Microwave vacuum freeze-drying process and equipment problems

Food Science and Technology Food Technology Microwave Vacuum Freeze-Drying Process and Equipment Problems DONG Tie-you, LI Su-yun, ZHANG Jian-long, ZHU Wen-xue (Department of Food and Biological Engineering, Henan University of Science and Technology, Luoyang 471003) Problem of Insufficient Cooling Capacity, Vacuuming Capacity of Vacuum System, and Problems such as theory, process, and equipment for safe operation of the entire system, and the basic technical methods and means for solving the problems, such as corresponding design, technology, and reasonable operating procedures, are also provided for the improvement and optimization of microwave vacuum freeze-drying processes and equipment. The basis.

0 Introduction The application of microwave vacuum freeze-drying technology will help extend the shelf life of foods, maintain the original flavor and nutrient content of foods, as well as the physiological activity of food ingredients, enhance the functionality of health foods, and improve fund projects: Natural Science Research in Henan Province Fund Project (0211062000); Project funded by the Ministry of Education and the Henan Provincial Personnel Department for Returning Students to Study Abroad, 'Innovation Talent Fund Project of the Department of Education of Henan Province.

Product processing and storage.

Because of the added value of agricultural products, they have a wide range of uses in high-end foods, health functional foods, and agricultural product processing industries. However, because the equipment required for this process is a high-tech product with high technical content, the investment in equipment is large. In addition, the physicochemical property parameters and process parameters of different food materials closely related to the microwave vacuum freeze-drying process are still not complete, and the relevant equipment and theoretical research is not yet very mature, thus limiting the application of this technology in China's food industry (1) . There is no doubt that the solution of these problems will effectively promote the application and promotion of microwave vacuum freeze-drying technology in China.

Based on a microwave vacuum freeze-drying experimental device, this study discussed the structure of microwave vacuum freeze-drying device, food technology and food technology issues, and microwave vacuum freeze-drying process.

1 Characteristics of microwave vacuum freeze-drying Microwave vacuum freeze-drying is a new technology that combines high-efficiency microwave radiation heating technology with vacuum freeze-drying technology. Microwave vacuum freeze-drying is the use of microwave radiation in the frozen state of the material to be dried, in the role of high-frequency alternating electromagnetic vibration of the material (mainly water) molecules and mutual friction so that the electromagnetic energy is converted into moisture in the material sublimation Required sublimation heat. In addition to the advantages of ordinary vacuum freeze-drying, microwave vacuum freeze-drying has other special advantages.

1.1 Electromagnetic waves in the fast drying range. The transmission characteristics of microwaves are that reflections, transmissions, and absorption occur in obstacles (objects to be irradiated or objects to be heated). The strength of these phenomena is related to the nature of the material. /01. It is the transmission of microwaves. Sexuality, microwaves can directly penetrate the material to achieve a balanced heating inside and outside, unlike the traditional vacuum drying, which transfers heat to the tray through the heating element and then transfers it to the heated material through the tray. With microwave heating, the heating time can be greatly shortened, and its drying speed and thermal efficiency are 4'20 times that of conventional heating methods.

The penetrating power of microwave can be expressed by the penetration depth. The penetration depth refers to the attenuation of incident energy to 1/e, and the value can be calculated as follows: r* relative dielectric constant tan dielectric loss factor.

It can be seen that the depth of penetration is proportional to the wavelength (ie, inversely proportional to the frequency) and is inversely proportional to the square root of the relative permittivity and dielectric dissipation factor. For example, if the 95” water is irradiated with microwaves at a frequency of 915 MHz, the depth of penetration is 29.5 cm, while at 2450 MHz microwave irradiation, it is only 4.8 cm, but it is difficult to describe the vacuum freeze-drying process completely using a mathematical model. The main reason is that the process of microwave vacuum freeze-drying heat and mass transfer is an unsteady solid-vapor phase transition process, but also involves the density, thermal conductivity, mass transfer coefficient, moisture content, etc. of various lyophilized materials. Physical parameters, large quantities, and wide areas, and these data are still unsound.In addition, there are many issues that need to be studied in depth, such as the measurement principles and methods related to microwave freeze-drying, and measuring instruments. Microwave vacuum freeze-drying only requires 12C is OK, but this does not mean that any material can be dried by microwave vacuum freeze-drying.For example, for some high moisture content materials with the moisture content of 85%95, if you use the microwave vacuum freeze-drying method, all the Removing water is not economical, and the efficiency is not high.

