Vacuum drying in the food industry has a wide range of important applications, in recent years, vacuum technology and microwave heating technology and other drying technology, the emergence of some new type vacuum drying device, proposed our country should speed up the development of new ideas and development of vacuum microwave drying device.
0. introduction, at present, an important trend of food processing technology is to maintain the nutrition and color and aroma of food, and the selection of drying process and equipment has great influence on the nutrition, color and aroma of food products. Food drying has many features and "food" related, it is different from the dry chemical products, the need to consider the loss of food hygiene and nutrition, flavor and color change and so on, there are strict limits on drying time and temperature; food drying is different from the pharmaceutical product drying, because food is often of low value-added products, and the drug is generally high value-added products, the former must consider the economic drying process. Around the "quality and economy", food drying technology and equipment have made many progress in recent years, and vacuum combined with other drying methods or heating technologies endows the vacuum drying device with new connotation and vitality.
1. characteristics of the vacuum vacuum drying of food drying is a basic principle based on the saturated vapor pressure and temperature are closely related, in a vacuum, the boiling point of water is reduced, the operation is operating under vacuum at low temperature can be avoided under high temperature, nutrients vitamins and other damage, while improving the drying speed. In addition in the vacuum system, the volume of air within the unit is lower than the content in the atmosphere, in this relatively hypoxic environment of food drying can reduce or even avoid the chance of oxidation of fat in foods, pigment or other browning oxidation, so the vacuum drying is a good food quality.
2., the traditional vacuum drying equipment has been widely applied in the fields of food, pharmacy, chemical industry and so on. Vacuum drying equipment has been developed and introduced in China, and its structural form is varied. In the food industry, the main forms are box type, double cone vacuum dryer, belt vacuum dryer and so on. These traditional vacuum drying equipments mainly use hot air, steam or electricity to heat, and transmit heat from outside to the interior of materials by heat conduction, convection or radiation.
2.1 chamber vacuum dryer is the oldest and the simplest vacuum dryer, and there are many hollow heating plates in the vacuum drying box. The heating plate is usually heated by steam, and can also be heated by electric heating or other radiation. The material is placed in the metal plate on the heating plate, and the heat passes through the heat conduction to the interior of the material to heat and evaporate the moisture. Box type vacuum dryer is still widely used in practice. It is suitable for the drying of liquid, slurry, powder and bulk food.
2.2 double cone vacuum desiccator double cone vacuum dryer is the conical container of rotary symmetrical jacket, the internal angle of the cone itself through the material is continuously stirred by rotary joints from one side of the rotary shaft into the steam or heat carrier, and with the help of another end of the shaft in the heart of the exhaust exhaust pipe with filter. The double cone vacuum dryer can achieve high vacuum, the internal structure is simple, the cleaning is easy, and the material can be completely discharged.
2.3 vacuum belt drier vacuum belt dryer is made up of a continuous stainless steel belt. The steel belt is around the heating drum and cooling drum, and the structure is multi-layered, which constitutes the dryer body and is then incorporated into the closed vacuum chamber. The material thin tile moved with the belt type heating plate, because under the vacuum condition, the material in the heating plate is boiling foaming, the finished product is porous; the whole system is a closed operation, good sanitary conditions, between the actual operating vacuum 100~10Kpa, the heating temperature is 150 degrees Celsius, the condition (drying temperature and time) between freeze drying and spray drying, freeze drying and the quality of the finished product is very close, but the freeze drying is the gap operation of vacuum belt dryer is a continuous operation, especially suitable for heat sensitive and easily oxidized in the drying of food, liquid or slurry material uniformly used, commonly used in the food drying Orange Juice tomato juice, instant tea, etc..
2.4 vacuum drum dryer vacuum drum dryer drum will be sealed in a vacuum chamber, the vacuum drum dryer, feeding and unloading and scraping etc. must be controlled from the drying chamber, so the drying cost is very high, so it can only be used for heat sensitive food very dry, if the juice, yeast, baby food etc..
Vacuum drying device of vacuum drying device in 3. most traditional heating or heat convection heat conduction, the way, the heating speed is slow and uneven, in recent years, researchers of vacuum technology and microwave heating technology and other drying technology, the emergence of some new type vacuum drying device.
3.1 the vacuum freeze-dried water has three kinds of aggregation states, that is, liquid, solid and vapour. With the constant decrease of pressure, the change of freezing point is little, but the boiling point is getting lower and lower and 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 converted directly into the vapor state without the liquid. The triple point pressure of water is 610.5Pa, and the temperature of three-phase point is 0.0098 degrees. When the pressure is lower than the three-phase point pressure, 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 the vacuum chamber and is between the two heating plates, which is heated by low temperature and far infrared to ensure uniform drying. The temperature of the heating plate is precisely controlled according to the heating curve of the drying process. The choice of vacuum degree should ensure that all the water in the whole drying process is insoluble in the form of ice. For most vegetables and meat, 0.5~1Pa is selected, corresponding to the sublimation temperature of 25 degrees. The vacuum freeze drying food is dehydrated at very low temperatures, nutrition flavor loss, can maximize the retention of the original nutrition and flavor, rehydration is excellent, complete recovery in a few seconds to a few minutes, its color, flavor, and fresh products of the same base. Vacuum freeze drying products: there are many coffee, instant tea, fruit juice, herbs, vegetables are onions, garlic, ginger, mushrooms, meat, shellfish etc.. Vacuum freeze drying one-time investment is large, so many food manufacturers prohibitive, such as type RAY50 ATLAS Danish freeze-drying equipment (freeze-dried area 45m2) offer up to $1 million 50 thousand, the lyophilized machine has just started, and the quality of foreign products is still a big gap, in addition to the vacuum freeze drying production cost is high, this is because of the need to maintain a high vacuum and low temperature (-25 C), long drying time, high energy consumption, these factors make the freeze drying in the food industry with great resistance.
