CN113558166A - Microwave-assisted drying method for agricultural products - Google Patents
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- CN113558166A CN113558166A CN202010350795.4A CN202010350795A CN113558166A CN 113558166 A CN113558166 A CN 113558166A CN 202010350795 A CN202010350795 A CN 202010350795A CN 113558166 A CN113558166 A CN 113558166A
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- A23—FOODS OR FOODSTUFFS; TREATMENT THEREOF, NOT COVERED BY OTHER CLASSES
- A23L—FOODS, FOODSTUFFS, OR NON-ALCOHOLIC BEVERAGES, NOT COVERED BY SUBCLASSES A21D OR A23B-A23J; THEIR PREPARATION OR TREATMENT, e.g. COOKING, MODIFICATION OF NUTRITIVE QUALITIES, PHYSICAL TREATMENT; PRESERVATION OF FOODS OR FOODSTUFFS, IN GENERAL
- A23L3/00—Preservation of foods or foodstuffs, in general, e.g. pasteurising, sterilising, specially adapted for foods or foodstuffs
- A23L3/005—Preservation of foods or foodstuffs, in general, e.g. pasteurising, sterilising, specially adapted for foods or foodstuffs by heating using irradiation or electric treatment
- A23L3/01—Preservation of foods or foodstuffs, in general, e.g. pasteurising, sterilising, specially adapted for foods or foodstuffs by heating using irradiation or electric treatment using microwaves or dielectric heating
Abstract
A microwave-assisted drying method for agricultural products combines staged temperature-controlled hot air drying with microwave collaborative drying, and comprises a first stage, a second stage and a third stage of microwave-assisted hot air drying steps, wherein wind speed is controlled to be constant, microwave power is set to be 0.8-1.5W/g, materials are grouped according to a first layering thickness and are respectively put into microwave-hot air drying equipment, the temperature is set to be a first-stage drying temperature, and first-stage drying is carried out until the water content of the materials reaches a first conversion point; fully mixing the materials after the first-stage drying, grouping according to the thickness of a second layer, setting the temperature as the second-stage drying temperature, and drying in the second stage until the water content of the materials reaches a second conversion point; and after the materials which are dried in the second stage are fully mixed, grouping according to the thickness of the third layer, setting the temperature as the third-stage drying temperature, drying the materials in the third stage until the moisture content of the wet basis of the materials to be dried is lower than 8%, and stopping drying.
Description
Technical Field
The invention relates to an agricultural product processing technology, in particular to a microwave-assisted drying method for agricultural products by material overlapping and variable temperature control.
Background
Drying is an effective way for agricultural product storage, and the aim of long-term storage is achieved by reducing the moisture content in the agricultural product.
At present, the methods for drying agricultural products mainly comprise hot air drying, freeze drying, spray drying, infrared drying, microwave drying and the like. The hot air drying has the advantages of low equipment investment and simple operation, but the drying efficiency is low, the drying time is long, the energy consumption is high, the product quality is reduced due to the long drying time, the crusting phenomenon appears when the surface of the material is hardened, and the internal moisture is difficult to remove.
Microwave drying is a drying mode which is started in recent years, and the material is heated from the inside of the material in the form of electromagnetic waves, so that moisture in the material is vaporized, and the moisture migrates from inside to outside, thereby drying the material. The microwave drying can reduce the temperature difference of the inner layer and the outer layer of the material in the drying process, effectively reduce the generation of thermal stress cracks of the material, and is beneficial to ensuring the quality of the dried material, but also has the problem of uneven drying caused by different temperature rise of each part of the material in the drying process.
The application number is 201510654273.2, the name is a Chinese invention patent application of 'a three-stage normal pressure microwave spouted drying method for rapidly preparing fruit and vegetable crisp grains', and discloses a drying method for fruit and vegetable crisp grains by controlling parameters such as microwave power, pulse frequency, intermittence time and the like, which comprises three stages, wherein the microwave power is firstly high and then low, the pulse frequency is firstly low and then high, the intermittence time is gradually shortened, the energy utilization rate can be effectively improved in the whole drying process, and the economic benefit is high.