The correct method is to reduce the moisture to 30% and 60% beforehand, and then use microwave food technology to vacuum freeze-dried to remove the remaining moisture. For example, some soups with higher moisture content should be concentrated, pre-frozen, and other pretreatments before drying. In conclusion, microwave freeze drying can be combined with other drying methods to optimize the drying process, reduce energy consumption and costs. For food materials, different microwave vacuum freeze-drying processes should be established according to different types, and the process parameters should be optimized to minimize product costs. Due to the different dielectric constants of different types of materials, the microwave reflection characteristics exhibited by the drying chamber under the same conditions are different. Therefore, when replacing the processed materials, the relevant parameters of the microwave system should be adjusted to maximize the Play the advantages of microwave vacuum freeze-drying equipment.

2.3 Equipment problems and countermeasures Microwave vacuum freeze-drying equipment is mainly composed of refrigeration system, vacuum system, microwave heating system, electrical control and computer data sampling system (see). It is a combination of microwave technology and refrigeration technology and vacuum technology.

I. Vacuum freeze-drying electric heating work chamber; 2. Purifier; 3.a hot plate; 4. Refrigeration compressor group; 5. Vacuum pump; 6. Vacuum freeze drying microwave heating work chamber; 7. Vacuum degree data acquisition system; 8. Heating plate temperature data acquisition system; 9. Material parameter (quality) data acquisition system; 10. Capturer mixing data acquisition system; 11. Microwave system data; 12. Computer control system; 13. Connection flange microwave vacuum Freeze-drying experimental device schematic The most common and most prominent problem of microwave vacuum freeze-drying equipment is energy consumption, microwave leakage, and difficult temperature control, which have seriously affected the promotion of the technology. These problems mainly have the following aspects: 2.3.1 Microwave heating system problems Mainly refers to the uniformity of microwave heating, temperature controllability and safety. The uniformity of microwave heating affects the drying speed of the material and the uniformity of the finished product after drying. Many microwave equipment manufacturers claim that microwaves have uniform heating characteristics. However, the microwave has its heating uniform side and uneven heating side. The former is reflected in the fact that for a single material, the microwave is heated as a whole, there is no problem of gradual warming from the outside to the inside, and heating uniformity is better. However, if the size of the material is large or the thickness of the material layer is large, such as exceeding the depth of penetration H+, there will be an uneven distribution of energy in the interior of the individual material or in the thickness of the material layer. In addition, the microwave energy passes through the incident and reflection* in the drying chamber. Regardless of the degree of improvement, there is still the non-food technology uniformity problem of microwave energy distribution to obtain the most uniform heating effect possible, in addition to solving the shape and size design of the cavity of the drying chamber. In addition to the coupling problems, the radiation and absorption characteristics of microwaves must be fully considered and used in the layout of materials and the design of transportation methods to improve the uniformity of drying.

The present microwave freeze-drying device cannot yet determine the significant shortfall when drying has ended. As a result, drying may end prematurely and the moisture content of the product may not meet the requirements. It may also cause excessive drying time and waste of energy. It has also been proposed to use fuzzy control to control the temperature, which is one of the topics to be further studied.

In addition, microwave leakage must be strictly prevented. In the system's inlet and outlet, and the external communication with the outside world, it is necessary to consider the shielding or attenuation of microwaves, and it does not interfere with the operation of other detection elements by microwaves. These can guarantee the microwave leakage control within the scope of international safety standards through a good structural design.

2.3.2 Refrigeration system problems The prone problem with refrigeration systems is the low efficiency of the water trap. It is mainly due to the small heat exchange area of ​​the water trap, the low refrigerating temperature and the small cooling capacity. The fundamental reason is that the design of the refrigeration system is unreasonable. This problem can be solved by improving the design of the refrigeration system, such as using two-stage refrigeration, increasing the cooling capacity of the refrigeration system, using a refrigerant with a lower evaporation temperature, and the evaporation temperature of -40! The piston compressor with the same power under the same working condition can have a higher cooling capacity than the screw compressor by 300. 2.3.3 Vacuum system problems The main problems that easily arise in vacuum systems are high energy consumption, long vacuuming time and vacuum degree. stable. These problems can be solved by adopting a tiered combined vacuum pumping system, ie, pre-extraction units with high pumping speeds and low-power maintenance units are divided, and the pre-extraction units pump the vacuum degree to a predetermined vacuum level, and then maintain it with a low-power unit. The degree of vacuum required for sublimation drying. This configuration can guarantee a short pre-draw time, ensure the quality of the material, and also greatly reduce the energy consumption of the unit.

