CN115574547A - Ultrasonic-assisted freeze drying method - Google Patents

Abstract

The invention discloses an ultrasonic-assisted freeze drying method, which belongs to the field of freeze drying and comprises the following steps: s1, placing a sheet-shaped material to be frozen on a temperature control plate in a vacuum tank; s2, cooling and applying first ultrasonic waves in the cooling process; s3, detecting whether the temperature of the flaky material to be frozen is reduced to a first preset temperature, if so, entering S4, and if not, continuing; s4, vacuumizing the vacuum tank to reduce the pressure of the vacuum tank to a preset pressure; s5, applying second ultrasonic waves; s6, after the first preset time, detecting whether the pressure rising rate of the vacuum tank is lower than a first pressure rising rate, if so, entering S7, otherwise, continuing; s7, heating and preserving heat after the temperature reaches a second preset temperature; s8, applying third ultrasonic waves in the heating and heat preservation processes; and S9, after second preset time, detecting whether the pressure rising rate of the vacuum tank is lower than a second pressure rising rate, if so, stopping, and otherwise, continuing. The invention not only can save energy and time, but also can improve the appearance and structural integrity of the product.

Description

A kind of ultrasonic assisted freeze-drying method

technical field

The invention relates to the technical field of freeze-drying, in particular to an ultrasonic-assisted freeze-drying method.

Background technique

Freeze-drying technology is a process of freezing fresh food and pharmaceutical raw materials, and then reducing the surface pressure, so that the water in it can be directly sublimated into gas. Since the water in the process is directly gasified without melting, it will not cause tissue collapse. And it can maintain the shape of the tissue before freeze-drying, so the freeze-dried material has a good quality. In addition, due to the low drying temperature, the active ingredients in the material are preserved, which can maximize the quality of the product.

However, freeze-drying technology also has disadvantages, such as time-consuming freeze-drying process, waste of electric energy, and high production costs. In the prior art, there is a technical solution to apply ultrasonic waves during the freezing process to save freeze-drying time and thus save electric energy. For example, in a freeze-drying method and supporting equipment disclosed in Document 1 (CN201410201099.1), it has been mentioned that ultrasonic-assisted Freeze-dried carrots.

However, the timing of applying ultrasonic waves is in the pre-cooling stage, so ultrasonic waves can only play a role in the freezing process, and have little effect on the long-term primary drying and secondary drying processes. It can be seen that the use of ultrasonic waves is relatively elementary.

Contents of the invention

Aiming at the problem of low effect of ultrasound in the freeze-drying technology in the prior art, the object of the present invention is to provide an ultrasound-assisted freeze-drying method.

To achieve the above object, the technical solution of the present invention is:

An ultrasonic-assisted freeze-drying method, comprising the following steps:

freezing:

S1. Place the sheet-like material to be frozen on the temperature control plate in the vacuum tank;

S2. Cool the temperature control board through the refrigeration system, and apply the first ultrasonic wave to the sheet-shaped material to be frozen through the ultrasonic vibrator during the cooling process;

S3. Detect whether the temperature of the flaky material to be frozen is lowered to the first preset temperature, if so, stop the step of S2 and enter into S4, otherwise continue to S2;

Once dry:

S4. Vacuumize the vacuum tank through the vacuum system, and reduce the pressure of the vacuum tank to a preset pressure;

S5. Applying a second ultrasonic wave to the sheet-like material to be frozen through the ultrasonic vibrator;

S6. After the first preset time, close the vacuum tank and detect whether the pressure increase rate of the vacuum tank is lower than the first pressure increase rate, if yes, stop drying once and enter S7, otherwise continue to S5;

Secondary drying:

S7. Heat the temperature control board through the heating system, and keep it warm after the temperature of the temperature control board reaches the second preset temperature;

S8. During the heating and heat preservation process, apply a third ultrasonic wave to the sheet-shaped material to be frozen through the ultrasonic vibrator;

S9. After the second preset time, close the vacuum tank and detect whether the pressure increase rate of the vacuum tank is lower than the second pressure increase rate, if yes, stop the secondary drying, otherwise continue to S8.

