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[Abstract]:
Freeze of vacuum freezing drying is the wet material to the eutectic point temperature, make the moisture in the material into solid ice, then under the high vacuum environment, through to the materia
Vacuum freeze drying
 
Freeze of vacuum freezing drying is the wet material to the eutectic point temperature, make the moisture in the material into solid ice, then under the high vacuum environment, through to the material is heated, ice sublimation directly into water vapor, reoccupy water vapor condenser will water vapor condensation in the vacuum system, in order to gain technology of dry products. Due to the drying process was conducted in low temperature, vacuum, water directly from a solid sublimation of gaseous material, thus can keep the maximum dry material color, aroma, taste, shape and nutrition ingredients, and dried products show porous sponge structure, easily soluble in water (even) in the ice water, able to quickly recover, seal after long shelf life. At present, vacuum freeze drying technology in biological engineering, pharmaceutical industry, food industry, materials science, agricultural and sideline products deep processing, and even antique calligraphy and painting repair and specimens of plant and animal production and other fields has a broad application.
 
In the food industry, especially in the food of high value and high value-added processing, vacuum freeze drying is high content of science and technology of drying technology, involves the wide range of knowledge of a foreign technology, is considered to be the most excellent, one of the most advanced drying technology. Due to the drying process was conducted in low temperature, vacuum, the moisture in the material directly from solid sublimate to gas, thus it can be maximally keep dry material color, aroma, taste, shape and nutrition ingredients, and the complex water performance is good. Because of the advantages that the technology can't compare with other drying methods, it is gradually applied to many industries, especially in food industry. And the technology of freeze-dried products are in good agreement to "green food", "health food", "convenience foods" three major development trends, therefore, freeze-dried food industry focus on industries by people gradually.
 
1. Vacuum freeze drying equipment
 
Has achieved large-scale freeze-drying equipment used for food, generally USES the automatic control system, at the same time to guarantee the quality of freeze-dried products and energy saving, usually adopts freeze-drying equipment and other drying equipment together a combination of freeze-drying equipment, for example, spray freeze-drying equipment and so on.
 
Vacuum freeze dryer is mainly composed of vacuum freeze drying oven, vacuum system, refrigeration system, heating system and automatic control system. The drying oven is a vacuum closed container, which is the heat transfer and mass transfer in the drying process. His performance directly affects the performance of the freeze-drying equipment. If a steam sterilization function is carried out, the freeze-dried box should also be a low-pressure internal pressure vessel. The shape of the drying chamber is mainly round and rectangular. The circular drying chamber is easy to operate, high strength and low production cost, but its internal space utilization is low. The vacuum system includes a vacuum main pump and a booster pump. The vacuum pump is used to remove air vapor from the system. When the vacuum pump works, open the isolation valve, the air and water vapor in the vacuum drying chamber are captured by the steam trap condenser and enter the vacuum pump, which is discharged from the exhaust vent of the vacuum pump. In order to prevent the gas from being captured by the steam trap condenser, it still contains a very small amount of water vapor into the pump, and the system valve is equipped with gas ballast. An oil mist trap is installed in the exhaust port of the vacuum pump to prevent the exhaust gas from polluting the indoor environment. Is the role of cold trap adsorption into ice water vapor condensation in the drying chamber, in order to avoid into the vacuum pump, on the one hand, can reduce the work burden of vacuum pump, on the other hand can guarantee drying chamber with low vacuum
 
Vacuum freeze drying system is mainly composed of refrigeration system, vacuum system, heating system, drying system and control system, etc, freeze drying room: freeze drying chamber with horizontal cylindrical, box and other types. Drying chamber to cooling to - 40 ℃ or lower temperature, and heating to + 50 ℃ or so. Dry room design has several layers to a dozen layers of dry shelf, to place dozens to hundreds of drying plates. Drying chamber with a pipe connected to the condenser at low temperature and vacuum valve, to eliminate indoor water vapor, as well as the pipe connected to the switch, refrigerating machine, in order to obtain the material needed for sublimation heat and low temperature environment. Condenser at low temperature, low temperature condenser is a sealed container, with a pipe connected to the machine, vacuum pump, heat exchanger, in order to get low temperature vacuum environment and condenser defrosting heat supply. Within the condenser temperature must be kept below the temperature of the material in drying chamber, general condenser temperature should be kept in - 40 ℃ to 50 ℃ or so, make a lot of water vapor condensed in the condenser chamber condenser defrosting device also has at low temperature, discharge valve, hot air blowing device, etc., used to melt condensation frost and internal moisture, discharge and then blow dry indoor.
 
Vacuum system consists of freezing drying chamber, low temperature condenser, vacuum valve and pipe, vacuum pump and vacuum instrument. The vacuum system of freeze-drying equipment requires good sealing performance. The vacuum pump adopts spiral or slide valve type oil seal mechanical pump, the vacuum system of the multi-stage steam jet pump and the vacuum system of the turbo-water ring pump in the center of the rotz pump.
 