The Chinese patent application with the application number of 201810819759.0 and the name of 'a lotus seed drying method' discloses a method for drying large-particle lotus seeds by low-temperature hot air in the early stage and by stages in a temperature-controlled microwave termination drying mode in the later stage, so that the drying efficiency and the product quality are improved, and a processing method is provided for efficient and high-quality production of the lotus seeds.
The Chinese patent application with the application number of 201710130185.1 and the name of 'multi-stage variable-temperature drying method for lemon slices' discloses that the drying temperature of each stage is controlled to be different through three-stage drying, the temperature is firstly high and then low, the early-stage temperature is high, the drying speed is accelerated, the later-stage temperature is low, the scorching phenomenon is prevented, the dried lemon slices are good in quality, and the reliability and the stability in the drying process are obviously improved.
The application number is 201511001523.9, the name is 'method for producing fruit and vegetable crisp chips by microwave-assisted pressure difference flash evaporation drying', Chinese patent application discloses that different drying modes are combined, the fruit and vegetable subjected to vacuum drying pretreatment is dried by pressure difference flash evaporation and microwave assistance, and the purpose of fruit and vegetable dehydration is achieved by utilizing the pressure difference. Different drying methods are combined at different stages, and in the drying process, the characteristics of high efficiency of microwave drying are utilized, and pressure difference flash evaporation is utilized to dehydrate fruits and vegetables, and the crisp taste can be kept.
However, the single drying mode in the prior art has problems of self, cannot meet the drying processing requirements of agricultural products, the hot air drying temperature is too low, the time consumption is too long, the temperature is too high, along with the loss of moisture, the material components such as protein, starch, fat and the like are denatured in the later drying period, and the content of nutrient substances is reduced; the microwave penetration ability is strong, begins the drying from the material is inside, can effectively reduce the inside and outside temperature difference of material, accelerates the moisture diffusion rate, but has the energy consumption height, and drying efficiency is low, dry inhomogeneous scheduling problem.
Disclosure of Invention
The invention aims to solve the technical problem of providing a microwave-assisted drying method for agricultural products by material overlapping and variable temperature control, which can effectively solve the problems of low drying efficiency, long time, poor drying uniformity and the like of a single drying method in the prior art, improve the utilization rate of energy consumption and reduce the energy consumption.
In order to achieve the purpose, the invention provides a microwave-assisted drying method for agricultural products, wherein a material overlapping and variable-temperature control mode is adopted, and materials to be dried are treated by combining staged temperature-controlled hot air drying and energy-carrying microwave synergistic drying, and the microwave-assisted drying method comprises the following steps:
s100, performing first-stage microwave-assisted hot air drying, grouping the selected materials to be dried according to the thickness of a first layer, respectively putting each group of the materials to be dried into microwave-hot air drying equipment, setting the temperature as a first-stage drying temperature, controlling the wind speed of hot air to be constant, performing microwave drying, setting the microwave power to be 0.8-1.5W/g, and performing first-stage drying on the materials to be dried until the water content of the materials to be dried reaches a first conversion point;
s200, performing secondary microwave-assisted hot air drying, fully mixing the materials to be dried which are dried in the first stage, grouping the materials according to the thickness of a second layer, respectively putting the materials into microwave-hot air drying equipment, setting the temperature as the secondary drying temperature, controlling the wind speed of hot air to be constant, performing microwave drying, keeping the microwave power the same as that in the step S100, and performing secondary drying on the materials to be dried which are dried in the first stage until the water content of the materials to be dried reaches a second conversion point; and
s300, third-stage microwave-assisted hot air drying, fully mixing the materials to be dried after the second-stage drying, grouping according to the thickness of a third layer, respectively putting the materials into the microwave-hot air drying equipment, setting the temperature to be a third-stage drying temperature, controlling the wind speed of hot air to be constant, performing microwave drying, keeping the microwave power the same as that in the step S100, performing third-stage drying on the materials to be dried after the second-stage drying is completed until the moisture content of a wet basis of the materials to be dried is lower than 8%, and stopping drying.
