Effects of freezing and thermal pretreatments on drying of Vaccinium bracteatum Thunb leaves: Drying mechanism, physicochemical properties and ability to dye glutinous rices

摘要

The influences of freezing pretreatment, thermal pretreatments (water blanching and steam blanching) and their combinations on hot-air drying kinetics of Vaccinium bracteatum Thunb (VBT) leaves at 60 degrees C and key quality attributes were investigated. Generally, all the applied pretreatments accelerated VBT leaf drying process. The sequential freezing (8 h) and water blanching (1 min) pretreatments shortened the drying time by 65.5%. The diffusion modeling results revealed that all the pretreatments intensified water diffusion and external water exchange with hot air during drying. Freezing pretreatments had no preservative effect on phenolics, iridoids, and antioxidant capacity of VBT leaves after dehydration, whereas both thermal pretreatments significantly improved dried VBT leaf quality in these three aspects. On the contrary, thermal pretreatments had destructive effects on activities of polyphenol oxidase and beta-glucosidase. Due to the low beta-glucosidase activity in dried VBT leaves with thermal pretreatments that participated in the transformation of iridoids to generate dark colorants eventually, the extracts from dried VBT leaves with only thermal pretreatments had weak ability to dye glutinous rice. However, this problem was fixed once extra beta-glucosidase was added before rice cooking. Overall, the obtained results can provide guidance for the preservation and drying of VBT leaves that has been poorly studied. (C) 2020 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.