Design And Test Of The Wireless Monitoring System For The Moisture Content Of Fruits And Vegetables During The Freeze Drying

Vacuum freeze-drying technology not only maximizes the retention of nutrients and active substances of fruits and vegetables, but also maintains the original color, flavor and shape, so it has broad application prospects in fruit and vegetable processing industry. But vacuum freeze-drying is high energy consumption, time-consuming, expensive, which causes the new technology promote slowly in the field of fruit and vegetable drying industry, therefore, how to reduce energy consumption is widely concerned. In recent years, we found that the relative dielectric constant and the moisture content of fruits and vegetables have excellent correlation during the freeze-drying. The relative dielectric measurement system can be used to monitor moisture content of fruits and vegetables, In view of the freeze-drying processing production equipment of fruit and vegetable requirements, wireless measure will bring great convenience to the actual production application. After a lot of research, this paper design a online radio detection system which can measure the relative constant of fruits and vegetables during freeze-drying, and the system was been used to monitor the moisture content of fruits and vegetables during freeze-drying. The main contents include:(1) Designed an online and wireless system which can monitor moisture content of fruits and vegetables during freeze-dried. The system includes a measuring device, a transceiver and PC software. Measuring device comprises a capacitive sensor, a temperature sensor and ZigBee end-node, which measures the relative dielectric constant of fruits and vegetables and interior temperature of lyophilized, and transmits the measured data to the transceiver. Transceiver includes ZigBee coordinator node and serial-to-USB circuit, which receive the measurement data from ZigBee end-nodes and sends data to the PC, so as to establish communication between the host computer and the measuring device. PC software receives data and real-time data processes, display, save, and draw the figure.(2) Evaluation the accuracy of the monitoring system at room temperature and high temperature. At room temperature, RCL tester and monitoring systems were used to measure eight different sizes of ceramic capacitors, measurements show that the relative error of the monitoring system is less than 3%, so the system has very high measurement accuracy at room temperature. To evaluate the temperature stability of the monitoring system, respectively, using the system to measure different sizes of plate capacitor at room and high temperature. The results show that the system relative error is less than 0.5%, so the system has good thermal stability at high temperature. Therefore, the system can accurately detect the capacitance, and has good temperature stability, so it can be applied to measure the relative dielectric constant of fruit and vegetable in freeze-dried cartridge.(3) Temperature of drying oven and the moisture content of fruits and vegetables affect the relative permittivity. Apples and potatoes as materials were vacuum freeze-drying in the drying temperature 60,65, 70,75 and 80℃, measuring material’s the relative dielectric material when it’s moisture content was 10, 20,30,40 and 50%. The results show that the dielectric constant and the temperature has relatively excellent correlation at same moisture content, and the temperature is a key factor of the relative permittivity, In the same water content, the higher the temperature the greater the relative dielectric constant, and when the moisture content is high, the greater the temperature effects on the relative dielectric constant.(4) At a given temperature, establish the correlation model between the relative permittivity and moisture of apples and potatoes. Apples and potatoes as tested materials, the system measure the moisture content and relative permittivity of the material during vacuum freeze-drying at 60,70,80℃. The relative permittivity and moisture linear fitting results show that excellent relative between dielectric constant and moisture content of the material at each temperature, the correlation coefficient R2> 0.93. Therefore, we can use this system based on the relative dielectric constant of fruits and vegetables to measure moisture content on-line during freeze-drying.The measurement system realizes real-time and wireless measure relative dielectric constant of fruits and vegetables. It provides a new technology and equipment for exploring low energy consumption of fruits and vegetables during vacuum freeze-drying.