3.2 spray continuous vacuum dryer is also called (Filtermat spray dryer)
Injection type continuous vacuum dryer and Filtermat spray dryer, combined the equivalent of vacuum belt dryer and spray dryer. Niro
Development of Hudson, this kind of dryer successfully solves the problem with sticky food -- such as dry food material with high sugar, high fat or high acid content, high viscosity materials by spray dryer traditional happens wall sticking phenomenon, difficult to dry. During the drying process, the material is sprayed vertically into the spray drying chamber through the pressure nozzle, and the hot air is also sprayed down. The semi dry powder material is gathered on the moving mesh belt, and the exhaust gas is discharged by the fan. The dry material is further moved, cooled and collected on the mesh belt. The spray tower to maintain medium vacuum, air temperature is only 100 degrees Celsius, the temperature of hot air and spray drying of 150 DEG C, the heat sensitive material loss is less, while reducing the height of the spray tower.
3.4 microwave vacuum drying microwave wavelength is 1.0~0.001m, frequency 300~300000MHz, penetrating electromagnetic radiation, microwave drying principle is: microwave generator of microwave radiation to dry materials, when the microwave into the material, through the water and other polar molecules with microwave frequency for synchronous rotation, such as dry vegetable products by 915MHz the vegetables in the microwave, is polar water molecules per second rotation 915 million, water and other polar molecules rotate at high speed so that the material instantaneous friction heat, resulting in material surface and internal temperature, so that a large number of water molecules from the material to escape, to achieve the effect of drying. The traditional heating method such as steam, hot air and electric heating is the use of the principle of radiation heat conduction, convection, heat transfer from the internal external material, from the outside to the inside need time, thermal conductivity of materials to the worse the longer it takes, so the heating speed is slow and uneven heat, high energy consumption. Microwave heating is the heated object itself as the heating body, it is called internal heating method, microwave through the food from the food from all sides, and also does not require heating, heat transfer medium, nor by convection, while the temperature rise inside and outside food, heating fast and uniform, only a fraction of the traditional heating method or a few one of the points, and can retain the original flavor of food and vitamins in food. The experiment shows that the dried vegetable chlorophyll, vitamins and other nutrients remaining 3%, shade is retained 17%, air retained 40% rapid drying, microwave drying can keep 60%~90%, vacuum freeze drying can keep 97%{2}. Microwave vacuum drying is an organic combination of microwave technology and vacuum technology, giving full play to the characteristics of fast and uniform microwave heating and low vaporization point in vacuum. It is a promising drying technology. Microwave vacuum drying technology in France, Japan, the United States in recent years has been from the laboratory to industrial production, deep processing of this technology is very suitable for heat sensitive food.
The University of California in cooperation with a company, the use of microwave vacuum drying of Seedless raisins, grapes to keep original shape and color, to avoid the traditional process (65 DEG C hot air drying for 24 hours) the product color, shape, flavor and nutrition changes the shortcomings of the product quality is greatly improved. The microwave vacuum drying room (2450MHz, 48Kw) made by France international microwave company has a diameter of 1.5m and a length of 12m. It can process instant orange powder. The product not only keeps its original color and fragrance, but also has much higher vitamin retention than spray drying. In recent years, the demand for high-grade dehydrated vegetables at home and abroad, and the production of dehydrated vegetables, the drying process is the key to product quality, dry preparation of dehydrated vegetables although good quality by vacuum freezing, but the vacuum freeze drying equipment is expensive, high production cost; since 80s has been the use of vacuum microwave -- hot air (45%~55%) the production of dehydrated vegetables, production and freeze drying technology in the quality of the products, and less investment, the total cost has decreased substantially.
4. concluding remarks
Vacuum drying with low drying temperature, drying indoor relative hypoxia, can prevent fat oxidation, a series of advantages such as brown pigment, suitable for heat sensitive food drying equipment, drying, in addition the cost costs are relatively low, vacuum drying in the food drying point has important status.
The combination of vacuum drying and microwave heating technology or other drying methods, there are a lot of new vacuum drying device, vacuum drying gives new connotation and vitality.
Vacuum microwave drying has advantages of microwave heating and vacuum drying, the drying technology is a very promising, suggestions for China to speed up the development, the development of vacuum microwave drying device.