Influence Of Drying Temperature On Dielectric Properties And Dehydration Characteristic Of Electric Field Pretreated Fruits And Vegetables

Vacuum freeze-drying, which could maintain the original quality of materials, holds significant promise in the field of fruit and vegetable processing. However, the drying time, energy consumption, and production costs have become bottlenecks, which need to be resolved urgently in practical production. In this dissertation, the influence of high-voltage pulse electric field (HPEF) pretreatment on fruits and vegetables was investigated to optimize the drying process and improve the drying rate. A novel approach was proposed to monitor the moisture content of fruits and vegetables online with the dielectric properties, which indicated the dehydration characteristics and were valuable for freeze-drying process monitoring of HPEF pretreated fruits and vegetables. Temperature is one of the most critical parameters to affect the dielectric properties. Therefore, this dissertation focused on the influence of drying temperature on dielectric properties and moisture diffusion in dehydration process of HPEF pretreated fruits and vegetables, and analyzed the correlation between the dielectric properties and the moisture content of fruits and vegetables at different temperature drying process. The main contents include:(1) Two-probe electrode device was designed with3532-50LCR meter to measure the dielectric parameters of the fruits and vegetables such as impedance, conductivity, and capacitance at the middle time of the drying process in the frequency range of100～1000kHz.(2) Influence of drying temperature on dielectric properties of HPEF Pretreated fruits and vegetables was studied. Apple, potato, white radish, and carrot were set as test materials. Experiments were conducted on the parallel and perpendicular to the fiber direction of organizational structure opposite characteristics significantly fruits and vegetables. Each pretreated samples was sampled at60℃,70℃,80℃,90℃and100℃for drying process in turn, and the dielectric parameters were measured. The results showed:Drying temperature had varying degrees of impact on impedance, conductivity, and capacitance of fruits and vegetables. Impedance increased with the rising drying temperature, while conductivity was reduced. The conductivity increased with the extension of drying time first increased slowly and then gradually decreased. Capacitance showed decreasing trend with the drying temperature increasing. Sampling in a different direction for impedance, conductivity and Capacitance may also be affected.(3) Experimental study on the dehydration properties and the variation of moisture content of fruits and vegetables were designed with different drying temperature. The test has shown:the pretreated fruits and vegetables were more prone to dryness, and the drying rate increase rapidly with temperature. In the same drying conditions, the dehydration rate was faster in the vertical fiber direction, which can reduce the freeze-drying time in production.(4) Combined with fruits and vegetables’equivalent electrical model, the relationship was discussed between dielectric characteristics and the drying temperature and dewatering characteristics. The linear regression model of capacity and moisture content under different drying temperature was established. Analysis results showed that both of them were significantly positive correlation. Dielectric constant and the capacitance of fruits and vegetables decreased with the increasing drying temperature, which had the same change tendency with moisture content.This dissertation investigated the influence of drying temperature on dielectric properties and drying characteristics of HPEF pretreated fruit and vegetables. Results can be the basis for using dielectric characteristics real-time online monitoring moisture content in the drying process.