RU2728169C1 - Method and apparatus for microwave-vacuum drying of a food product - Google Patents

Abstract

FIELD: food industry.SUBSTANCE: invention relates to the food industry. Disclosed is a method for microwave vacuum drying of tea, according to which inside vacuum drying chamber tea is placed and subjected to microwave heating in vacuum. Simultaneously with UHF heating of tea in vacuum, repeated operation of inlet-outlet of controlled portion of gas from said chamber is performed, note here that said gas is introduced inside chamber from below relative to tea arranged therein, and is removed from above from above. A device for the method implementation is additionally produced.EFFECT: invention is aimed at acceleration of tea drying process.14 cl, 2 dwg, 1 tbl, 5 ex

Description

Technology area

The present invention relates to the field of food, chemical and related industries and can be used in the processing of food products with ultrahigh frequency currents (microwave radiation) using a vacuum.

State of the art

It is known that the process of drying food raw materials or products in a vacuum chamber is more efficient and makes it possible to carry out drying at lower temperatures. It is also known that when drying an object under the influence of microwave radiation (microwave radiation), the food product is heated not only from the surface, but also in its volume, which also has a positive effect on the efficiency of the process, namely, it reduces the heating time of food. However, the drying chamber quickly fills up with water vapor, causing condensation to form on the colder parts of the food to be dried and parts of the drying chamber, which slows down the drying process. Some well-known technical solutions involve the collection and drainage of condensate into condensate receivers.

The combination of two techniques - microwave heating of water-containing products and their vacuum dehydration within the framework of one technological process - leads to an intensification of the drying process and faster removal of moisture from the bulk of the product. The known method of drying fruit and berry raw materials, mainly frozen, disclosed in the patent for invention of the Russian Federation No. 2 322 067 (publ. 20.04.2008). The known method involves drying the previously thawed raw materials in an apparatus with a microwave power supply and vacuum. However, as noted earlier, during microwave vacuum drying, moisture is released from the product in the form of condensate, which slows down the drying process of the product.

A known method of processing dry black tea, dry tea raw materials, disclosed in the patent for the invention of the Russian Federation No. 2 683 474 (publ. 03/21/2018). The known method includes a stage of additional drying, in which the food product (dry black tea) is subjected to thermal action by a microwave electromagnetic field in a drying chamber with constant pressure.

As in the case of conventional microwave drying, there are examples of technical solutions that offer collection and removal of steam to condense moisture. Known vacuum-electromagnetic wood dryer, disclosed in the patent for the invention of the Russian Federation No. 2 133 933 (publ. 27.07.1999). The known dryer contains a drying chamber, an electromagnetic energy generator, an electromagnetic energy input device and a vacuum pump. A steam condensation system is used to collect and remove moisture formed during product drying. Similarly, due to the use of a steam condensation system, moisture removal is carried out in a vacuum freeze drying unit, disclosed in the patent for invention of the Russian Federation No. 2 203 459 (publ. 27.04.2003). Also known is a method of vacuum drying of products of plant and animal origin, disclosed in the patent for invention of the Russian Federation No. 2 151 984 (publ. 06/27/2000). The known method involves placing the product in a vacuum chamber and heating it with microwave energy. The water evaporating from the volume of the product during its drying enters the water cooling system.

The known method and device for processing agricultural products, disclosed in the patent for the invention of the Russian Federation No. 2 242 906 (publ. 29.08.2002). The known method involves loading the raw material into the chamber, creating a vacuum, providing heating using microwave radiation and condensing steam.

However, all these solutions are characterized by a significant drawback - the removal of water vapor while maintaining a reduced pressure occurs due to its displacement from the entire working volume of the chamber by newly formed steam from the product. At the same time, water vapor inevitably comes into contact with less heated parts of the product and especially the walls and structural elements of the installation, which are not subject to microwave heating (which is one of the key parameters of the high efficiency of microwave technologies). It should be noted that in the case of other heating methods, the installation heats up together with the product and moisture condensation does not occur.

A known method for dehydration of a food product and a vacuum microwave device are disclosed in the patent for invention of the Russian Federation No. 2 442 084 (publ. 10.02.2012, application PCT WO 2008/094806, publ. 07.08.2008). The known method involves placing a food product of plant origin in the annular region of a rotating carousel and, as the carousel rotates, the food product is dehydrated in a vacuum microwave device. The rotating carousel has a plurality of partitions, the surfaces of which can be perforated. By perforating the baffles, steam generated during product dehydration is easier to remove. However, condensation can still form on colder surfaces inside the Vacuum Microwave Unit, slowing down the drying process.