From the vacuum freeze drying process, different products are different from the freeze-drying process due to the difference of variety, composition, moisture content, eutectic point and calving temperature. The same kind of sample, different freeze-drying machine or the same freeze-drying machine different installed capacity, its freeze-drying process is also different. Therefore, the study of freeze-drying process of various materials is the key to vacuum freeze-drying technology.
 
Vacuum freeze-drying is generally divided into three stages: pre-freezing stage, sublimation stage and analytical stage. Freeze-drying process must be developed in sections, and then the whole process can be formed to form the curve of temperature, pressure and time, namely the freeze-drying curve. For each new product, a freeze-drying curve must be worked out.
 
2. Pre-freezing stage
 
Precool temperature must be below the eutectic point temperature of the material, the best low 5 ℃ to 10 ℃. The freezing process of materials is an exothermic process. It takes some time. In order to freeze all products, it is generally kept at about 2 h after the product reaches the specified pre-freezing temperature. The key to the freezing process is the rate at which food is frozen. Freezing rate directly affects the drying rate and product quality. During the slow freezing, the crystal lattice of ice crystals is larger, and the crystal of ice crystals is hexagonal and symmetrical, which is beneficial to the sublimation of ice crystals in materials, but the product quality is poor. And the ice crystals formed in the frozen form are irregular bough or ball type, small gap, high resistance when sublimation, not conducive to freeze-drying. Therefore, it is necessary to find a suitable freezing rate to obtain better physical properties and solubility, and facilitate the sublimation of the drying process.
 
The sublimation stage, the first stage of drying. The temperature of the material in the sublimation phase should be lower than the total melting point. Too much lower, longer sublimation time, lower sublimation rate. Higher than total melting point temperature, the product melts, appears dry shrinkage phenomenon. Therefore, the temperature of the product should be controlled strictly below and close to the total melting point in production. The heat required for sublimation is usually derived from the shelf. In the sublimation process, the freezing temperature cannot exceed the eutectic temperature of the material, and the temperature of the drying layer cannot exceed the disintegration temperature of the material. Lyophilization process state of primary drying stage has kept a low temperature, phase ii due to the vacuum drying period, make the product temperature can increase shelf temperature, temperature, but it must be below its collapse that help reduce a small amount of residual moisture. During the drying process, when the temperature of the product is close to the temperature of the heating shelf, the pressure in the drying box is close to the pressure of the water catcher and the difference between the two is maintained.
 
The analytical phase, the second stage, is dry. There is no freezing area in the material of the analytical phase. The temperature of the material can rise rapidly to the highest allowable temperature, and the temperature is maintained until the freeze drying is completed. The laminar temperature (the temperature of the freeze-drying curve) is generally slightly higher than the product temperature. The specific value is related to the lyophilizer and is obtained by experiment. The stress of the analytical phase is generally 20 Pa - 30 Pa. The final phase of freeze-drying can be higher. The analytical time is determined by the variety and shape of the product, the requirements of residual water content and the performance of the lyophilizer. Parsing stage water vapor condenser temperatures will decline due to small amount of water vapor, when the freeze-drying chamber pressure down to about 20 pa nearby, is advantageous to the escape of water vapor from the product, but the product need to heat rapidly, more calories, heat transfer, pressure is too low, go against the pressure regulating sublimation method may be adopted to accelerate the parsing.
 
The packaging of freeze-dried foods is also critical. Because of freeze-dried food is under the low temperature low pressure processing, porous structure of freeze-dried food organization, total surface area of 100 ~ 150 times bigger than the old, the opportunity to contact with moisture and oxygen increased greatly, so be under vacuum conditions or kept in a sealed bags full of dry inert gas; When need to crushing and points the packing, should also be in filled with nitrogen gas environment below 25 ℃, relative humidity below 40%, temperature conditions. In order to keep the moisture content of dry food under 5%, the packing bag should be put into the desiccant to absorb the moisture. The packing material is good for air permeability, high strength and deep color. After packaging, vacuum seal is taken to facilitate long-term storage.
 
There are several problems to be noticed in vacuum drying.
 
A. The freezing temperature should be kept below the triple point. Otherwise, in a vacuum, the boiling of liquid causes the surface of the product to be uneven.
 
B. In the process of sublimation drying, sufficient vacuum is maintained. Lack of vacuum or heat supply may lead to incomplete water sublimation and high water content.
 
C, heating is too fast, the heating is not even or the pre-freezing is incomplete, then the product will be partially liquefied during the sublimation process, and the spray bottle phenomenon can be produced under the condition of decompression, so that the product surface is not smooth.
 
D, keep the sublimation temperature constant.
 
E, the quick freezing process, the faster, the smaller the ice crystals generated, the smaller the impact on the structure of the item.
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