The hottest food vacuum drying device and its prog

Food vacuum drying devices and their progress

Abstract vacuum drying has a wide range of important applications in the food industry. In recent years, vacuum technology has been combined with microwave heating technology and other drying technologies, and some new types of vacuum drying devices have emerged. It is proposed that China should speed up the development of vacuum microwave drying devices

0. Preface

at present, an important development trend of food processing technology is to maintain the nutrition, color and flavor of food to the greatest extent, and the selection of drying technology and equipment has a great impact on the nutrition, color and flavor of food products. Food drying has many characteristics related 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 the drying of pharmaceutical products, because food is often low value-added products, while drugs are generally high value-added products. The former must consider the economy of the drying process. Focusing on "quality and economy", food drying technology and equipment have made a lot of progress in recent years, and the combination of vacuum and other drying methods or heating technologies has given the vacuum drying device new connotation and vitality

1. Characteristics of food vacuum drying

vacuum drying is based on the following basic principle: the saturated vapor pressure of water is closely related to temperature. Under vacuum, the boiling point of water decreases, that is, operating under vacuum, that is, operating at low temperature, which can avoid the destruction of nutrients, vitamins, etc. under high temperature, and improve the drying speed at the same time. In addition, in the vacuum system, the content of air per unit volume is lower than that in the atmosphere. Food drying in this relatively anoxic environment can reduce or even avoid the oxidation opportunities of fat, pigment browning or other oxidative deterioration in food. Therefore, better food quality can be obtained by vacuum drying

2. The traditional vacuum drying device

vacuum drying is widely used in food, pharmaceutical, chemical and other industries. Various vacuum drying equipment with various structural forms have also been developed and introduced in China. The commonly used forms in the food industry mainly include box type, double cone type vacuum dryer, belt type vacuum dryer, etc. These traditional vacuum drying devices mainly use hot air, steam or electricity to heat, and use the principles of heat conduction, convection or radiation to transfer heat from the outside to the inside of materials

2.1 box vacuum dryer

box vacuum drying is the oldest and simplest vacuum dryer. There are many hollow heating plates in the vacuum drying box. The heating plates are generally heated by steam, and can also be heated by electric heating or other radiation. The material is placed on the heating plate in the metal plate, and the heat is transmitted to the interior of the material through heat conduction, so that the water is heated and evaporated. At present, the box vacuum dryer is still widely used in practice, which is suitable for the drying of liquid, paste, powder and granular food materials

2.2 double cone vacuum dryer

double cone vacuum dryer is to rotate the conical container with symmetrical jacket, make the materials continuously stirred by the inclination of the internal cone itself, send steam or heating carrier from one side of the rotary shaft through the rotary joint, and exhaust with the help of the filtered exhaust pipe set at the center of the shaft at the other end. The double cone vacuum dryer can achieve high vacuum, with simple internal structure and easy cleaning, and all materials can be discharged

2.3 vacuum belt dryer

the vacuum belt dryer is composed of a continuous stainless steel belt. The steel belt bypasses the heating drum and the cooling drum, and the structure is multi-layer, forming the main body of the dryer, which is then incorporated into the closed vacuum chamber. The materials are laid flat on the belt heating plate and move with it. Under vacuum conditions, the materials are bubbling on the force measuring instigator driven by the connected push rod on the heating plate, so the finished products are porous; The whole system is closed operation, with good sanitary conditions. The actual operating vacuum is between 100~10kpa, and the heating temperature is about 150 ℃. Its transportation conditions (drying temperature and time) are between freeze-drying and spray drying. The quality of finished products is very close to freeze-drying, but freeze-drying is a gap operation, and the vacuum belt dryer is a continuous operation, which is especially suitable for the drying of heat sensitive and highly oxidizable food. Liquid or slurry materials can be used uniformly, Dried orange juice, tomato juice and instant tea are commonly used in food

2.4 vacuum drum dryer

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 drying room, so the drying cost is very high, so it can only be used for the drying of very heat sensitive foods, such as juice, yeast, baby food, etc

3. New progress of vacuum drying devices

traditional vacuum drying devices are mostly heated by heat conduction, convection or thermal radiation, and the heating speed is slow and uneven. In recent years, researchers have combined vacuum technology with microwave heating technology and other drying technologies, and some new types of vacuum drying devices have emerged

3.1 vacuum freeze drying

water has three aggregation states, namely liquid, solid and vapor. With the continuous reduction of pressure, the freezing point changes little, while the boiling point side is getting 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 directly transformed into a vapor state without going through the liquid state. The three-phase point pressure of water is 610.5pa, and the three-phase point temperature is 0.0098 ℃. When the pressure is lower than the three-phase point pressure, solid ice can absorb heat and directly convert it into steam. This is the principle of freeze-drying. During vacuum freeze-drying, the product is placed in the vacuum chamber between two heating plates, and the far-infrared heating is excited by low temperature to ensure uniform drying. The temperature of the heating plate is accurately controlled according to the heating curve of the drying process. The vacuum degree should be selected to ensure the whole drying. According to these data, we can get a formula that all moisture in the drying process is in the form of ice and does not dissolve. For most vegetables and meat, it is appropriate to choose 0.5~1pa, corresponding to the sublimation temperature of about 25 ℃