Thus, only passive removal of water vapor from the chamber volume by maintaining a regulated reduced pressure (low, medium or high vacuum) is not able to prevent moisture condensation on the cold parts of the drying chamber and less heated parts of the dried product, in particular, its packaging. This, in turn, increases the risks for the safe operation of equipment, reduces the quality of the product obtained at the output of the process, and significantly increases the risks of deterioration in the quality characteristics of the product, including the risk of microbiological contamination.

Disclosure of the essence of the invention

The present invention is based on the technical problem of creating a simple and reliable technical means for carrying out an efficient process of drying food products.

The technical result achieved by the implementation of the present invention is to accelerate the drying process of food products.

In the first aspect of the invention, a method of microwave vacuum drying of a food product is disclosed, in which a food product is placed inside a vacuum drying chamber and subjected to microwave heating in a vacuum, while, in contrast to the prototype, simultaneously with microwave heating of the product in vacuum, a repeated operation of adding removal of a controlled portion of gas from said chamber, this gas being introduced into the chamber from below relative to the product placed in it, and removed from above relative to it.

In a second aspect of the present invention, a microwave vacuum drying apparatus is disclosed comprising a vacuum drying chamber for accommodating a food product, at least one microwave source configured to excite a microwave field within said chamber, a gas distribution system configured to gas introduction into the chamber from below relative to the product placed in it, and a gas outlet system, which is configured to remove the gas introduced into the chamber from above relative to the product placed in it and to provide a vacuum inside the chamber.

The present invention is illustrated in FIG. 1 and FIG. 2, which are schematic views of a microwave vacuum dryer.

Description of preferred embodiments of the invention

Microwave material processing demonstrates many promising advantages over traditional heating technologies, such as improved product quality, shorter process times, energy and energy savings due to higher efficiency, reduced environmental pollution and equipment costs, and more. high installation flexibility.

In the traditional heating process, heating sources are, for example, heating elements, resistances and infrared rays. Due to thermal radiation and / or convective heat exchange, their energy is transferred to the surface of the material and from there is transferred to its interior. Thermal conductivity, adsorption and specific heat of the material determine the heating process. Sensitive materials, under certain circumstances, do not allow high temperatures, and if the material also has poor thermal conductivity, then in this case a long process and overheating of the material are inevitable, therefore boundaries are set in the production of certain products using traditional heating technologies.

Microwave heating differs from traditional heating systems due to the fact that heat is directly generated in the bulk of the material. Dielectric heating of substances containing polar molecules occurs in the furnace. The electric component of electromagnetic waves enhances the movement of molecules with a dipole moment, and intermolecular friction leads to an increase in the temperature of the substance. The speed of microwave propagation is the speed of light in a vacuum or in air. If the microwave source is turned on, then it is directly present in the heated body and immediately begins energy conversion. With a quick shutdown, the heating process stops immediately. There are no lengthy furnace heating and cooling processes. Non-polar materials (eg air, Teflon, quartz glass) cannot convert energy and therefore do not heat them up. Microwaves penetrate these materials but are not attenuated (without energy conversion). In general, a heated material that is able to carry out energy conversion is considered "heaters" since the material itself is a heating source.

For the purposes of the present invention, a food product is understood to mean a significant group of products of plant origin, consumed by humans or used to improve the taste and aroma of food. Such products can be represented by grains, cereals, spices, berries, fruit and herbal raw materials, tea, tea leaves, tea semi-finished products, but are not limited to these examples of food products.

Microwave heating refers to a process in which energy with a frequency of 300 MHz to 300 GHz penetrates the material to be heated as an electromagnetic wave with a wavelength in the range of 1 m to 1 mm, and is then converted into heat.

The claimed method of microwave vacuum drying of food is carried out as follows.

Inside the vacuum drying chamber 1, food product 3 is placed on a grid tray 2. In particular, the food product 3 to be placed can be placed in a package, for example, in a cloth bag, or be packed in small containers, if the food product is represented by tea, then this is there may be tea bags. Before subjecting the food product 3 to microwave heating, a reduced pressure is provided in the chamber 1 using a vacuum pump.

The configuration of the drying chamber 1 and the layout in it of the microwave sources used for microwave heating are selected in such a way as to ensure the uniformity of the effect of microwave radiation on the dried food product 2.

Preferably, the drying pressure provided inside the drying chamber 1 is characterized by values that are close to the middle of the range of values typical for low vacuum conditions. In accordance with the state standard GOST 5197-85 “Vacuum technology. Terms and definitions ", these are values in the range from 100 kPa to 100 Pa (10 ³ ... 10 ⁰ mm Hg).

