Food vacuum drying equipment and its development

Vacuum drying has been widely used in food industry. In recent years, some new types of vacuum drying equipment have appeared due to the combination of vacuum technology, microwave heating technology and other drying technologies. It is proposed that China should speed up the development of vacuum microwave drying equipment.

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At present, an important development trend of food processing technology is to keep the nutrition and color and flavor of food to a great 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 associated with "food". It is different from the drying of chemical products. The former needs to consider food hygiene, nutrition loss, color and flavor changes, and has strict restrictions on drying temperature and time. Food drying is also different from the drying of pharmaceutical products, because food is often low value-added products, while drugs are generally high-value products, the former must The economy of drying process is considered. Around the "quality and economy", in recent years, food drying technology and equipment have made a lot of progress. The combination of vacuum and other drying methods or heating technology endows the vacuum drying device with new connotation and vitality.


The characteristics of food vacuum drying are based on the basic principle: the saturated vapor pressure of water is closely related to temperature. In vacuum, the boiling point of water is reduced, that is to say, the operation under vacuum is operated at low temperature, which can avoid the damage of nutrients, vitamins, etc. at high temperature, and improve the drying speed. In addition, in the vacuum system, the air content per unit volume is lower than that in the atmosphere. In this relatively anoxic environment, food drying can reduce or even avoid the oxidation of fat in food, browning of pigment or other oxidative deterioration, so the vacuum drying can obtain better food quality.

2. The traditional vacuum drying device is widely used in food, pharmaceutical, chemical and other industries. Various vacuum drying equipment has been developed and introduced in China, and its structure is various. In the food industry, the commonly used forms are box type, double cone type vacuum dryer, belt type vacuum dryer and so on. These traditional vacuum drying devices are mainly heated by hot air, steam or electricity. The heat is transferred from the outside to the inside of materials by using the principles of heat conduction, convection or radiation.

2.1 box type vacuum dryer box type vacuum drying is a kind of simple vacuum dryer with a long history. 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 electricity or other radiation. The material is placed in a metal plate on a heating plate, and the heat reaches the inside of the material through heat conduction, so that the water is heated and evaporated. Box vacuum dryer is still widely used in practice, which is suitable for drying liquid, paste, powder and bulk food materials.

2.2 double cone vacuum dryer the double cone vacuum dryer is to make the conical container with symmetrical jacket rotate. The material is continuously stirred by the inclination of the inner cone itself. The steam or heating carrier is sent from one side of the rotary shaft through the rotary joint, and the exhaust pipe with filter screen is set at the center of the other end of the shaft. The double cone vacuum dryer can achieve high vacuum degree, simple internal structure, 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, which bypasses the heating drum and cooling drum. The structure is multi-layer, forming the main body of the dryer, and then it is brought into the closed vacuum chamber. The material book is paved on the belt heating plate and then moves. Because the material is bubbling on the heating plate under vacuum condition, the finished product has porosity. The whole system is closed operation, sanitary condition is good, the actual operating vacuum degree is 100~10Kpa, the heating temperature is about 150 degrees Celsius, and its transportation condition (drying temperature and time) is between freezing drying and spray drying. The product quality is close to freeze-drying, but freeze-drying is intermittent operation, while vacuum belt dryer is continuous operation. It is especially suitable for drying heat sensitive and easily oxidized food. Liquid or slurry materials can be used uniformly. The 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, feeding, unloading and scraping must be controlled from the outside of the drying room. Therefore, the drying cost is very high, so it can only be used for drying very heat sensitive food, such as juice, yeast, baby food, etc.

The new development of vacuum drying device the traditional vacuum drying device is 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 equipment have emerged.

3.1 there are three aggregation states of vacuum freeze-drying water, namely liquid state, solid state and vapor state. With the decrease 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 the vapor state without passing through the liquid state. The three-phase point pressure of water is 610.5pa, and the temperature of three-phase point is 0.0098 ℃. When the pressure is lower than the pressure of three-phase point, the solid ice can absorb heat and directly transform into vapor water vapor. This is the principle of freeze-drying. During vacuum freeze-drying, the product is placed in the vacuum chamber between two heating plates. The far-infrared heating is used to stimulate the low temperature to ensure the drying uniformity. The temperature of the heating plate is controlled according to the heating curve of the drying process. The vacuum degree should be selected to ensure that all water in the drying process is in the form of ice, and for most vegetables and meats, 0.5 ~ 1pA is suitable for sublimation temperature of about 25 ℃. Because the vacuum freeze-drying food is dehydrated at a very low temperature, the loss of nutrients and flavor substances of food is less, so the original nutrition and flavor can be largely retained. The rehydration property is excellent, and the recovery can be completed in seconds to minutes. The color, taste and shape of vacuum freeze-dried food are exactly the same as those of fresh food. There are many vacuum freeze-drying products: coffee, instant tea, juice, herbal medicine, vegetables such as onion, garlic, ginger, ground Ru, meat, fresh shellfish, etc. The one-time investment of vacuum freeze-drying is very large, which makes many food manufacturers flinch. For example, the ray50 freeze-drying equipment of atlas in Denmark (the freeze-drying area is 45m2)