The microwave-assisted drying method for agricultural products, wherein step S100 is preceded by a preprocessing step: before drying begins, the fresh material to be dried is selected, washed and/or sliced.
The microwave-assisted drying method for agricultural products, wherein after the step S300, the method further comprises the following steps: after drying, the dried material was cooled to room temperature and stored after packaging.
The microwave-assisted drying method for the agricultural products is characterized in that the thickness of the first sub-layer is 10-30mm, and the thickness of the second sub-layer is 20-60 mm; the thickness of the third layer is 30-90 mm.
The microwave-assisted drying method for the agricultural products is characterized in that the wind speed of the hot wind is 1-1.5 m/s.
The microwave-assisted drying method for the agricultural products, wherein the thickness of the first layer is 5-15 mm; the thickness of the second sub-layer is 10-30 mm; the thickness of the third layer is 15-45 mm.
In the microwave-assisted drying method for agricultural products, the first conversion point is a dividing point of water with a moisture content of a wet base at a difficult flowing state and bound water; and the second conversion point is a dividing point of the moisture content of the wet base between free water and water which is difficult to flow.
In the microwave-assisted drying method for agricultural products, the first conversion point is 50% of the initial water content of the material to be dried; the second conversion point is 25% of the initial water content of the material to be dried.
The microwave-assisted drying method for the agricultural products is characterized in that the microwave power is 1.2W/g.
The microwave-assisted drying method for the agricultural products is characterized in that the primary drying temperature is high-grade temperature, and the temperature value of the high-grade temperature is 70-90 ℃; the second-stage drying temperature is a middle-grade temperature, and the temperature value of the middle-grade temperature is 50-70 ℃; the third-stage drying temperature is a low-grade temperature, and the temperature value of the low-grade temperature is 30-50 ℃.
The invention has the technical effects that:
the invention adopts the modes of material overlapping and staged variable-temperature drying, hot air drying is taken as the main mode, hot air microwave combined drying is taken as the auxiliary mode of whole-process microwave drying, the blast temperature is divided into three stages from high to low, the temperature of each stage is changed according to different water conversion points, the temperature change gradient is reduced, and the drying uniformity is obviously improved. The temperature of hot air changes in stages in the drying process, so that the drying uniformity and the quality of the dried product can be better ensured compared with constant-temperature drying. On the basis of staged temperature control hot air drying, the energy-carrying microwave synergistic drying is combined, so that the defects of hot air drying can be effectively overcome, and the drying efficiency is improved. For spouting the drying, the selection on the drying method is different, is favorable to the scale of equipment and technology more, can improve drying efficiency and drying uniformity, and efficiency is higher, can dry under the ordinary pressure, and the mode is simple, changes the operation.
The invention is described in detail below with reference to the drawings and specific examples, but the invention is not limited thereto.
Drawings
FIG. 1 is a flow chart of an embodiment of the present invention.
Wherein the reference numerals
S000-S400 steps
Detailed Description
The invention will be described in detail with reference to the following drawings, which are provided for illustration purposes and the like:
according to the invention, hot air drying is combined with microwave drying to dry agricultural products, and in the hot air drying process, microwave assistance is simultaneously utilized, heat is transferred to moisture in the materials in the hot air introducing process, so that the vaporization and evaporation of the moisture in the materials are accelerated, the moisture gradient inside and outside the materials is increased along with the increase of the moisture evaporation amount, the internal moisture is transferred outwards, and meanwhile, the microwave is added in the whole process to reduce the temperature difference between the inner layer and the outer layer of the materials, accelerate the outward transfer of the internal moisture in the materials, and avoid the phenomenon that the internal moisture is not dried yet and the crust phenomenon appears on the surface of the materials at the later stage of drying, so that the moisture transfer is hindered. The combined drying method of hot air and microwave can effectively improve the drying efficiency, ensure the quality of the dried material, and avoid the problems of long drying time, low drying efficiency, high energy consumption, inconsistent temperature in the drying process, uneven drying and the like; meanwhile, the materials with the same water content are mixed after each drying stage is completed by material overlapping and variable-temperature drying, and the temperature is controlled in each stage, so that the energy consumption can be effectively reduced, the energy consumption utilization rate is improved, the drying time is shortened, and the drying uniformity is effectively ensured. In the drying process, the thickness of the material is changed from three stages of 'thin-middle-thick' in different drying stages, and the drying temperature is changed from three stages of 'high-middle-low' at the same time; layering thickness corresponds high-grade temperature, can be when dry initial stage material moisture content is high, with higher speed the evaporation of water, middle level thickness corresponds middle-grade temperature, let the lower environment of material entering temperature through high temperature drying, let the material carry out the process of slow soviet and dry going on simultaneously, the low layer thickness corresponds low-grade temperature, the material has got into dry later stage this moment, the temperature reduces, guarantee material drying quality and nutrient substance's reservation, improve the product quality effectively and make the drying process change the control.
