Technology and principle of vacuum freeze drying of fruits and vegetables

Because the freeze-dried food avoids the defects such as discoloration, taste change, loss of nutrients and poor rehydration caused by the traditional dehydration technology, it has the advantages of keeping the original food shape, color, fragrance, taste, nutrition unchanged, good rehydration, light weight and normal temperature storage Therefore, the price of freeze-dried food in the international market is 4-6 times that of hot air dried food and 7-8 times that of quick frozen food It plays an irreplaceable role in mountaineering, navigation, exploration, field operations and other special occasions, and is also the main food for astronauts in space Freeze-dried food has reached a high level of popularity in some developed industrial countries, and the proportion of freeze-dried food in the United States and Japan has reached more than 40% According to the statistics of relevant departments, at present, the United States consumes 5 million tons of freeze-dried food every year, Japan 1.6 million tons, France 1.5 million tons, and other countries are also considerable Japan imports about 100 billion yen of freeze-dried food every year, while Hong Kong, Singapore and South Korea import 50 billion yen of freeze-dried food every year In Japan, the United States and Europe, about 6000 tons of lyophilized garlic powder is needed every year, which shows that the international market of lyophilized food is large With the sustainable development of China's economy and the continuous improvement of people's living standards, people's demand for food quality is higher and higher, especially for high-quality infant food and health food In addition, China's tourism, exploration, navigation industry will have a great development At the same time, the rapid rise of the catering industry and the acceleration of people's life rhythm, the demand for convenient instant food is growing Therefore, the development of freeze-dried food has a broad international and domestic market First, the basic principle of freeze-dried food production is the same as other drying methods To maintain the continuous sublimation drying, two basic conditions must be met, namely, the continuous supply of heat and the continuous elimination of generated steam In the initial stage, if the material temperature is relatively high, the latent heat required for sublimation can be obtained from the sensible heat of the material itself However, with the sublimation, the material temperature will soon fall to the temperature balanced with the partial pressure of the steam in the drying room At this time, if there is no external heating, the sublimation drying will stop In the case of external heating, if the steam generated by sublimation is not removed in time, the partial pressure of steam will rise, and the temperature of materials will rise accordingly When the freezing point of materials is reached, the ice crystal in materials will melt, and freeze-drying will not be carried out The process of supplying heat is a heat transfer process, and the process of removing steam is a mass transfer process Therefore, the sublimation drying process is essentially a process of heat and mass transfer at the same time Any process in nature has driving force The driving force of heat transfer in sublimation drying is the temperature difference between heat source and sublimation interface, while the driving force of mass transfer is the differential pressure between sublimation interface and steam trap (or cold trap) The greater the temperature difference, the faster the heat transfer rate; the greater the steam differential pressure, the faster the mass transfer (i.e steam removal) rate During freeze-drying, not only the good quality of the product should be maintained, but also the rapid drying rate should be achieved The latent heat needed for sublimation must be transferred from the heat source to the surface of the dried material through the external heat transfer process, and then through the internal heat transfer process to the actual occurrence of ice sublimation in the material The generated water vapor must reach the surface of the material through the internal mass transfer process, and then transfer to the steam trap (cold well) through the external mass transfer process Any process or several processes together may become the "bottleneck" of the drying process, which depends on the design of the freeze-drying equipment, operating conditions and the characteristics of the dried materials Only by improving the efficiency of heat and mass transfer and increasing the surface area of freeze-dried materials per unit volume, can the drying rate be faster       In the process of sublimation of ice crystal, every 1g of sublimation of ice crystal needs to absorb 2822.4j of heat If there is no heat source, the ice crystal will absorb heat from the product when sublimation, that is, to maintain the heat energy necessary for sublimation by reducing the temperature of the product When the temperature of the product is reduced, the saturated vapor pressure of the ice crystal is also reduced When it is reduced to the same pressure as that in the environment, the sublimation stops, because Therefore, in the process of freeze-drying, heat energy must be applied to the products However, it is not easy to transfer heat energy in a vacuum environment Therefore, people have designed the following heating methods: ① contact heat transfer mode: This is a simple heating method A heating multi-layer shelf is set in the drying room, on which is a drying plate with dried food Use the drying disk and shelf contact conduction heating under these circumstances The heating shelf and the drying plate, the drying plate and the dried food can not be completely in good contact, so when using this method for heating, the drying time is longer than other methods, but its advantage is that the drying is simple in structure and can make full use of space         ② Multiple heating mode: the contact conduction only heats one side of the food, and in this method, both sides of the dried food are in contact with the heating plate, so the heat transfer is good and the drying time can be shortened Before the dried food contacts with the heating plate, the metal mesh aluminum plate is used to clamp it, so as to open the channel of water vapor during sublimation and reduce its resistance, and then the hydraulic pressure and shelf are used The advantages of this method are that the drying time can be shortened, but in order to contact with the upper shelf, the shelf must be movable, so the hydraulic device must be used, which leads to the complex structure and reduces the utilization rate of the drying room, so