Preferably, in S3, the temperature of the sheet-like material to be frozen is detected by an infrared thermometer, and the temperature at the center of the sheet-like material to be frozen is used as a judgment basis.

Preferably, in S3, the eutectic point temperature of the sheet-like material to be frozen is obtained through the following steps:

Put the sheet-like material to be frozen into the differential scanning calorimeter, and lower the temperature at a cooling rate of 10°C/min, from 25°C to -70°C, and measure the eutectic point temperature;

On the basis of the measured eutectic point temperature, lower 5° C. as the first preset temperature described in S3.

Preferably, in S4, the preset pressure is 5-20Pa; in S7, the second preset temperature is 40°C.

Preferably, in S1, the thickness of the sheet-like material to be frozen is 5mm-10mm.

Preferably, in S2, the first ultrasonic wave is released in the form of a 5-min interval for every 1 s of work.

Preferably, in S5, the second ultrasonic wave is released in the form of a 3min interval every 5s.

Preferably, in S8, the third ultrasonic wave is released in the form of an interval of 1 min every 5 seconds.

Preferably, in S6, the first preset time is 1-4 hours, and the pressure increase rate of the vacuum tank is closed and detected every 1 minute, and the first pressure increase rate is 5 Pa/min.

Preferably, in S9, the second preset time is 2-5 hours, and the pressure rise rate of the vacuum tank is closed and detected every 10 minutes, and the second pressure rise rate is 10 Pa/min.

Preferably, in S9, when it is detected that the pressure increase rate of the vacuum tank is lower than the second increase rate, the secondary drying continued for 1h.

Preferably, the method further includes: S10. Turn off the vacuum system and the heating system, and vacuum-tightly pack the dried sheet-shaped material to be frozen after being taken out of the vacuum tank.

By adopting the above-mentioned technical scheme, the beneficial effect of the present invention is that: by using ultrasonic waves to process materials in the stages of freezing, primary drying, and secondary drying, so that: in the freezing stage, ultrasonic waves can effectively promote the formation of ice crystals between cells, while ultrasonic waves The effect is to increase the thermal conductivity and improve the heat transfer effect; in the primary drying stage, the ultrasonic power used is strengthened, the main function is to improve the heat transfer effect, and at the same time provide energy, so that the water molecules can obtain the kinetic energy of sublimation and speed up the primary drying speed; in the secondary drying stage , Ultrasound is used to improve the heat transfer effect, and at the same time provide energy, so that the crystallization water can overflow. Thereby, the freeze-drying time is greatly saved, and the product phase and structural integrity rate of the product can be improved.

Description of drawings

Fig. 1 is the method flowchart of embodiment one of the present invention;

Fig. 2 is the comparison chart of time-consuming under the method of embodiment 1 of the present invention and vacuum freeze-drying contrast;

Fig. 3 is a comparison chart of dry matter content and void ratio between the method of Example 1 of the present invention and hot air drying and vacuum freeze drying;

Fig. 4 is a comparison diagram of the texture of Radix Polygoni Multiflori slices under the method of Example 1 of the present invention compared with hot-air drying and vacuum freeze-drying;

Fig. 5 is a comparison diagram of the brightness of Radix Polygoni Multiflori slices under the method of Example 1 of the present invention compared with hot air drying and vacuum freeze drying;

FIG. 6 is a flow chart of the method of Embodiment 3 of the present invention;

Fig. 7 is a structural schematic diagram of the present invention;

Fig. 8 is the front view of the present invention;

Fig. 9 is a top view of the present invention;

Fig. 10 is a sectional view along line A-A in Fig. 9 .