Referring to fig. 1, fig. 1 is a flow chart of an embodiment of the present invention. The invention relates to a microwave-assisted drying method for agricultural products, which adopts a material overlapping and variable-temperature control mode, utilizes a staged temperature control hot air drying mode in combination with energy-carrying microwave cooperative drying to treat materials to be dried, microwave-hot air drying equipment can be used in the drying process, the main body of the equipment is a microwave-hot air drying device and comprises an openable drying chamber, a shell for sealing the equipment is arranged outside the drying chamber, a control part and a touch screen are arranged on the shell and used for controlling drying parameters, a microwave generating device is arranged at the top of the drying chamber, a material tray is arranged at the lower part of the drying chamber, the materials are subpackaged through the material tray, a hot air inlet corresponding to the material tray is arranged at the bottom of the drying chamber, a hot air control device is externally connected, and microwave drying is continuously carried out in the whole drying process. The microwave-assisted drying method for agricultural products of the embodiment can comprise the following steps:
step S100, performing first-stage microwave-assisted hot air drying, grouping the selected materials to be dried according to a first layer thickness (preferably 10-30mm), respectively putting each group of the materials to be dried into a drying bin of microwave-hot air drying equipment, setting drying parameters, setting the temperature to be a first-stage drying temperature (preferably 70-90 ℃), setting the air speed of hot air to be unchanged through a hot air control device, performing microwave drying, setting the microwave power to be 0.8-1.5W/g, preferably 1.2W/g, performing first-stage drying on the materials to be dried, determining the moisture content by measuring the mass change of the materials until the moisture content of the materials to be dried reaches a first conversion point, wherein the first conversion point is a boundary point of water with difficult flow and combined water, preferably 50% of the initial moisture content of the materials to be dried, stopping the first stage drying;
step S200, secondary microwave-assisted hot air drying, wherein the materials to be dried after the first-stage drying are fully mixed and grouped according to the thickness of a second layered layer (preferably 20-60mm), the grouped materials are respectively placed into a drying bin of microwave-hot air drying equipment, parameters are adjusted, the temperature is set to be the second-stage drying temperature (preferably 50-70 ℃), the wind speed of hot air is controlled to be unchanged, microwave drying is carried out, the microwave power is kept the same as that in step S100, the embodiment is kept at 1.2W/g, the materials to be dried after the first-stage drying is carried out in the second-stage drying, the quality change of the materials is measured until the moisture content of the materials to be dried reaches a second conversion point, the second conversion point is a boundary point of moisture base moisture content between free water and water which is difficult to flow, preferably 25% of the initial moisture content of the materials to be dried, stopping the second stage drying; and
step S300, third-stage microwave-assisted hot air drying, fully mixing the materials to be dried after the second-stage drying, grouping the materials to be dried according to the thickness (preferably 30-90mm) of a third layer, respectively putting the materials into a drying bin of microwave-hot air drying equipment, adjusting parameters, setting the temperature to be a third-stage drying temperature (preferably 30-50 ℃), controlling the wind speed of hot air to be unchanged, performing microwave drying, keeping the microwave power the same as that in step S100, keeping the microwave power at 1.2W/g in the embodiment, performing third-stage drying on the materials to be dried after the second-stage drying, measuring the mass of the materials until the moisture content of a wet basis of the materials to be dried is lower than 8%, and stopping drying.