2.3.4 Control system issues The proper control of the drying process is the fundamental purpose of the control system. At this stage, the microwave vacuum freeze-drying equipment cannot use different microwave powers according to the dielectric constant of the material and the different stages of drying. Therefore, an intelligent expert control system should be developed.

2.4 Other issues that should be noted 2.4.1 Each batch of dried material should be the same variety as much as possible, and different materials should not be mixed and dried. If it is necessary to mix and dry, the powder/liquid/or paste mixture can be thoroughly mixed to make it as uniform as possible.

2.4.2 Microwaves have penetrability. This does not mean that the size of the material is not limited, nor does it mean that materials of different sizes will have the same drying speed. It is therefore advisable to pre-treat the material to a granular or flake size that is as small as possible. For spherical products, the presence of Fe2+, the food technology food additive, increases the absorbance of the two pigments. However, due to the use of chemically pure reagents and the absorption of moisture, precipitation in the dye solution still occurs. There is a gel, so the measured absorbance of this dye solution is high. Does it mean that Fe2+ ions have a color-enhancing effect on the pigment, and it needs to be further studied.

4 Summary 4.1 Polar organic solvents such as water and ethanol are effective solvents and extractants of Rhodiola and red dates pigments, but with high concentrations of acidic ethanol (95% / will obtain higher purity Rhodiola pigment. Use 50% The acidic ethanol can obtain high purity red jujube pigment.

4.2 Rhodiola pigment at visible light 314nm, red jujube pigment has maximum absorption wavelength at 320nm.

4.3 Natural light has little effect on Rhodiola and red date pigments, and both of these pigments have good lightfastness.

4.4 At the temperature of 2080, the light absorption of Rhodiola rosea increased obviously. When it was above 80 (90, 100), the absorbance decreased, indicating that the pigment had good thermal stability when it was heated below 80°C. Jujube pigment has good thermal stability. It is not easily decomposed or discolored by heat.

4.5 pH value has a great influence on the stability of Rhodiola pigment. The pigment is stable under acidic conditions and unstable under alkaline conditions. PH (above page 45) should not exceed 20*; for sheet or block products, the thinnest thickness should not exceed 15mm10. When the material must appear in a larger size, the feasible method is at the end of drying When the microwave power is reduced, the temperature difference between the inside and the outside of the material is effectively reduced.

2.4.3 The material must not be mixed with metal objects. Because metal objects can disturb the distribution of the field intensity in the cavity, it may cause serious uneven distribution of energy, and even cause electric discharge and other serious problems that affect the drying efficiency and product quality.

2.4.4 During operation, care must be taken not to run without load, ie there must be a load in the heating chamber. If there is no material in the cavity, microwaves reflect the reflected wave and return to the magnetron through the coupling port, thereby destroying the magnetron or other electronic components.

3 Conclusion In summary, microwave vacuum freeze-drying is a relatively complex technology involving microwave technology, vacuum technology, cryogenic technology, fluid mechanics, microbiology, heat and mass transfer, and automatic control, and is an interdisciplinary development. The product. The system is prone to uneven drying problems, drying speed problems, microwave leakage problems, lack of cooling capacity, vacuum system vacuum capacity issues and the entire system of safe operation issues can be through the appropriate design, process, and reasonable Operating procedures and other technical means to solve.

It has little effect on the stability of jujube pigment. The pigment has good pH stability.

4.6 Rhodiola pigments and red dates pigments are relatively stable to carbohydrates such as glucose, maltose, lactose, and sucrose, and essential trace elements such as Ca2+, Mg2+, Mn2+, and Zn2+. Fe2+ ions have a great influence on these two pigments and should be avoided. Contact with iron.

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