In the figure: 1-vacuum tank, 2-cover, 3-temperature control board, 4-vacuum pump, 5-vacuum valve, 6-pressure detection device, 7-ultrasonic vibrator, 8-semiconductor refrigeration module, 9-infrared thermometer , 10-insulation layer, 11-pipeline, 12-support conduit.

detailed description

The specific embodiments of the present invention will be further described below in conjunction with the accompanying drawings. It should be noted here that the descriptions of these embodiments are used to help understand the present invention, but are not intended to limit the present invention. In addition, the technical features involved in the various embodiments of the present invention described below may be combined with each other as long as they do not constitute a conflict with each other.

It should be noted that, in the description of the present invention, the orientation or positional relationship indicated by the terms "upper", "lower", "left", "right", "front", "back" etc. The description of the structure of the invention is only for the convenience of describing the invention, but does not indicate or imply that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and thus should not be construed as limiting the present invention.

For the "first" and "second" in this technical solution, it is only to distinguish the names of the same or similar structures, or the corresponding structures with similar functions, not to arrange the importance of these structures, nor to sort or compare them size, or other meanings.

In addition, unless otherwise clearly specified and limited, the terms "installation" and "connection" should be interpreted in a broad sense, for example, the connection can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection, or It can be an electrical connection; it can be a direct connection, or an indirect connection through an intermediary, or an internal connection between two structures. Those of ordinary skill in the art can understand the specific meanings of the above terms in the present invention in accordance with the general idea of the present invention and in connection with the specific circumstances of the context of this solution.

Embodiment one

An ultrasonic-assisted freeze-drying method, which includes three stages of freezing, primary drying, and secondary drying, as shown in Figure 1, the specific steps are as follows:

freezing:

S1. Place the flake material to be frozen on the temperature control plate in the vacuum tank.

In this embodiment, the freeze-drying of Radix Polygoni Multiflori is taken as an example for illustration. The Radix Polygoni Multiflori is cut into thin slices with a thickness of 5mm-10mm, preferably 8mm, and evenly arranged in the middle of the surface of the temperature control plate, so that the Radix Polygoni Multiflori Shouwu slices (that is, sliced materials to be frozen) are in good contact with the surface of the temperature control plate. Of course, in other embodiments, it can also be other types of Chinese medicinal materials, or food, such as ginseng, Tian Qi, shiitake mushrooms, strawberries, matsutake, truffles, and medicines that need to be freeze-dried.

S2. Cool down the temperature control plate through the refrigeration system, and apply the first ultrasonic wave to the sheet-shaped material to be frozen through the ultrasonic vibrator during the cooling process.

In this embodiment, the refrigeration system is configured as a semiconductor refrigeration module, which applies cold energy to the temperature control board through its cold end, and its hot end faces the outside of the vacuum tank. There are multiple ultrasonic vibrators, such as four, and the four ultrasonic vibrators are evenly installed around the center of the temperature control plate, so as to apply ultrasonic waves to the slices of Baishouwu uniformly. Under the control of the ultrasonic generator, the four ultrasonic vibrators The vibrators usually work at the same time, and the first ultrasonic waves generated are released in the form of 5 minutes of 1 second of work.

S3. Detect whether the temperature of the flaky material to be frozen has dropped to the first preset temperature, if yes, stop the step of S2 and enter into S4, otherwise continue to S2.

In this embodiment, the temperature of the slices of Baishouwu (sheet-like material to be frozen) is detected by an infrared thermometer installed in a vacuum tank, and the temperature at the center of the slice of Baishouwu (sheet-like material to be frozen) As the actual temperature, the determination of S3 is sequentially performed.

And, the first preset temperature of Baishouwu slices (flaky material to be frozen) is obtained through the following steps:

Put the sheet-like material to be frozen into the differential scanning calorimeter, and lower the temperature at a cooling rate of 10°C/min, from 25°C to -70°C, and measure the eutectic point temperature; Basically, lower 5°C as the first preset temperature described in S3. For example, if the measured eutectic point temperature of Radix Polygoni Multiflori is -13.5°C, in this embodiment, it is reduced by 5°C on the basis of -18.5°C as the actual eutectic point temperature (i.e. the first preset temperature) and compare it with the temperature at the center of the Baishouwu slice (flaky material to be frozen).