Wherein, before step S100, a preprocessing step S000 may be further included: before drying begins, the fresh materials to be dried are selected, cleaned and/or sliced, and dust is cleaned. The step S300 may be followed by a packaging and storing step S400: after drying, the dried material was cooled to room temperature and stored after packaging.
In the embodiment, the thickness of the first sub-layer is preferably 10-30mm, and the thickness of the second sub-layer is preferably 20-60 mm; the third layer thickness is preferably 30-90 mm. The wind speed of the hot wind is preferably 1-1.5 m/s. In another embodiment, the first sublayer thickness may be 5-15 mm; the thickness of the second sub-layer can be 10-30 mm; the third partial layer may have a thickness of 15-45 mm. The first-stage drying temperature is high-grade temperature, and the temperature value of the high-grade temperature is preferably 70-90 ℃; the second-stage drying temperature is a middle-grade temperature, and the temperature value of the middle-grade temperature is preferably 50-70 ℃; the third-stage drying temperature is a low-grade temperature, and the temperature value of the low-grade temperature is preferably 30-50 ℃. The variable temperature drying temperature of the three stages changes from high to low, and each stage corresponds to a corresponding temperature; the material thickness corresponding to the three drying stages is changed from thin to thick, and each stage corresponds to the thickness of the corresponding material layer.
The experimental procedures used in the following examples are conventional unless otherwise specified. The experimental raw materials and the related equipments used in the following examples are commercially available unless otherwise specified.
Example 1
The method for microwave-assisted drying of agricultural products is adopted to carry out material overlapping and variable-temperature control drying on the peanuts. The specific process is as follows:
(1) before drying begins, cleaning fresh peanuts to be dried, and selecting clean peanuts with full seeds;
(2) grouping the selected peanuts, dividing the materials into 4 groups according to the layering thickness of 15mm, controlling the wind speed to be 1m/s, setting the temperature according to three gears of high-medium-low, and preparing to carry out variable temperature drying in three stages;
(3) respectively putting each group of materials into a drying bin of hot air-microwave drying equipment, setting the temperature as 90 ℃ of primary drying temperature and setting the microwave power as 1.2W/g, starting the first-stage drying until the water content reaches about 30%, and stopping the first-stage drying;
(4) fully mixing the peanuts which are dried in the first stage, dividing the peanuts into 2 groups according to the thickness of the material as the thickness of a middle layer, respectively putting the peanuts into a drying bin of hot air-microwave drying equipment for drying, setting the drying temperature to be 70 ℃ of the second-stage drying temperature, keeping the microwave power at 1.2W/g, starting the second-stage drying until the water content reaches 15%, and stopping the second-stage drying;
(5) fully mixing the peanuts which are dried in the second stage, layering the peanuts according to the thickness of a thick layer of 45mm, putting the mixture into a drying bin of hot air-microwave drying equipment, setting the temperature as 50 ℃ of third-stage drying temperature, keeping the microwave power at 1.2W/g, starting drying in the third stage until the moisture content of a wet base is lower than 8%, and stopping drying;
(6) and cooling the dried peanuts to room temperature, packaging and storing.
The obtained dried peanuts have higher quality, higher drying rate and better drying effect than peanuts dried under the conditions of single temperature and uniform thickness. The method not only improves the drying efficiency, but also greatly improves the drying quality of the dried product.
