Research progress of microwave drying technology and equipment for agricultural products

Abstract: Taking the microwave drying technology as the research object, the research status of microwave drying equipment in agricultural products processing was expounded, and some technical problems such as uneven drying and partial burning in microwave drying were put forward. Then, the research progress of hot air microwave fluidized drying, vacuum microwave drying, freeze microwave drying and other combined technologies and equipment were introduced. Finally, by analyzing the advantages and disadvantages of different processing technology, it is pointed out that microwave combined drying technology is the development trend of  microwave drying industry for agricultural products. 

microwave drying equipment

Key words: agricultural products; microwave drying; equipment; research progress 0 Introduction

Drying is an important means to ensure the quality of agricultural products. It is also a common food processing technology. China's agricultural products are relatively scattered, and the processing level is backward, resulting in a large number of losses in the transportation, storage and marketing links after harvest. Timely and effective post-harvest drying is of great significance to reduce the production costs and economic losses of grain, fruits and vegetables, edible fungi and other agricultural products.

In recent years, the demand for dehydrated fruits and vegetables has been increasing rapidly both in China and in the international market. The dried products after dehydration are not only convenient for transportation, storage and sale, but also improve the added value of agricultural products, and have great market potential and development space. It is particularly important to improve the drying technology level of agricultural products.

microwave drying industry for agricultural 

Combined drying test of hot air and microwave

Microwave drying equipment has a significant advantage in drying efficiency, and there is a certain difference in drying quality between microwave drying equipment and hot air drying equipment. Lu Weiqiao et al. analyzed the quality of ginger slices dried by microwave and hot air through scanning electron microscope. The results showed that the structure of ginger slices dried by microwave changed, the cell wall became confused under microwave, and mixed with starch granules. The change of structure would affect the rehydration ability and rehydration ability of the products. After the physical properties.

Microwave drying equipment

Microwave drying is the whole volume heating. If the water is not dispersed in time, the material will deteriorate due to overheating. Microwave drying speed is too fast, the final moisture content will be difficult to control, these technical defects to a single microwave drying challenges. Hot air drying at the beginning and the final stage can save energy and control the final state of the product. In the process of atmospheric pressure, hot air can effectively remove the free water from the surface of the material, and the dehydration process is mild and slow, which can ensure that the cytoskeleton of the material is not destroyed. The microwave drying process can accelerate the migration of moisture in the material. The combination of microwave and hot air in the middle stage can improve the working efficiency and drying quality

2.1 hot air atmospheric pressure microwave drying

Hot-air microwave drying under atmospheric pressure is to optimize the drying process combination by synergistic dehydration of internal and external materials with microwave and hot air. The drying time of carrot by microwave was shortened by 25%-90% compared with that without microwave, and the better drying quality was obtained at low power. The microwave combination drying of Agaricus bisporus can not only shorten the processing time, but also improve the product quality and rehydration ability

Microwave drying of garlic 

Thermophysical properties and porous properties of Soybean under microwave drying mechanical equipment

Abstract: The temperature and quality changes of Soybean samples during microwave drying were measured in real time by using

microwave drying machine

test system. The variations of physical properties such as density, thermal conductivity, specific heat and thermal diffusivity of samples with different moisture content during drying were analyzed. 

From the point of view of porous media, the variations of pore properties such as porosity, fractal dimension, pore diameter and specific surface area in different drying nodes (100%, 85%, 70%, 55%, 40%) were studied. Law. By changing the microwave heating power, the influence of microwave power on the heat and mass transfer of samples is analyzed. The results showed that during the microwave drying process, a dense membrane was formed in the cortex cells of soybean, which hindered the water migration. When the moisture content was reduced to 30%, the coke appeared inside the sample, and then the cells in the center collapsed completely. But the density of the whole drying process is not large, and the reduction is about 5 kg /m3

. Soybean microwave drying equipment

Temperature dependent properties of wood during microwave drying

It can be said that the microwave radiation power has a significant effect on the temperature variation characteristics of wood. The higher the power is, the faster the heating rate is, and the shorter the isothermal time is. The thicker the sample is, the slower the heating rate is, but the difference is not big. The temperature distribution inside the wood during the microwave drying process is abnormal. The temperature distribution inside the wood is uniform, and the temperature gradient from the center to the wood surface is not ten. The temperature of wood increased during the late stage of drying.

