Vacuum dryer equipment and progress
Vacuum drying has a wide range of important applications in the food industry. In the past few years, vacuum technology combined with microwave heating technology and other drying technologies have emerged, and some new types of vacuum drying devices have emerged. It is proposed that China should accelerate the development of vacuum microwave drying. A new perspective on the device.
0.
Foreword At present, an important development trend of food processing technology is that Zui maintains the nutrition and color aroma of foods to a large extent, and the choice of drying process and equipment has a great influence on the nutrition, color and flavor of food products. Food drying has many characteristics similar to "food". It is different from the drying of chemical products. The former needs to consider food hygiene, nutrient loss, color and flavor changes, etc., and has strict restrictions on drying temperature and time; food drying is different from medicine. Drying of the product, because the food is often a low value-added product, and the drug is generally a high value-added product, the former must consider the economics of the drying process. Around the "quality and economy", in recent years, food drying technology and equipment have made a lot of progress, and vacuum combined with other drying methods or heating technology, giving the vacuum drying device a new connotation and vitality.
1.
Vacuum drying characteristics of foods Vacuum drying is based on the basic principle that the saturated vapor pressure of water is closely related to temperature. Under vacuum, the boiling point of water is reduced, that is, operating under vacuum, that is, operating at low temperatures, avoiding high temperatures. Under the destruction of nutrients such as vitamins, and at the same time increase the drying speed. In addition, in the vacuum system, the content of air per unit volume is lower than that in the atmosphere, and drying the food in the environment of relative hypoxia can alleviate or even avoid the chance of oxidation of fat in the food, pigment browning or other oxidative deterioration. Therefore, vacuum drying is used to obtain better food quality.
2.
The vacuum drying of traditional vacuum drying equipment has been widely used in food, pharmaceutical, chemical and other industries. Various vacuum drying equipments have been developed and introduced in China, and their structural forms are various. Commonly used forms in the food industry are box-type, double-cone vacuum dryers, belt vacuum dryers, and the like. These conventional vacuum drying devices mainly use hot air, steam or electric heating to transfer heat from the outside to the inside of the material by means of heat conduction, convection or radiation.
2.1 Box vacuum dryer Box vacuum drying is a long history of Zui, but also a simple vacuum dryer in Zui. There are many hollow heating plates in the vacuum drying box. The heating plate is usually heated by steam. Electric heating or other radiant heating. The material is placed on a hot plate on a metal plate, and heat is transferred to the inside of the material through heat conduction, so that the water is heated and evaporated. Box vacuum dryers are still commonly used in practice and are suitable for drying liquid, slurry, powder and bulk food materials.
2.2 Double cone vacuum dryer The double cone vacuum dryer rotates the conical container with a symmetrical jacket. The material is continuously stirred by the inclination of the inner cone itself, and is fed by one side of the rotary shaft through the rotary joint. Steam or heat the carrier and vent it by means of an exhaust pipe with a filter at the center of the other end shaft. The double cone vacuum dryer can achieve a higher degree of vacuum, the internal structure is simple, the cleaning is easy, and the materials can be completely discharged.
2.3 Vacuum belt dryer The vacuum belt dryer is composed of a continuous stainless steel belt. The steel belt bypasses the heating roller and the cooling drum. The structure is multi-layered to form the main body of the dryer and then incorporated into the closed vacuum chamber. . The material book is laid on the belt heating plate and moves accordingly. Because the material is boiled on the heating plate under vacuum conditions, the finished product has porosity; the whole system is closed operation, sanitary condition is good, practical operation The vacuum is between 100~10Kpa, the heating temperature is about 150°C, and the running condition (drying temperature and time) is between freeze drying and spray drying. The quality of the finished product is very close to freeze drying, but the freeze drying is the gap operation and vacuum. The belt dryer is a continuous operation, which is especially suitable for the drying of heat-sensitive and highly oxidizable foods. The liquid or slurry materials can be used uniformly. The dried orange juice, tomato juice, instant tea and the like are commonly used in foods.
2.4 Vacuum drum dryer The vacuum drum dryer seals the drum in the vacuum chamber. In the vacuum drum dryer, the feeding, unloading and scraping must be controlled from the outside of the drying chamber. Therefore, the drying cost is high, so only Can be used for the drying of very heat sensitive foods, such as juice, yeast, baby food, etc.
3.
New developments in vacuum drying devices Traditional vacuum drying devices are mostly heated by heat conduction, convection or heat radiation. The heating rate is slow and not uniform. In recent years, researchers have combined vacuum technology with microwave heating technology and other drying technologies. Some new types of vacuum dryers have emerged.