Example 2
By adopting the microwave-assisted drying method for the agricultural products, the apples are subjected to material overlapping and variable-temperature control drying to prepare apple slices. The specific process is as follows:
(1) before drying, cleaning, airing and slicing fresh apples to be dried, wherein the thickness of apple slices is preferably 3-5 mm;
(2) grouping the cut apple slices, dividing the materials into 4 groups according to the layering thickness of 10mm, controlling the wind speed to be 1.5m/s, setting the temperature according to three gears of high-middle-low, and preparing to carry out variable temperature drying in three stages;
(3) respectively putting each group of materials into a drying bin of hot air-microwave drying equipment, setting the temperature as 70 ℃ of primary drying temperature, preferably 1.2W/g of microwave power, starting the first-stage drying until the water content reaches about 30%, and stopping the first-stage drying;
(4) fully mixing the apple slices dried in the first stage, uniformly dividing the apple slices into 2 groups according to the thickness of the middle layer as the thickness of the material, respectively putting the apple slices into drying bins of hot air-microwave drying equipment for drying, setting the drying temperature to be 50 ℃ of the second stage drying temperature, keeping the microwave power at 1.2W/g, starting the second stage drying until the water content reaches 15%, and stopping the second stage drying;
(5) fully mixing the apple slices subjected to the second-stage drying, layering according to the thickness of a material layer being a thick layer of 30mm, putting the apple slices into a drying bin of hot air-microwave drying equipment, setting the temperature to be 30 ℃ of third-stage drying temperature, keeping the microwave power at 1.2W/g, starting the third-stage drying, and stopping drying until the moisture content of a wet base is lower than 8%;
(6) and cooling the dried apple slices to room temperature, packaging and storing.
The obtained dried apple slices have higher quality, higher drying rate and better drying effect than those dried under the conditions of single temperature and uniform thickness. The method not only improves the drying efficiency, but also greatly improves the drying quality of the dried product.
Example 3
The dried sweet potatoes are prepared by carrying out material overlapping and variable-temperature control drying on the sweet potatoes by adopting the agricultural product microwave-assisted drying method. The specific process is as follows:
(1) before the drying starts, cleaning, peeling, airing and slicing fresh sweet potatoes to be dried, wherein the thickness of the dried sweet potatoes is preferably 2-3 mm;
(2) dividing the cut sweet potato strips into groups, dividing the materials into 4 groups according to the layering thickness of 15mm, controlling the wind speed to be 1.2m/s, setting the temperature according to three gears of high-medium-low, and preparing to carry out three-stage variable temperature drying;
(3) respectively putting each group of materials into a drying bin of hot air-microwave drying equipment, setting the temperature as 90 ℃ of primary drying temperature and the microwave power as 1.2W/g, starting the first-stage drying until the water content reaches about 30%, and stopping the first-stage drying;
(4) fully mixing the sweet potato strips which are dried in the first stage, dividing the sweet potato strips into 2 groups according to the thickness of the middle layer as the thickness of the material, respectively putting the sweet potato strips into drying bins of hot air-microwave drying equipment for drying, setting the drying temperature to be 70 ℃ of the second stage drying temperature, keeping the microwave power at 1.2W/g, starting the second stage drying until the water content reaches 15%, and stopping the second stage drying;
(5) fully mixing the sweet potato strips subjected to the second-stage drying, layering according to the thickness of a material layer being 45mm, putting the sweet potato strips into a drying bin of hot air-microwave drying equipment, setting the temperature to be 50 ℃ of third-stage drying temperature, keeping the microwave power at 1.2W/g, starting the third-stage drying, and stopping drying until the moisture content of a wet base is lower than 8%;
(6) and cooling the dried sweet potatoes to room temperature, packaging and storing.
The dried sweet potato slices have higher quality and better drying effect than sweet potato slices dried under the conditions of single temperature and uniform thickness. The method not only improves the drying efficiency, but also greatly improves the drying quality of the dried product.
The material drying process is divided into three stages, the thickness and the temperature of the material in each stage are different, the microwave power is the same, and the wind speed is the same; setting the temperature of three gears of high, medium and low, and carrying out variable temperature drying on the materials at three-level temperature of high-medium-low; in the drying process, dividing the material into three thicknesses of thin, medium and thick according to the thickness of the material layer, and horizontally dividing the three thicknesses into a plurality of groups; drying the material at the first stage at a first drying temperature corresponding to the thin layer thickness, at the second stage at a second drying temperature corresponding to the middle layer thickness, and at a third drying temperature corresponding to the thick layer thickness; until the water content of the material is less than 8 percent after drying. The invention has the advantages of high drying speed, good product quality of the dried product, good drying uniformity and low drying energy consumption. Compared with single-stage hot air-microwave drying, the drying time is saved by about 25-33%, and the drying energy consumption is saved by about 20-30%.