High-voltage vacuum devices - magnetrons 5 are used as sources of electromagnetic microwave radiation 5. Magnetrons 5 can operate at various frequencies from 0.5 to 100 GHz, with powers from several watts to tens of kilowatts in continuous mode, and from 10 W to 5 MW in in the pulse mode with pulse durations mainly from fractions to tens of microseconds. Microwaves enter the drying chamber 1 through a waveguide - a channel with metal walls that reflect microwave radiation. Preferably, the walls of the drying chamber 1 are also made of metal in order to ensure the reflection and direction of the microwaves on the product 3, thereby avoiding energy losses and not exposing the operating personnel to microwave radiation.

Food 3 heats up due to the presence of dipole water molecules, at one end of which there is a positive electric charge and at the other - negative. Under the influence of an electric field, dipole molecules line up along its lines of force. At an oscillation frequency of 1000 MHz, the electric field 2 * 10 ⁹ times per second changes its direction and the orientation of water molecules changes with the same frequency. As a result of such fluctuations, product 3 heats up and water evaporates.

With the combined use of microwave heating and vacuum drying technologies, the heating rate of product 3 and the intensity of moisture release from it increase. This leads to a rapid filling of the drying chamber 1 with water vapor. Moisture partially condenses on the colder walls and mechanisms of the chamber 1, as well as parts of the product to be dried 3 that are colder due to the persisting uneven heating. Passive removal of water vapor under vacuum conditions is not able to prevent moisture condensation on the cold parts of the chamber 1 and less heated parts of the product 3 and, in particular, its packaging. This leads to an increase in the risk for the safe operation of equipment, adversely affects the uniformity of drying, reduces the consumer qualities of the resulting product and its standardized characteristics, including, can lead to a risk of microbiological contamination.

In order to improve the quality of drying by removing moisture condensing inside the drying chamber, the present invention proposes, simultaneously with microwave heating of a food product in a vacuum, to carry out a repeated operation of injecting and removing a controlled portion of gas from the drying chamber 1, while the gas is introduced into the drying chamber 1 from below and, in particular, from the sides with respect to the food product 3 placed in the drying chamber 1, and discharged from above relative to it. For this, the drying chamber 1 additionally comprises a gas distribution system, which is configured to introduce gas into the drying chamber 1 from below and, in particular, from the sides with respect to the product 3, and a gas outlet system that is configured to remove the gas introduced into the drying chamber from above relative to the product. In this case, a part of the gas outlet system is a vacuum pump, which not only removes the gas introduced into the chamber 1, but also provides a vacuum inside it.

The gas distribution system includes a gas supply valve 6 and perforated gas supply pipes 7 interacting with it, and the pipes 7 are located inside the drying chamber 1 from the bottom and, in particular, from the sides relative to the food product 3 placed in the chamber 1. The gas outlet system includes a vacuum the pump and the perforated gas outlet pipes 8 interacting with it through the vacuum line valve of the vacuum pump 4, and the pipes 8 are placed in the chamber 1 from above relative to the product 3. In this case, the mentioned vacuum pump not only provides the removal of the gas introduced into the chamber 1, but also maintains the regulated vacuum level. The use of perforated pipes 7 and 8 allows you to evenly distribute the gas along the entire length and, respectively, the volume of the chamber 1, and remove it, eliminating the occurrence of "dead" zones that contribute to the formation of condensate on the surfaces of the chamber 1 and product 3. Gas supply through the mentioned pipes from below and on the sides from the product to be dried allows you to create a “gas pipe” around such a product and the moisture evaporated from it, providing isolation of the equipment from water vapor and transfer of moisture directly into the gas outlet pipes.

It should be noted that this solution makes it possible to replace the widespread mixing of dried products in a perforated drum during microwave drying, as, for example, disclosed in RF patent No. 2 442 084, with a static arrangement of products inside the chamber. In the proposed method, the uniformity of heating, the removal of moisture and the efficiency of drying is ensured by the stable entrainment of water vapor from the chamber 1, from the surface of the product 3 and the transfer of moisture from the inner regions of the dried product 3 to its surface due to the difference in the partial pressures of water vapor along the thickness of the heated material.

As the gas introduced into the drying chamber 1, atmospheric air, nitrogen or other inert gas or carbon dioxide, if it is necessary to exclude oxidation processes during drying, or water vapor, can be used. It is also possible to supply humidified air if the task is to remove unpleasant aromas from the surface and volume of the product - water molecules displace unwanted tastes from the product, which are carried away with the total volume of gas. In this case, the degree of humidification of the air should not be high, so that, in addition to removing unwanted aromatic compounds, the final product also loses its own moisture to the required moisture values.

