Study On Vacuum Far-infrared Drying Of Lemon Slices

The Far-infrared drying in vacuum is a combination drying method which is used to dry heat—sensitive and oxygen—sensitive materials. It has the advantages of vacuum drying and rapid heating characteristics of far infrared. For different materials, the lacks of data such as suitable operating pressure, radiation distance, penetration depth and the absorption wavelength range limit the vacuum far infrared drying equipment in the application of dry food and biological materials.Lemon is rich in VC, VA, VB1, VB2, VP and limonene, and also contains a plenty of trace elements, such as Ca, P, Fe, Zn, Mg, etc. At the meantime, Lemon plays a role in the prevention and adjuvant therapy to various diseases. There are about 300.000 tons annual output of lemon in China. Dride lemon slices are more convenient to store and transport. Currently, drying methods of lemon slices are given priority to hot air drying and freeze-drying. Dried lemon slices have poor Vitamin C content, dark color, low commercial value in hot air drying, on the contrary, have high quality in freeze-dried lemon. But freeze drying needs a long drying time and high cost.By analyzing the drying kinetics characteristics of lemon slices under the conditions of vacuum with infrared drying, the paper establishes the drying kinetic model of lemon slices, and determines the related parameters of the drying kinetic model by linear regression analysis. Through the studies of drying kinetics characteristics, RGB intensity and Vitamin C retention rate of lemon slices under the different conditions of vacuum with infrared drying, t the paper obtains optimal drying process parameters of which the temperature at 60℃, pressure at 2kpa, radiation distance at 140mm and slice thickness at 4mm. Through analysis of RGB intensity mean of lemon during the process of the far-infrared drying in vacuum, the study finds that the merits of the color of lemon the final product depends on its later stage of drying, namely the drying terminal segment is a critical period of controlling lemon slices products.The fresh lemon slices were dried by three methods:hot air drying, far-infrared drying in vacuum and vacuum freeze-drying in this paper. After comparative analysis of drying time, average value of RGB intensity and Vitamin C retention rate, a conclusion was drawn out in terms of performance:Vacuum freeze-drying> Vacuum far infrared drying>Hot air drying.In order to solve the uneven problem and reduce lemon slice’s degree of browning in the later stage of the infrared drying in vacuum, two kinds of modified drying methods (variable temperature far-infrared drying in vacuum and vacuum far-infrared-hot air drying) are introduced. The comparative analyses of drying time, average value of RGB intensity and Vitamin C retention rate prove that the two modified drying process can solve the problem effectively, and the lemon slices dried by far infrared vacuum-hot air drying method is slightly better than the variable temperature vacuum far-infrared drying.