| Chinese date is a kind of fruit that is unique in China. It's nutrient-rich, high of medical value. It is considered a good tonic fruit. The commonly used drying methods are drying under roof, drying in the sunlight and roasting. It needs 10 to 15days of drying of Chinese date in the sunlight which is easy influenced by the weather and is of high rate rotting. The temperature can be controlled when roasting in oven with good quality of product. However it needs 24 to 48 hours with low energy efficiency and large amount of labor. In this experiment, Chinese date bought in Seattle of United States was used as the main material. Combined radio frequency and hot air(RFHA)drying of Chinese date was studied. The final aim was to reduce drying time, improve energy efficiency, improve product quality.A open-ended coaxial probe dielectric property measurement device was used to obtain Chinese date dielectric properties over the frequency range of 1～1800 MHz. Influences of frequency, moisture content and temperature on Chinese date dielectric properties were analyzed.Study showed that dielectric constant and dielectric loss factor of Chinese date decreased with the increase of frequency under the moisture content from 20% to 60%(w. b.). Dielectric constant increased with the increase of temperature. Dielectric loss factor increased quickly with the increase of temperature at radio frequency(RF)range. It changed barely with the increase of temperature at microwave(MW)frequency range. Therefore, it can be concluded that higher loss factor can appear at the site of higher moisture content and temperature, which caused faster heating and temperature uneven. Further more, temperature runaway occurred in RF heating.Penetration depths of MW and RF in Chinese date were calculated under different temperature and moisture content. Penetration depths of RF decreased steadily with the increase of moisture content and temperature. While the penetration depths of MW changed very little.Choosing pre-dried Chinese date as the test material, RFHA drying of Chinese date was conducted using a 6kW, 27.12MHz pilot-scale RF system(COMBI 6-S, Strayfield, UK). Fiber optic probes and infrared camera technology were used for temperature measurement. Initial current of different electrode space study showed that when the electrode space was greater than 15cm, there was a gently change of initial current. The drying effect of hot air temperature was greater than the electrode space. The optimum drying conditions were electrode space 20.5cm, air temperature 45℃. During the process of RFHA drying, material temperature at the middle layer was the highest, then that of top layer, ten the bottom layer. Material at the bottom layer lost the most of water among three layers. That at the top lost the least. During RFHA drying of Chinese date, the temperature of material went up fast at beginning thirty minutes, and then it increased slowly to the end. The drying rate remained nearly constant during the process of RFHA drying. The moisture content(w. b.)went down to 25.68% after 360min drying, which reached the national standard of China. It was tried of hot air drying of Chinese date at 60℃using a kind of tunnel dryer. The result showed that hot air drying took 3 times of the time that RFHA drying needed. So it come the conclusion that RFHA drying technology can significantly shorten the drying time, can improve production efficiency, and reduce energy consumption.At present, there was rarely literature on RF drying of food and agricultural products in china and abroad. It can not been seen any research on dielectric properties and RF drying of Chinese date in the literature. The trial was the first RFHA drying research on dates. It was the first study of dielectric properties of dates. It can provide technical parameters and theoretical basis on RFHA drying of Chinese date. |
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