthraquinones in rhubarb. This study provides a comprehensive understanding of how the drying process influences the functional components of rhubarb and offers valuable insights for determining the optimal moisture content in industrial drying processes.