The use of ultrasound at this stage can effectively promote the formation of intercellular ice crystals in the slices of Baishouwu, so that more water can be frozen into ice crystals at this stage, so that it can be removed in a subsequent drying; at the same time, the intervention of ultrasound can also It can improve the thermal conductivity, that is, improve the contact effect between tissues through vibration, and then improve the heat conduction effect, so that the cold energy can be transferred between tissues faster and better.

Once dry:

S4. Vacuumize the vacuum tank through the vacuum system, and reduce the pressure of the vacuum tank to a preset pressure.

In this embodiment, the vacuum system is configured as a system including a vacuum pump, a vacuum valve, a pressure detection device and necessary pipelines. The vacuum pump is used to evacuate the vacuum tank to maintain the vacuum degree inside the vacuum tank, and the vacuum valve can be used to control the vacuuming conveniently and flexibly, and the pressure detection device can accurately observe the vacuum degree inside the vacuum tank. And the specific configuration preset pressure is 5-20Pa, usually, the pressure of the vacuum tank is maintained at the level of 10Pa.

S5. Applying a second ultrasonic wave to the sheet-shaped material to be frozen through the ultrasonic vibrator.

After the pressure of the vacuum tank reaches the above-mentioned preset pressure, under the control of the ultrasonic generator, the ultrasonic vibrators arranged in the vacuum tank usually work at the same time, and the second ultrasonic waves generated are released in the form of 3 min intervals every 5 seconds. Usually, The ultrasonic power was maintained at the level of 100W.

With such a setting, by using higher-power ultrasonic waves in a drying process, the heat transfer effect between tissues can be effectively improved. At the same time, ultrasonic waves can also provide sound wave energy, so that water molecules can obtain sublimation kinetic energy, thereby speeding up the drying speed.

S6. After the first preset time, close the vacuum tank and check whether the pressure increase rate of the vacuum tank is lower than the first pressure increase rate, if yes, stop drying once and enter S7, otherwise continue to S5.

Among them, the first preset time is usually configured as 1-4h, and the specific time length is determined according to the thickness of Baishouwu slices (flaky materials to be frozen). The larger the thickness, the longer the first preset time, that is, the vacuum maintenance time and The intervention time of the second ultrasonic wave is longer, for example, in this embodiment, the first preset time is configured as 1 hour. In addition, in this embodiment, the vacuum tank is closed by closing the vacuum valve, and the pressure rise rate of the vacuum tank is closed and detected every 1 minute, and the first pressure rise rate is set to 5 Pa/min.

Secondary drying:

S7. The temperature control board is heated by the heating system, and the temperature control board is kept warm after the temperature of the temperature control board reaches the second preset temperature.

In this embodiment, the temperature control board is heated by changing the direction of the current or changing the direction of the semiconductor refrigeration module, and the second preset temperature is set to 40°C, and the temperature is still detected by the infrared thermometer.

S8. During the heating and heat preservation process, the third ultrasonic wave is applied to the sheet-like material to be frozen through the ultrasonic vibrator.

In this embodiment, under the control of the ultrasonic generator, the ultrasonic vibrators arranged in the vacuum tank usually work at the same time, and the generated third ultrasonic wave is released in the form of 1 minute interval every 5 seconds, and the ultrasonic power is maintained at the level of 100W.

With such a setting, by using higher-power ultrasonic waves in the secondary drying process, the heat transfer effect between tissues can be effectively improved, and at the same time, the ultrasonic waves are also used to provide sound wave energy, so that the crystallization water can be energy-captured and overflowed more efficiently.

S9. After the second preset time, close the vacuum tank and detect whether the pressure increase rate of the vacuum tank is lower than the second pressure increase rate, if yes, stop the secondary drying, otherwise continue to S8.