The mode of gas injection and its removal from the drying chamber 1 is selected in such a way as to achieve the entrainment of all moisture released from the product in the form of vapor without condensing it in liquid form. For this, when carrying out drying, the following requirements must be observed:

- the power level of the vacuum pump is either kept constant or increased;

- the volume of the controlled portion of the gas introduced into the drying chamber 1 can be reduced or kept constant;

- the pressure in the drying chamber 1 is allowed to increase when the product is processed by microwave radiation by no more than 5 - 7.5 kPa.

Drying time depends on the amount of raw materials loaded and the depth of the created vacuum. The food product is dried to a moisture content of 3-8%.

To ensure the automation of the technological process, the microwave vacuum drying apparatus can be configured with the possibility of setting the modes of the operation of gas inlet / outlet to / from the drying chamber, for example, using a previously created software library of modes.

The essence of the present invention is illustrated by several examples, which should not be construed as limiting its scope, but are intended to make it clearer for a specialist.

Example 1: Drying Granulated Black Tea

50 kg of granulated black tea, packed in a 2-layer woven polypropylene bag, is placed in the apparatus on a grid tray. The initial moisture content of the tea was 10%.

The gas mixture supply valve is opened 100% and the vacuum pump is turned on at 20% power. Gradually close the gas mixture supply valve until the pressure in the apparatus reaches 70.9 kPa.

Fix the feed valve.

The magnetrons are turned on for 1 minute, and during this period the power of the vacuum pump is increased by 2%.

Then turn off the magnetrons and after 1 minute turn on the magnetrons for 1 minute and increase the power of the vacuum pump by another 2%.

The cycle is repeated 25 times.

Then it is kept maintaining the achieved minimum pressure for 10 minutes.

Then turn off the vacuum pump and wait for the pressure in the chamber to rise to the current atmospheric pressure (10 min). Take out the bag with the product and leave to cool at room temperature in a dry ventilated area.

The final moisture content of the tea was 4%.

Example 2: Drying Black Leaf Tea

20 kg of black leaf tea, packed in a woven polypropylene bag, are placed in the apparatus on a grid tray. The initial moisture content of the tea was 8%.

Open the gas mixture supply valve to 100% and turn on the vacuum pump at 10% power. Gradually close the gas mixture supply valve until the pressure in the apparatus reaches 50.6 kPa.

Fix the feed valve.

The magnetrons are turned on for 1 minute, and during this period the power of the vacuum pump is increased by 5%.

Then turn off the magnetrons and after 2 minutes turn on the magnetrons for 1 minute and increase the power of the vacuum pump by another 5%.

The cycle is repeated 8 times.

Then it is kept maintaining the achieved minimum pressure for 10 minutes.

Then turn off the vacuum pump and wait for the pressure in the chamber to rise to the current atmospheric pressure (10 min). Take out the bag with the product and leave to cool at room temperature in a dry ventilated area.

The final moisture content of the tea was 3%.

Example 3: Drying CTC Black Tea

25 kg of CTC black tea packed in a woven polypropylene bag is placed in the apparatus on a grid tray. The initial moisture content of the tea was 9%.

The vacuum pump is turned on at 100% power with the gas mixture supply valve fully closed and wait until the pressure in the apparatus reaches 5 kPa.

Then, the gas mixture supply valve is gradually opened until a stable pressure of 10.1 kPa is reached and the magnetrons are turned on until the pressure rises to 15.5 kPa. Then turn off the magnetrons and wait for the pressure to drop to 10.1 kPa and repeat the cycle 12 times.

Withstand maintaining the attained minimum pressure for 10 min.

Turn off the vacuum pump and wait for the pressure in the chamber to rise to the current atmospheric pressure (10 min). Take out the bag with the product and leave to cool at room temperature in a dry ventilated area.

The final moisture content of the tea was 4%.

Example 4: Drying black tea bags

15 kg of black tea packed in paper bags and packed in boxes of 100 bags are placed into the machine on a grid tray. The initial moisture content of the tea was 11%.

The vacuum pump is switched on at 15% power with the gas mixture supply valve fully open. Gradually close the gas mixture supply valve until the pressure in the apparatus reaches 506 kPa.

Turn on magnetrons for 30 seconds, turn off for 1 minute and repeat the cycle 10 times.

Then turn off the vacuum pump and wait for the pressure in the chamber to rise to the current atmospheric pressure (10 min). Take out the bag with the product and leave to cool at room temperature in a dry ventilated area.

The final moisture content of the tea was 6%.

Example 5: Drying a Fresh Herbal Tea Leaf

20 kg of fresh herbal tea leaves, packed in a woven polypropylene bag, are placed in the apparatus on a grid tray. The initial moisture content of the tea was 70%.

