Influence of combined freeze-drying and far-infrared drying technologies on physicochemical properties of seed-used pumpkin

A promising drying technology, namely combined freeze-drying and far-infrared drying (FD-FIRD) was employed to dehydrate seed-used pumpkin (SUP) and its effects on physical characteristics (color, microstructure, texture, shrinkage), bioactive components (phenolics and carotenoids) of SUP were evaluated. The low-field nuclear magnetic resonance (LF-NMR) was used to analyze moisture state and distribution during FD process, which is beneficial for choosing appropriate moisture transfer point. Results showed that most free water (87.29 %) was removed when drying time reached 25 h, thus retaining original tissue structure of sample with almost no deformation during later FIRD process. Therefore, FD for 25 h followed by FIRD for 2 h (FD25-FIRD2) sample exhibited similar appearance with FD sample. The free phenolics content of FD-FIRD samples was 14.97 %-26.60 % higher than that of FD depending on FIRD duration. Especially, p-coumaric acid content of FD25-FIRD2 sample increased of 32.23 %. Besides, FD-FIRD performed higher retention for carotenoids showing 3.00-3.39 folds than FIRD.