Influence of different drying methods coupled with different process modes on physicochemical qualities and anthraquinones contents of Rheum palmatum L

Rhubarb is a popular food in Europe with laxative properties attributed to anthraquinones. Industrially drying fresh Rheum palmatum easily lose color characteristics and bioactive anthraquinone (AQ) content. Hot air drying (HAD), sun drying (SD) and smoke pre-drying followed by hot air drying (SMD-HAD) were employed to preserve physicochemical qualities and AQ content of R. palmatum. Different growing years, process modes (segment length, diameter, drying temperature and whether or not peeled) were compared among different drying methods. Compared with HAD and SD, SMD-HAD maintained optimal levels of AQ content and shiny color, it also significantly shortened the drying time by 10–15 days than SD. The segment length of 5 cm and the whole rhizome of 4-year-old plants were suitable to meet the industrial drying. Treatments of keeping the peel showed the higher total and free AQ content (2.84% and 0.33%) than treatments of removing the peel (1.51% and 0.23%). Moreover, AQ content in periderm was much higher than in phloem + xylem part and pith part, this discovery will present theoretical basis for revising the stipulation that Rhubarb should be scraped in Chinese pharmacoeia. It can be concluded that SMD-HAD is an efficient technique for the industrial drying production of high-quality R. palmatum. •Smoke pre-drying followed by hot air drying is suitale for drying R. palmatum.•Reserving periderm can obtained high anthraquinones content.•Below 50 °C was a good choice to maintain high quality of R. palmatum.