because the vacuum freeze-dried food is dehydrated at a very low temperature, the loss of nutritional components and flavor substances of the food is less, which can maximize the crack resistance of the concrete. Judging by the cracking degree of the side of the specimen, it retains the original nutrition and flavor, has excellent rehydration, and can be restored within seconds to minutes. Its color, taste and shape are basically the same as those of the fresh product. There are many vacuum freeze-drying products: coffee, instant tea, fruit juice, herbs, etc. vegetables include onions, garlic, ginger, grinding Ru, meat, fresh shellfish, etc. Vacuum freeze-drying has a large one-time investment, which discourages many food manufacturers. For example, the quotation of ray50 freeze-drying equipment (freeze-drying area 45m2) of atlas in Denmark is as high as 1.05 million US dollars. Domestic freeze-drying machines have just started, and there is a big gap in quality with foreign products. In addition, the production cost of vacuum freeze-drying is also high, because it needs to maintain a high vacuum and low temperature (-25 ℃), the drying time is long, and the energy consumption is high, These factors bring great resistance to the use of freeze-drying in the food industry

3.2 jet continuous vacuum dryer is also known as (filtermat spray dryer)

jet continuous vacuum dryer is also known as filtermat spray dryer, which is equivalent to the combination of belt vacuum dryer and spray dryer. This kind of dryer developed by Niro Hudson company successfully solves the drying problem of sticky food materials, such as food materials with high sugar content, high fat content or high acid content. Materials with high viscosity will stick to the wall with the traditional spray dryer, which is difficult to dry. During the drying process, the materials are sprayed vertically downward to the spray drying chamber through the pressure nozzle, and the hot air is also sprayed downward. The semi dry powder materials are gathered on the moving belt, and the tail gas is also discharged by the fan. The dried materials are further moved, cooled and collected on the belt. Due to the medium vacuum degree maintained in the spray tower, the hot air temperature only needs to be about 100 ℃, while the general hot air temperature of spray drying is about 150 ℃, so the loss of heat sensitive materials is less, and the height in the spray tower is reduced

3.4 microwave vacuum drying

microwave is a penetrating electromagnetic radiation with a wavelength of 1.0~0.001m and a frequency of 300~300000mhz. The best method is to adopt polyurethane rigid foam/polyisocyanate. The principle of microwave drying is: the microwave generator radiates the microwave onto the dried material. When the microwave is injected into the material, the polar molecules such as water rotate synchronously with the microwave frequency, For example, if 915MHz microwave is used for drying vegetable products, the polar water molecules in the vegetables rotate 915million times per second. As a result of such high-speed rotation, the materials instantly generate friction heat, causing the surface and interior of the materials to rise at the same time, so that a large number of water molecules escape from the materials to achieve the effect of material drying. Traditional heating methods, such as steam, hot air, electric heating, use the principles 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 outside to the inside. The worse the thermal conductivity of the material, the longer the time required. Therefore, the heating speed is slow and the heat is uneven, and the energy consumption is high. Microwave heating is to make the heated object itself become a heating body, so it is called internal heating method. The microwave passes through the food from all directions, and the inside and outside of the food are heated at the same time, without heat transfer medium or convection. The temperature inside and outside the food rises at the same time, and the heating speed is fast and uniform. It only needs a fraction or dozens of traditional heating methods, and can better retain the vitamins in the food and the original color, flavor and flavor of the food. Experiments show that the chlorophyll, vitamins and other nutrients of fresh vegetables dried in the sun are only 3%, 17% can be retained in the shade, 40% can be retained in the hot air rapid drying, 60% - 90% can be retained in the microwave drying, and 97% can be retained in the vacuum freeze drying

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 of water under vacuum conditions. It is a very promising drying technology. Microwave vacuum drying technology has been promoted from laboratory to industrial production in France, Japan and the United States in recent years. This technology is very suitable for the deep processing of heat sensitive food

the University of California, in cooperation with a company, used microwave vacuum drying seedless raisins, which maintained the original shape and color of grapes, avoided the shortcomings of the traditional process (65 ℃ hot air drying for 24 hours) in the past, and the product quality was greatly improved. The microwave vacuum dryer (2450MHz, 48kW) manufactured by France international microwave company has a microwave vacuum drying chamber with a diameter of 1.5m and a length of 12m. It processes instant orange powder. The product not only retains its original color and flavor, but also retains its vitamins much higher than spray drying

in recent years, there is a great demand for high-grade dehydrated vegetables at home and abroad, and the drying process of dehydrated vegetables is the key to determine the product quality. Although the quality of dehydrated vegetables prepared by vacuum freeze-drying is good, the vacuum freeze-drying equipment is expensive and the production cost is high; Since the 1980s, vacuum microwave hot air (45%~55%) has been used abroad to produce dehydrated vegetables, which is equivalent to the products produced by freeze-drying process in terms of quality, with less one-time investment and a significant decrease in the total cost

4. Conclusion

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