Research On Far-infrared Vacuum Drying Of Vegetables And Fruits

This paper has mainly researched the effects of far-infrared-vacuum drying technology on vegetables and fruits, simulated its drying process, and optimized the drying parameters for vegetables and fruits.A lab-scale far-infrared-vacuum dryer was designed and construct, and the materials could rotate in the dryer to make the drying process more uniform. The viewing mirror on the dryer could bear the high temperature, and the radiant distance and radiant power were alterable according to need.The kinetics of far-infrared-vacuum drying of thin layer sliced vegetables and fruits were studied by introducing an empirical mathematical models using carrot as test material. The empirical mathematical models were obtained by using polybasic linear regressing of the data produced in the lab-scale far-infrared-vacuum drying. The empirical mathematical models and the data showed that the drying rate was mainly influenced by the radiant power density and vacuum pressure, and the radiant distance exhibited little influence on it. In the drying process, the larger the radiant power density, the faster the drying rate. The vacuum pressure had prominent influence on the drying rate when the vacuum degree was higher than 0.07 MPa.The temperature distribution and changes during far-infrared-vacuum drying of slices vegetables and fruits were also investigated using carrot as test material, and related mathematical models were developed. The research showed that the distribution and changes of the temperature in samples were strongly related to the thickness of the slices dried in far-infrared-vacuum dryer. When the thickness was less than the critical value, most of the moisture evaporated and flowed out of the sample with little resistance and the surface temperature was almost the same with the core temperature, and when it was larger than the critical value, the core temperature was higher than the surface temperature because of the higher resistance. The whole drying process of the far-infrared-vacuum drying can be divided into 3 periods; they were warming-up period, constant temperature period and the heating-up period. In the warming-up period, the heat mainly resulted in the samples temperature rising. In the constant temperature period, the temperature almost kept constant and it mainly controled by the saturated vapor temperature under certain vacuum pressure, the bigger the vacuum pressure, the lower the temperature. However in the heating-up period, the temperature of the sample rised rapidly because of the resistance and little moisture available, and its change was controled by the outer drying condition such as far-infrared-vacuum radiant power density and vacuum pressure.A series of far-infrared-vacuum drying technology were done following orthogonal experiments, and qualities of samples were evaluated and tested by rehydration rate andβ-carotene retention. The result suggested that the larger the radiant power density, the shorter the drying process and worse the qualities. For the drying time and rehydration rate, power density had the most important effect and the vacuum pressure followed, and the radiant distance had the least effect on them. For theβ-carotene retention, the vacuum pressure had a most important effect and power density followed, the radiate distance had the least effect on them. The optimized drying parameters for vegetables and fruits which mainly concerned their qualities were: drying power density, 2 W/g; radiate distance, 155 mm; and vacuum pressure, 0.07 MPa. This paper also researched combination of far-infrared-vacuum with conventional air drying or vacuum drying using Chinese Chives as test material, and the results suggested that the combination of far-infrared-vacuum drying and other drying method could improve the product's quality such as appearance, rehydration rate and higher pigment retention.