the equipment cost is high In addition, for the non plane and non-uniform dried food, there is a disadvantage that the effect cannot be fully exerted         ③ Heating method with nail plate: This is the deformation of the above-mentioned multiple heating method This method uses two heating plates with multiple nails to clamp the dried food in the middle for heating The heating contact area of this method is expanded to the inside of the dried food, so the heat supply can be effectively carried out Using this method, the drying time can be greatly shortened, which is the desired way However, on the contrary, the operation before and after drying is complex, which requires manpower and time, and also involves sanitation problems, so it is rarely used in practical scale devices         ④ Radiation heating method: this method is to place the heated and dried food on the drying plate or drying net, and then insert between two heating plates, so that it does not contact with the heating plate, while the heating plate radiates to supply heat, so that the heating plate can be heated to a high temperature above the allowable temperature, while the temperature of the dried food is kept within the allowable temperature, so that the drying time can be shortened And the shape of the dried food will not be affected if it is not set The operation before and after drying is also very easy, especially in the large continuous drying device, which is more effective The appropriate control mode has been designed, and the radiation energy conversion efficiency of the heating plate has been improved, and the drying time has been shortened to the extent that it can match the multiple heating Therefore, the heating mode has evolved into the basic form of freeze-drying food equipment ⑤ microwave heating mode: microwave irradiation can heat different shapes of food inside and outside, greatly shortening the drying time (about 10% - 20%) In addition, the utilization rate of drying chamber is also high Although microwave heating has obvious advantages, so far there is no successful example in industry This is because the energy generated in the form of microwave is expensive, and the cost is 10-20 times that of steam In addition, the microwave heating process is difficult to control If there is enough heat supply, there will be a small amount of ice melting at the sublimation interface, and the dielectric constant of water is much larger than that of ice Water will absorb more heat to make the temperature rise and make more ice melt, eventually leading to drying failure ⑥ infrared heating: install an infrared generator in the drying room to generate infrared radiation However, due to its high maintenance cost, it is rarely used in freeze-dried food To sum up, various heating methods have their own characteristics Based on the continuous understanding of the essence of freeze-drying process, people have explored a variety of combinations of heating and radiation, such as conduction radiation heating, conduction microwave heating, radiation microwave heating and so on Its purpose is to improve drying rate and reduce energy consumption on the premise of ensuring product quality 2 Production equipment of freeze-dried food The main equipment of freeze-dried food production is the vacuum freeze-drying unit for food The performance, energy consumption and operation automation of the unit determine the technical level of freeze-dried food production enterprises The freeze-drying unit for food is divided into intermittent type and continuous type Continuous units are rare in domestic enterprises The intermittent lyophilizer is composed of five parts: drying box, heating system, vacuum system, refrigeration system and control system (1) drying box: there are two main forms of cylinder and square, each of which has its own advantages Cylinder is easy to manufacture, but there are many spaces that cannot be utilized; square is on the contrary, with high space utilization, but it is difficult to manufacture ⑵ heating system: most of them adopt radiation heat transfer The radiation plate is made of anodized aluminum The heat transfer medium includes heat transfer oil, saturated water, secondary steam, organic solvents such as propylene glycol and glycerin The heat source is high-pressure water steam (3) vacuum system: mechanical vacuum pump is used, including roots pump + oil seal pump and roots pump + roots pump + air jet + water ring pump The main difference between the two is that the former can not pump steam, so it requires high efficiency of the cold well, with the advantages of low energy consumption, while the latter, on the contrary, can pump a small amount of water steam, with the disadvantages of high energy consumption         (4) refrigeration system: it is composed of refrigeration unit and cooling exhaust pipe, which is used as water vapor catcher (also known as cold trap) according to its relative position with radiant panel assembly It has the following forms: ① bottom mounted type: placed at the bottom of radiant panel assembly; ② side mounted type: placed on both sides of radiant panel assembly; ③ rear mounted type: placed behind radiant panel assembly; ④ additional type: placed on radiant panel assembly The component is not in the same container, another container is placed, and there is a short and thick pipe connection between the two containers (5) control system: there are two categories: manual control and automatic control Automatic control is divided into two categories: instrument automatic control and PLC programmable control PLC programmable control is advanced Now there is a special freeze-drying program controller It is designed on the basis of PLC according to the freeze-drying requirements It can automatically complete the complex control operation in the freeze-drying process, and has the function of storing data 3 Configuration of simple freeze-dried food production line (1) pre-treatment equipment: ① multi purpose slicer: 1, with a production capacity of 200-500kg / h, which can be made in China If conditions permit, slicers made in Taiwan, Japan and the United States can be purchased, which is advanced by the United States of America ② cooling tank: the cooling tank can be replaced by the cleaning tank after being green In the simple production line, the oscillating drain machine can be used for cooling the tap water If possible, the centrifuge can be used ③ quick freezing storehouse: the configuration of quick freezing storehouse is suitable for the processing capacity of lyophilizer, and it is better to meet the requirements of 2-3 shifts of freeze-drying with daily production ingredients The cold source is set separately or semi separately (the low-pressure circulating part is independent) to