The quotation is as high as 1.05 million US dollars. The domestic freeze-drying machine has 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. This is due to the need to maintain a high vacuum and (- 25 ℃) low temperature, long drying time and high energy consumption. These factors bring great resistance to the use of freeze-drying in the food industry.

3.2 spray continuous vacuum dryer is also called (Filtermat spray dryer).

The spray continuous vacuum dryer is also called Filtermat spray dryer, which is equivalent to the combination of belt vacuum dryer and spray dryer. Niro

The dryer developed by Hudson has successfully solved the drying problem of sticky foods such as high sugar content, high fat content or high acid content food materials. The sticky material will be sticky and dry with traditional spray dryer. 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 net belt. Due to the maintenance of medium vacuum in the spray tower, the hot air temperature is only about 100 degrees Celsius, while the general spray drying hot air temperature is about 150 degrees Celsius. Therefore, the loss of heat sensitive material is low, and the height of the spray tower is reduced.
3.4 microwave vacuum drying microwave is a penetrating electromagnetic radiation wave with wavelength of 1.0-0.001m and frequency of 300-30000mhz. The principle of microwave drying is: microwave generator radiates microwave to dry materials. When microwave is injected into the material, polar molecules such as water will rotate synchronously with the frequency of microwave. For example, if 915MHz microwave is used to dry vegetables, the electrode in vegetables will be changed As a result of such high-speed rotation of polar molecules such as water, the friction heat of materials will be generated instantaneously, and the surface and interior of materials will be heated at the same time, so that a large number of water molecules will escape from the materials to achieve the drying effect of materials. Traditional 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 materials. It takes a certain time from the surface to the inside. The worse the thermal conductivity of materials is, the longer the time is required. Therefore, the heating speed is slow and 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. Microwave passes through the food from all directions, and the food is heated inside and outside at the same time. It does not need heat transfer medium or convection. The temperature inside and outside of the food rises at the same time. The heating speed is fast and uniform. It only needs a fraction or a few of the traditional heating method, and can keep food in good condition The original color and flavor of vegetable and food. Some experiments showed that only 3% of the nutrients such as chlorophyll and vitamins were left in the sun dried fresh vegetables, 17% in the shade drying, 40% in the hot air drying, 60% ~ 90% in the microwave drying, and 97% in the vacuum freeze-drying. Microwave vacuum drying is an organic combination of microwave technology and vacuum technology, which makes full use of the characteristics of fast and uniform microwave heating and low vaporization point of water under vacuum conditions. It is a promising drying technology. In recent years, microwave vacuum drying technology has been promoted to industrial production from laboratory in France, Japan and the United States, which is suitable for the deep processing of heat sensitive food.

The University of California cooperated with a company to use microwave vacuum drying seedless raisin, which kept the original shape and color of grape, avoided the shortcomings of traditional technology (drying in hot air at 65 ℃ for 24 hours) in color, shape, flavor and nutritional components, and greatly improved the quality of the product. The microwave vacuum drying room (2450MHz, 48Kw) manufactured by France international microwave Co., Ltd. has a diameter of 1.5m, a length of 12M, and processing of instant orange powder. The product not only keeps its original color and fragrance, but its vitamin retention is much higher than that of spray drying. In recent years, there is a great demand for high-grade dehydrated vegetables at home and abroad, and the drying process is the key to determine the quality of dehydrated vegetables. 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, foreign countries have used vacuum microwave hot air (45% ~ 55%) to produce dehydrated vegetables The quality of the products produced by freeze-drying process is equivalent to that produced by freeze-drying process, but the one-time investment is less, and the total cost is greatly reduced.

Conclusion (1)

Vacuum drying has a series of advantages, such as low drying temperature, relatively hypoxia in drying room, avoiding fat oxidation and browning of pigment. It is suitable for drying heat sensitive food materials. In addition, the equipment cost and drying cost are relatively low. Vacuum drying plays an important role in food drying. （2）

Combining vacuum drying with microwave heating technology or other drying methods, many new vacuum drying devices appear, which endow vacuum drying with new connotation and vitality. （3）

Vacuum microwave drying has absorbed the advantages of both microwave heating and vacuum drying. It is a promising drying technology. It is suggested that China should speed up the research and development of vacuum microwave drying equipment.