The present invention may be embodied in other specific forms without departing from the spirit or essential attributes thereof, and it should be understood that various changes and modifications can be effected therein by one skilled in the art without departing from the spirit and scope of the invention as defined in the appended claims.
Claims (10)
1. A microwave-assisted drying method for agricultural products is characterized in that a material overlapping and variable-temperature control mode is adopted, and materials to be dried are processed by combining staged temperature-control hot air drying and energy-carrying microwave synergistic drying, and the microwave-assisted drying method comprises the following steps:
s100, performing first-stage microwave-assisted hot air drying, grouping the selected materials to be dried according to the thickness of a first layer, respectively putting each group of the materials to be dried into microwave-hot air drying equipment, setting the temperature as a first-stage drying temperature, controlling the wind speed of hot air to be constant, performing microwave drying, setting the microwave power to be 0.8-1.5W/g, and performing first-stage drying on the materials to be dried until the water content of the materials to be dried reaches a first conversion point;
s200, performing secondary microwave-assisted hot air drying, fully mixing the materials to be dried which are dried in the first stage, grouping the materials according to the thickness of a second layer, respectively putting the materials into microwave-hot air drying equipment, setting the temperature as the secondary drying temperature, controlling the wind speed of hot air to be constant, performing microwave drying, keeping the microwave power the same as that in the step S100, and performing secondary drying on the materials to be dried which are dried in the first stage until the water content of the materials to be dried reaches a second conversion point; and
s300, third-stage microwave-assisted hot air drying, fully mixing the materials to be dried after the second-stage drying, grouping according to the thickness of a third layer, respectively putting the materials into the microwave-hot air drying equipment, setting the temperature to be a third-stage drying temperature, controlling the wind speed of hot air to be constant, performing microwave drying, keeping the microwave power the same as that in the step S100, performing third-stage drying on the materials to be dried after the second-stage drying is completed until the moisture content of a wet basis of the materials to be dried is lower than 8%, and stopping drying.
2. The microwave-assisted drying method of agricultural products of claim 1, further comprising a preprocessing step before step S100: before drying begins, the fresh material to be dried is selected, washed and/or sliced.
3. The microwave-assisted drying method of agricultural products of claim 1 or 2, further comprising a packaging and storing step after step S300: after drying, the dried material was cooled to room temperature and stored after packaging.
4. The microwave-assisted drying method of agricultural products of claim 3, wherein the first sub-layer is 10-30mm thick and the second sub-layer is 20-60mm thick; the thickness of the third layer is 30-90 mm.
5. The microwave-assisted drying method of agricultural products according to claim 4, wherein the wind speed of the hot wind is 1-1.5 m/s.
6. A microwave-assisted drying method for agricultural products according to claim 1 or claim 2, wherein the first sub-layer is 5-15mm thick; the thickness of the second sub-layer is 10-30 mm; the thickness of the third layer is 15-45 mm.
7. The microwave-assisted drying method for agricultural products according to claim 6, wherein the first switching point is a dividing point of water with a moisture content of a wet base at a non-flowable state and a bonded water; and the second conversion point is a dividing point of the moisture content of the wet base between free water and water which is difficult to flow.
8. The microwave-assisted drying method for agricultural products of claim 7, wherein the first transition point is 50% of the initial moisture content of the material to be dried; the second conversion point is 25% of the initial water content of the material to be dried.
9. The microwave-assisted drying method of agricultural products according to claim 6, wherein the microwave power is set to 1.2W/g.
10. The microwave-assisted drying method for agricultural products of claim 6, wherein the primary drying temperature is a high-grade temperature, and the high-grade temperature has a temperature value of 70-90 ℃; the second-stage drying temperature is a middle-grade temperature, and the temperature value of the middle-grade temperature is 50-70 ℃; the third-stage drying temperature is a low-grade temperature, and the temperature value of the low-grade temperature is 30-50 ℃.
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