Developing effective radio frequency drying processes for tiger nuts: Dynamic analysis of moisture state, dielectric properties and quality

Abstract

Radio frequency (RF) heating technology is widely applied to dry agricultural products due to the advantages of fast and volumetric heating, but it lacks systematic and in-depth studies on combination drying methods. This study aimed to investigate the drying mechanism of tiger nuts with hot-air drying (HD), radio frequency drying (RFD), radio frequency assisted hot-air drying (RFHD), and radio frequency-vacuum drying (RFVD), in terms of analyzing dynamic changes of dehydration kinetics, moisture migration, and dielectric properties during the drying processes, as well as quality attributes of dried tiger nuts. The results indicated that the RF drying time (including RFD, RFHD and RFVD) was significantly reduced by 38.60–54.39%, compared with HD. Although RF drying had a positive effect on physical qualities, it caused clear changes to the microstructure. Among all drying methods, the free water content (A23) in the tiger nuts decreased as drying processed, while the immobilized water content (A22) increased first and then decreased. The A23 exhibited strong positive correlations with dielectric constant (ε′) and loss factor (ε″). Correspondingly, ε′ and ε″ gradually decreased due to a reduction in the A23 value. The results may comprehensively understand the RF drying mechanism of tiger nuts to provide high-quality products and optimize the drying process.