Among them, the second preset time is usually configured as 2-5h, and the specific time length is determined according to the thickness of the Baishouwu slice (flaky material to be frozen). The larger the thickness, the longer the second preset time, that is, the heating time and the second preset time. The intervention time of the third ultrasonic wave is longer, for example, in this embodiment, the second preset time is configured as 2 hours. In addition, in this embodiment, the vacuum tank is closed by turning off the vacuum pump, and the pressure rise rate of the vacuum tank is closed and detected every 10 minutes, and the second pressure rise rate is set to 10 Pa/min.

So far, the ultrasonic-assisted freeze-drying process is over, and slices of Radix Polygoni Multiflori with low water content and high quality are obtained.

As shown in FIG. 2 , it shows a time-consuming schematic diagram of each stage of vacuum freeze-drying and ultrasonic-assisted freeze-drying in this embodiment. It can be seen that in the freezing stage, this embodiment takes 68 minutes, while vacuum freeze-drying takes 143.3 minutes; in the primary drying stage, this embodiment takes 273 minutes, and vacuum freeze-drying takes 623 minutes; It took 253 minutes; for the overall drying process, the time used in this example was 458 minutes, and the vacuum freeze-drying time was 1020 minutes, which was only 44.9% of the vacuum freeze-drying time used in this example.

As shown in FIG. 3 , it shows a schematic diagram of the differences in dry matter content and porosity among the three methods of ultrasonic-assisted freeze-drying, vacuum freeze-drying, and hot-air drying in this embodiment. It can be seen that there is no significant difference in dry matter content in this embodiment, vacuum freeze-drying and hot-air drying, which shows that the three drying methods can be fully dried, but the porosity of the three drying methods is significantly different, which is due to During the hot air drying process, the expansion of pores and the fragmentation of materials occur, resulting in a decline in quality. However, this embodiment can effectively avoid the above problems and make the structure of the obtained product more effective.

As shown in Figure 4, it shows the schematic diagram of the differences in the weight of the core and the weight of the outer cortex of the Radix Polygoni Multiflori slices by the three methods of ultrasonic-assisted freeze-drying, vacuum freeze-drying, and hot-air drying in this embodiment. It can be seen that the drying method of this embodiment is obviously better than the other two drying methods in terms of the weight of the core and the weight of the outer cortex of the Baishouwu slices.

In addition, the puncture hardness of the core and outer cortex of the slices of Baishouwu obtained by the three drying methods were measured by a texture analyzer, and it was found that hot air drying would cause the slices of Baishouwu to harden, which is due to the dehydration process. Fiber coking caused by vacuum freeze-drying and ultrasonic-assisted drying in this embodiment both reduce tissue hardness, thereby achieving better protection of the structure, which is conducive to the leaching of Chinese herbal medicines, and ultrasonic-assisted freeze-drying in this embodiment It is possible to obtain Baishouwu slices with better toughness.

As shown in FIG. 5 , it shows the brightness of three different drying methods of ultrasonic-assisted freeze-drying, vacuum freeze-drying, and hot-air drying in this embodiment measured by an automatic colorimeter. It can be seen that the brightness of the slices of Radix Polygoni Multiflori after hot air drying is 72.28; the brightness of slices of Radix Polygoni Multiflori after vacuum freeze-drying is 91.67; the brightness of the slices of Radix Polygoni Multiflori after ultrasonic-assisted freeze-drying in this embodiment is 93.10; is the mean of three different treatments. The results showed that the hot air drying caused the brightness of the slices of Baishouwu to decrease, while the vacuum freeze-drying and the ultrasonic-assisted freeze-drying of this embodiment both increased the brightness of the slices of Baishouwu. This is because hot-air drying produces adverse reactions such as oxidation, while vacuum freeze-drying has less impact. The ultrasonic-assisted freeze-drying in this embodiment has a shorter time and better quality than the above two methods.

Embodiment two

It differs from Embodiment 1 in that: in S9, when it is detected that the pressure rise rate of the vacuum tank is lower than the second pressure rise rate, the secondary drying for 1 hour is continued, that is, the steps of S7 and S8 are continued and Last for 1h.