Distribution and variation of water content field in wood during

  microwave drying equipment

During microwave drying of wood, the moisture distribution in wood interior is very different from that in conventional drying. In conventional drying, the moisture movement speed of interior is usually less than that of evaporation on the surface of wood, and the surface layer reaches below the fiber saturation point rapidly, and the central part is larger. Water content gradient. In the process of microwave vacuum drying, the moisture content distribution in wood showed different laws.

microwave drying of wood 

Analysis of moisture migration mechanism of wood during microwave drying

The microwave drying equipment test showed that the temperature distribution in wood was uniform during the microwave vacuum drying process, and there was no fixed distribution pattern of overall internal high, external high and external low or external high and internal low. Uniform, there is no obvious integral moisture gradient in the direction of wood thickness; with the decrease of wood moisture content, the moisture distribution inside the wood is more uniform; in the early stage of wood drying, there is moisture accumulation on the surface of the wood.

microwave drying equipment

In the course of microwave drying, when the moisture content of wood is below the fiber saturation point, there is no obvious internal high and external low moisture content field and fixed internal high and external low temperature field.

Therefore, the effect of moisture gradient on moisture diffusion and heat diffusion can be neglected during microwave drying of wood. Moisture migration in wood is mainly driven by the total pressure gradient, which is removed by osmotic flow in the form of liquid water or steam. When the moisture content of wood is below the fiber saturation point, the absorbed water in the wood itself does not move, but moves toward the adjacent area.

microwave drying of wood 

Study on microwave drying of wood

Wuxi No.2 Furniture Factory developed a wood microwave dryer in May 1977. Through several years of production practice, the technical performance of the microwave drying equipment is basically stable, and the drying production is relatively normal. Sometimes it can maintain two shifts of production, and has accumulated certain production experience, which is beneficial to the research and popularization of wood microwave drying technology in China. Contribution.

wood microwave dryer 
In order to summarize the experience, make the drying standard and promote the drying production, Northeast Forestry College cooperated with Wuxi Furniture Factory II. During 1981-1982, the productive technology experiment of wood microwave drying was carried out under the production conditions of the factory.

The test equipment and methods The test equipment is a microwave dryer with a Novo cavity, which consists of a microwave source, a resonant cavity, an energy suppressor, a humidity exhaust device and a transmission system.

microwave drying equipment 

Study on microwave drying

Based on the test results and years of production experience, the microwave output power listed in the microwave drying reference list of some commonly used tree species and sawn timber specifications is proposed. For a channel

microwave dryer 

like Wuxi Furniture Factory II, which consists of four two resonant cavities in series, the first three (1-3) can be of medium power. Begin to gradually increase to the maximum upper limit, the last (fourth) take the minimum power (lower limit), to avoid serious mismatch, damage pipe 2.

The results showed that the quality index of wood microwave drying could be added with carbonization index on the basis of convective drying quality index (average final moisture content, drying uniformity, moisture content gradient, stress deviation, visible defects such as cracking, bending, etc.). There are 385 samples in this experiment, only 9 of them are carbonized, accounting for 2.3%. If the moisture content gradient on the thickness of the wood according to the above drying criteria, the result of this test is satisfactory.

It reflects the obvious law that the water content of surface layer is higher than that of core layer (see Table 1).This is the characteristic of microwave drying different from convection drying. The distribution of final moisture content of 01.2.6 Xin Lan song 48 was uniform along the length of test material.

wood microwave drying