3.1 Vacuum Freeze-dried water has three states of aggregation, namely liquid, solid and vapor. As the pressure continues to decrease, the freezing point does not change much, while the boiling side is getting lower and lower, getting closer to the freezing point. When the pressure drops to a certain value, the boiling point is combined with the freezing point, and the solid ice can be directly converted into a vapor state without passing through the liquid state. The triple point pressure of water is 610.5Pa, and the triple point temperature is 0.0098 °C. When the pressure is lower than the triple point pressure, the solid ice can absorb heat directly into steam vapor, and the principle of freeze drying is here. When vacuum freeze-drying, the product is placed in a vacuum chamber between two heating plates, and the far-infrared heating is accelerated by low temperature to ensure uniform drying. The temperature of the heating plate is precisely controlled according to the heating curve of the drying process. The degree of vacuum should be chosen to ensure that all water in the entire drying process is not melted in the form of ice. For most vegetables and meats, 0.5 to 1 Pa is selected, which corresponds to a sublimation temperature of about 25 °C. Since the vacuum freeze-dried food is dehydrated at a very low temperature, the loss of nutrients and flavors of the food is small, and the original nutrition and flavor can be retained to a large extent, and the rehydration is excellent, and the restoration can be completed in seconds to minutes. Its color, taste, shape and freshness are basically the same. There are many vacuum freeze-dried products: coffee, instant tea, juice, herbs, etc. Vegetables include onions, garlic, ginger, grind, meat, and fresh shellfish. Vacuum freeze-drying has a large investment, which makes many food manufacturers discouraged, such as Danish ATLAS RAY50 freeze-drying equipment (freeze area 45m2
The price is as high as 1.05 million US dollars. The domestic freeze-drying machine has just started. There is still a big gap between the quality and foreign products. In addition, the production cost of vacuum freeze-drying is also higher, because it is necessary to maintain a high vacuum and (-25 °C) low temperature, long drying time, high energy consumption, these factors make the use of freeze-drying in the food industry to bring great resistance.
3.2 jet continuous vacuum dryer, also known as (Filtermat spray dryer)
The jet continuous vacuum dryer, also known as the Filtermat spray dryer, is equivalent to a combination of a belt vacuum dryer and a spray dryer. Niro
The dryer developed by Hudson has successfully solved the problem of viscous foods such as foods with high sugar content, high fat content or high acid content. The viscous materials are treated with traditional spray dryers. Sticking to the wall occurs and drying is difficult. During the drying process, the material is sprayed vertically downward through the pressure nozzle to the spray drying chamber, and the hot air is also sprayed downward. The semi-dry powder material is collected on the moving mesh belt, and the exhaust gas is also discharged by the fan. The dried material is on the mesh belt. Move, cool, and collect further. Since the medium temperature is maintained in the spray tower, the hot air temperature is only about 100 ° C, and the general spray drying hot air temperature is about 150 ° C, so that the heat sensitive material loss is small, and the height in the spray tower is lowered.
3.4 Microwave vacuum drying Microwave is a wavelength of 1.0~0.001m, frequency is 300~300,000MHz, and has penetrating electromagnetic radiation wave. The principle of microwave drying is: microwave generator radiates microwave to dry material, when microwave is injected into material Internally, the polar molecules such as water rotate synchronously with the frequency of the microwave. For example, the dried vegetable products use 915MHz microwave, the polar water molecules in the vegetables rotate 915 million times per second, and polar molecules such as water As a result of such high-speed rotation, the material instantaneously generates frictional heat, which causes the surface and interior of the material to simultaneously heat up, so that a large amount of water molecules escape from the material, and the material is dried. Conventional heating methods such as steam, hot air, electricity, etc. use the principle of heat conduction, convection, and radiation to transfer heat from the outside to the inside of the material. It takes a certain time from the inside and the outside, and the time required for the heat conduction performance of the material is worse. Long, so the heating rate is slow and the heat is not uniform, and the energy consumption is high. Microwave heating is to make the object to be heated become a heating element. Therefore, it is called an internal heating method. The microwave passes through the food from all sides, and the food is heated inside and outside. No heat transfer medium or convection is required, and the temperature inside and outside the food rises at the same time. The heating rate is fast and uniform, only a fraction or a few of the traditional heating method, and can better preserve the original color and flavor of the vitamins and foods in the food. Experiments have shown that dried vegetables have only 3% of chlorophyll and vitamins, 17% of dry and dry, 40% of hot air drying, 60% to 90% of microwave drying, and vacuum freeze drying. Keep 97%{2}. Microwave vacuum drying combines microwave technology and vacuum technology to give full play to the characteristics of fast and uniform microwave heating and low water vaporization point under vacuum conditions. It is a promising drying technology. Microwave vacuum drying technology has been promoted to industrial production in France, Japan, and the United States in recent years. This technology is very suitable for deep processing of heat sensitive foods.
The University of California collaborates with a company to dry the seedless raisins using microwave vacuum to maintain the original shape and color of the grapes, avoiding the changes in color, shape, flavor and nutrients of the traditional process (24°C hot air for 24 hours). Disadvantages, product quality has greatly improved. The microwave vacuum drying chamber (2450MHz, 48Kw) manufactured by France International Microwave Co., Ltd. has a diameter of 1.5m and a length of 12m. It processes instant orange powder. The product not only retains the original color and fragrance, but also retains more vitamins than the spray. dry. In recent years, the demand for high-grade dehydrated vegetables is very large at home and abroad. The production of dehydrated vegetables is the key to determining the quality of the products. The vacuum dehydration is used to prepare dehydrated vegetables. Although the quality is good, the vacuum freeze-drying equipment is expensive. High cost; since the 1980s, foreign countries have adopted vacuum microwave-hot air (45%~55%) to produce dehydrated vegetables, which are equivalent in quality to the products produced by freeze-drying process, while the one-time investment is small, and the total cost is relatively large. decline.
4. Conclusion (1)
Vacuum drying has a low drying temperature, relatively low oxygen in the drying chamber, can avoid fat oxidation, pigment browning and other advantages, suitable for drying of heat sensitive food materials, in addition, equipment cost, drying cost is relatively low, vacuum drying in The middle point of food drying has an important position. (2)
Vacuum drying combined with microwave heating technology or other drying methods has led to a number of new vacuum drying devices that give vacuum drying a new connotation and vitality. (3)
Vacuum microwave drying absorbs the advantages of both microwave heating and vacuum drying. It is a promising drying technology. It is recommended that China accelerate the development and development of vacuum microwave drying equipment.
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