Embodiment three

The difference between it and Embodiment 1 or 2 is that: as shown in FIG. 6 , the method further includes S10.

S10. Turn off the vacuum system and the heating system, and vacuum-tightly pack the dried flake material to be frozen from the vacuum tank.

Embodiment Four

An ultrasonic-assisted freeze-drying device, which is used to implement the method of any of the above embodiments, as shown in Figure 7-10, the device includes a vacuum tank 1, a cover 2, a temperature control plate 3, a vacuum pump 4, and a vacuum valve 5 , pressure detection device 6, ultrasonic vibrator 7, semiconductor refrigeration module 8, infrared thermometer 9 and insulation layer 10.

Wherein, the vacuum tank 1 has a cylindrical structure as a whole, and the vacuum tank 1 specifically includes a tank body and a cover 2 detachably and fixedly connected to the top of the tank body, that is, the top of the tank body is open, and the tank body is closed by the cover 2 . Correspondingly, an insulating layer made of insulating material is laid on the outer wall of the tank body and the cover 2 . Wherein, a temperature control board 3 for carrying materials is fixedly installed inside the vacuum tank 1 . In this embodiment, specifically, an opening matching the temperature control board 3 is opened at the bottom of the tank body, and the temperature control board 3 is fixedly installed in the opening. And the temperature control plate 3 is configured as a rigid plate structure made of heat-conducting material, such as a stainless steel plate, and is preferably configured as a circle.

The vacuum pump 4 is arranged outside the vacuum tank 1, and its suction end is connected with the tank side wall of the vacuum tank 1 through a pipeline 11, and a vacuum valve 5 and a pressure detection device 6 are also installed on the pipeline 11, wherein the pressure detection The device 6 is preferably configured as a pressure gauge, or may also be configured as a digital display pressure gauge in other embodiments. The vacuum pump 4 is used to vacuumize the vacuum tank 1, thereby maintaining the vacuum degree inside the vacuum tank 1, and the vacuum valve 5 can be used to control the vacuuming conveniently and flexibly, and the pressure detection device 6 can accurately observe the vacuum inside the vacuum tank 1. Vacuum.

There are multiple ultrasonic vibrators 7 , for example four, and all the ultrasonic vibrators 7 are fixedly installed inside the vacuum tank 1 , and may also be installed outside the vacuum tank. In addition, a plurality of overtime wave oscillators are evenly arranged around the center of the temperature control plate 3 in the circumferential direction. In this embodiment, each ultrasonic vibrator 7 is fixedly installed on the surrounding edges of the temperature control plate 3 , so as to better apply ultrasonic waves to the materials carried on the temperature control plate 3 .

The ultrasonic generator (not shown in the figure) is arranged outside the vacuum tank 1, and the ultrasonic generator is electrically connected with each ultrasonic vibrator 7 through wires, so as to control the ultrasonic vibrator 7 to emit ultrasonic waves of applicable frequency.

The semiconductor refrigeration module 8 is fixedly installed on the bottom of the vacuum tank 1, and the semiconductor refrigeration module 8 is connected with the temperature control board 3, so as to release cold and heat to the temperature control board 3, so as to achieve the purpose of controlling the temperature of the temperature control board 3 . In this embodiment, the semiconductor refrigeration module 8 is fixedly installed on the bottom side of the temperature control board 3 , and the bottom side of the temperature control board 3 is additionally provided with an insulation layer 10 , such as a polyurethane foam insulation layer 10 . The semiconductor refrigeration module 8 works by receiving electric current, and switches the cooling mode and the heating mode according to the change of the current direction.

The infrared thermometer 9 is fixedly installed in the vacuum tank 1, and the infrared thermometer 9 is used to detect the temperature of the temperature control board 3 and the materials it carries. In this embodiment, the infrared thermometer 9 is arranged at a position directly above the center of the temperature control plate 3, for example, the infrared thermometer 9 is fixedly connected to the side wall of the vacuum tank 1 through a support conduit 12, and the support conduit 12 runs through the side wall of the vacuum tank 1 , and a cable for connecting with the infrared thermometer 9 passes through the supporting conduit 12 .

