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Citrus belongs to Rutaceae and is the largest fruit in the world. In China, citrus planting area is quite extensive, mainly concentrated in Hunan, Zhejiang, Fujian, Guangdong, Jiangxi and other places, is the third largest export trade agricultural products after wheat and maize. At present, China's citrus production ranks first in the world, accounting for 11.7% of the world's total output. After the production and processing of canned oranges and fruit juices, citrus peels are produced in large quantities. Citrus peel residue is a by-product of citrus industry, accounting for 30%~45% of the total citrus. The treatment of peel residue has become the most difficult problem in citrus processing industry in China. The peel residue is produced more than 500 tons every year in China. The traditional method is drying orange peel or Chinese patent medicine, and more than 80% orange peel is discarded without any treatment, resulting in waste of resources and environmental pollution. According to the research, citrus peels contain a large amount of water, cellulose and lignin, but also contain many kinds of bioactive ingredients such as essential oils, pigments, flavonoids, pectin and so on. These ingredients can be widely applied in food, medicine and chemical industry.
Using essential oil distillation microwave heating machine , the higher the frequency, the faster the heating speed, but the higher the frequency, the shorter the wavelength, and the smaller the penetration capacity. The heating speed of 2450 MHz microwave heating machine is faster than that of 915 MHz microwave, but the penetration depth of 915 MHz microwave is greater than that of the former. Therefore, for thicker materials, in order to achieve uniform heating, smaller frequency should be chosen, but thinner materials, higher frequency can be chosen to improve heating speed.In addition, the frequency also affects the dielectric loss coefficient. At room temperature, the dielectric loss coefficient of pure water at 2450 MHz microwave is twice that of 915 MHz microwave.
Distillation in water is the direct immersion of raw materials into water for distillation. The content of high boiling point aromatic components in the product obtained by this method is low. Water distillation is to place raw materials on porous baffles and heat the saturated steam generated by water through the raw materials. This method is not suitable for easily caking or powder raw materials, but the product quality is better than water distillation. Water vapor distillation is to place the raw material on a multi-layer porous partition board, through which the steam from the nozzle pipe passes through the raw material, and carry out steam distillation. The requirements for raw materials are the same as those for water distillation. As the temperature of steam can be adjusted at will, the product obtained is the best of the three methods. This experiment introduces the distillation method in water. Water molecule is easy to penetrate into the pericarp cell tissues, displace essential oil, make essential oil diffuse into water, form oil-water azeotrope under the action of water vapor and distill the oil-water mixture at the same time, and separate the oil layer after standing, then orange peel essential oil can be obtained.
Steam distillation is a commonly used method to extract essential oil from citrus peel.The principle is that citrus essential oil can be separated by distillation of oil and water under the action of water vapor, which is based on the permeation of water molecules, low boiling point of essential oil and volatilization with water vapor.Steam distillation has been used in the extraction of essential oil from citrus peel because of its simple equipment, easy operation, low cost and high oil yield. However, due to the long time and high temperature of steam distillation, it is easy to decompose the thermosensitive compounds in essential oils, thus reducing the quality of essential oils. Some studies have shown that the time of high temperature distillation can be greatly shortened by using ultrasonic treatment before steam distillation, and the extraction rate is nearly 1.5 times higher than that of direct distillation.
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