Open the gas mixture supply valve by 20% and turn on the vacuum pump at 20% power.

The magnetrons are turned on for 1 minute, and during this period the power of the vacuum pump is increased by 2% and the gas mixture supply valve is closed by 2%.

Then turn off the magnetrons and after 2 minutes turn on the magnetrons for 1 minute, increase the power of the vacuum pump by another 2% and close the gas mixture supply valve by 2%.

The cycle is repeated 40 times.

Then it is kept, maintaining the achieved minimum pressure, for 30 minutes.

Then turn off the vacuum pump and wait for the pressure in the chamber to rise to the current atmospheric pressure (10 min). Take out the bag with the product and leave to cool at room temperature in a dry ventilated area.

The final moisture content of the tea was 8%.

Specific technical parameters characterizing the drying process of food in accordance with the present invention in comparison with other widely used drying methods are presented in table 1.

Table 1 - Comparison of different drying methods

Indicator names Drying of ICB Convective drying Freeze drying Microwave drying Drying in accordance with the claimed invention Drying time (hour) Up to 4 8-10 10-20 Up to 4 1 Achieved residual moisture,% 3-4 8 3.5 2.5-4 3-8 Storage ability More than 1 year 0.3-0.5 years More than 1 year More than 1 year More than 1 year

Thus, with the help of a small and inexpensive design change, an effective method and device for microwave vacuum drying of food products was obtained, devoid of the disadvantages arising from moisture condensation during the intensification of drying processes.

The proposed technical change provides a solution to several problems simultaneously:

- ensuring the constancy of the driving force of the microwave drying process - the difference in the concentration of water vapor in the center of the volume of the product and on its surface

- carryover of moisture from the near-surface layers of the product;

- prevention of moisture condensation on the inner surface of the drying chamber;

- heating of the material in those areas where the humidity is highest and drying is required in the first place;

- uniform heating and distribution of residual moisture over the volume of the product;

- reduction of time and energy consumption for drying a unit of material;

- reducing the risk of local overheating of the product.

List of items in drawings

1 Vacuum drying chamber 2 Lattice tray 3 Food product 4 Vacuum line valve of the vacuum pump five Magnetron 6 Gas supply valve 7 Perforated gas tubes 8 Perforated flue pipes

Claims (14)

1. A method of microwave vacuum drying of tea, in which tea is placed inside a vacuum drying chamber and subjected to microwave heating in a vacuum, characterized in that simultaneously with microwave heating of tea in a vacuum, a repeated operation of adding and removing a controlled portion of gas from said chamber is carried out , and this gas is introduced into the chamber from below with respect to the tea placed in it, and is discharged from above with respect to it. 2. A drying method according to claim 1, wherein the tea is placed in a package. 3. The drying method according to claim 1, further comprising introducing gas into the chamber from the sides relative to the tea placed therein. 4. The drying method according to claim 1, wherein the volume of the controlled portion of gas introduced into the drying chamber is reduced or kept constant. 5. The drying method of claim 1, wherein the power level of the vacuum pump is either kept constant or increased. 6. The method according to claim 1, wherein the pressure in the drying chamber is allowed to increase when the tea is treated with microwave radiation by no more than 5-7.5 kPa. 7. The drying method according to claim 1, wherein the gas is atmospheric air. 8. The drying method according to claim 1, wherein the gas is humidified atmospheric air. 9. The drying method according to claim 1, wherein the gas is a gas that prevents oxidative processes from occurring inside the tea to be dried due to temperature exposure. 10. The drying method according to claim 9, wherein the gas that prevents oxidative processes inside the tea being dried is nitrogen or an inert gas or carbon dioxide. 11. The drying method of claim 1, wherein the gas is humidified air. 12. Apparatus for microwave vacuum tea drying, consisting of a vacuum drying chamber designed to accommodate tea, at least one microwave radiation source configured to excite a microwave field inside said chamber, a gas distribution system, which is configured to introduce gas into the chamber from below with respect to the tea placed in it, and a gas outlet system, which is configured to remove the gas introduced into the chamber from above relative to the tea placed in it and provide a vacuum inside the drying chamber, while the gas distribution system includes a gas supply valve and perforated tubes interacting with it, and the gas outlet system includes a vacuum pump and perforated gas outlet tubes interacting with it. 13. The apparatus of claim 12, wherein the gas distribution system is further configured to inject gas into the chamber from the sides relative to the tea placed therein. 14. Apparatus according to claim 12, which is configured to set the modes of operation of gas inlet / outlet to / from the drying chamber.

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