The embodiments of the present invention have been described in detail above with reference to the accompanying drawings, but the present invention is not limited to the described embodiments. For those skilled in the art, without departing from the principle and spirit of the present invention, various changes, modifications, substitutions and modifications to these embodiments still fall within the protection scope of the present invention.

Claims (12)

1. An ultrasonic-assisted freeze drying method is characterized in that: the method comprises the following steps:

freezing:

s1, placing a flaky material to be frozen on a temperature control plate in a vacuum tank;

s2, cooling the temperature control plate through a refrigerating system, and applying first ultrasonic waves to the flaky material to be frozen through an ultrasonic vibrator in the cooling process;

s3, detecting whether the temperature of the flaky material to be frozen is reduced to a first preset temperature, if so, stopping the step S2 and entering S4, otherwise, continuing to the step S2;

primary drying:

s4, vacuumizing the vacuum tank through a vacuum system, and reducing the pressure of the vacuum tank to a preset pressure;

s5, applying second ultrasonic waves to the flaky material to be frozen through an ultrasonic vibrator;

s6, after the first preset time, closing the vacuum tank and detecting whether the pressure rising rate of the vacuum tank is lower than a first pressure rising rate, if so, stopping primary drying and entering S7, otherwise, continuing S5;

secondary drying:

s7, heating the temperature control plate through a heating system, and preserving heat of the temperature control plate after the temperature of the temperature control plate reaches a second preset temperature;

s8, in the heating and heat preservation processes, applying third ultrasonic waves to the flaky material to be frozen through the ultrasonic vibrator;

and S9, after a second preset time, closing the vacuum tank and detecting whether the pressure rising rate of the vacuum tank is lower than a second pressure rising rate, if so, stopping secondary drying, and otherwise, continuing S8.

2. The method of claim 1, wherein: in S3, the temperature of the flaky material to be frozen is detected by an infrared thermometer, and the temperature of the center of the flaky material to be frozen is used as a judgment basis.

3. The method of claim 2, wherein: in S3, the eutectic point temperature of the flaky material to be frozen is obtained through the following steps:

putting the flaky material to be frozen into a differential scanning calorimeter, cooling at the cooling rate of 10 ℃/min, cooling from 25 ℃ to-70 ℃, and measuring to obtain the eutectic point temperature;

and on the basis of the measured eutectic point temperature, reducing the temperature by 5 ℃ to be used as the first preset temperature in the S3.

4. The method of claim 1, wherein: in S4, the preset pressure is 5-20Pa; in S7, the second preset temperature is 40 ℃.

5. The method of claim 1, wherein: in S1, the thickness of the flaky material to be frozen is 5mm-10mm.

6. The method of claim 1, wherein: in S2, the first ultrasonic wave is released in the form of a pause of 5min per 1S of operation.

7. The method of claim 1, wherein: in S5, the second ultrasonic wave is released in the form of a pause of 3min per 5S of operation.

8. The method of claim 1, wherein: in S8, the third ultrasonic wave is released in the form of an interval of 1min per 5S of operation.

9. The method of claim 1, wherein: in S6, the first preset time is 1-4h, the vacuum tank is closed every 1min, the pressure rising rate of the vacuum tank is detected, and the first pressure rising rate is 5Pa/min.

10. The method of claim 1, wherein: in S9, the second preset time is 2-5h, the pressure rising rate of the vacuum tank is closed and detected every 10min, and the second pressure rising rate is 10Pa/min.

11. The method of claim 1, wherein: in S9, when it is detected that the pressure rising rate of the vacuum tank is lower than the second pressure rising rate, the secondary drying is continued for 1 hour.

12. The method of claim 1, wherein: the method further comprises the following steps:

and S10, closing the vacuum system and the heating system, taking out the dried flaky material to be frozen from the vacuum tank, and then carrying out vacuum sealing packaging.

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