Freeze-thaw and ultrasound pretreatment before microwave combined drying affects drying kinetics, cell structure and quality parameters of Platycodon grandiflorum

•Freeze-ultrasound + microwave combined drying was proposed as a new drying method.•Freeze-ultrasound markedly improved the drying efficiency of P. grandiflorum.•Ultrasound pretreatment effectively maintain original appearance of samples.•Freeze and freeze-ultrasound treatments was better in preserving active compounds. Platycodon grandiflorum (Jacq. A.DC.) is a kind of plant with high medical application value, and drying is an effective method to preserve its medical and commercial values. In this study, effects of freeze-thaw and ultrasound pretreatment before drying on drying kinetics, cell structure and quality parameters of Platycodon grandiflorum (P. grandiflorum) were investigated. Four P. grandiflorum samples were dried following control and pretreated with freeze-thaw (FT), ultrasound (US), freeze- ultrasound (FUS). Pretreatments could largely improve the mass transfer and 20 min was chosen as the pretreatment time for the subsequent experiments. The cell structure and biomechanical properties of cell wall were affected by different pretreatment as evidenced by optical microscopic analysis and pectin methylesterase (PME) activity results. Dynamic mechanical analyzer (DMA) data indicated that the viscoelastic properties varied after pretreatment and during drying process. Base on the results of drying rate, color evaluation, high performance liquid chromatography (HPLC), and gas chromatography–mass spectrometry (GC–MS), ultrasound pretreatment (US) was better in maintaining the original appearance, while the FT and FUS methods were ideal for preserving the Platycodin D and aroma compounds and enhancing the drying rate.