Vacuum drying has been widely used in food industry. In recent years, with the combination of vacuum technology, microwave heating technology and other drying technologies, some new types of vacuum drying devices have appeared.0.
Introduction at present, an important development trend of food processing technology is * big limit to keep the food nutrition and tasty, and the choice of drying technology and equipment has a great influence on the food product's nutrition, color, flavor.Food drying has many characteristics related to "food". It is different from the drying of chemical products. The former needs to consider food hygiene, nutritional loss, color and flavor changes, etc.Food drying is different from the drying of medical products, because food is often a low added value product, and drugs are generally high added value products, the former must consider the economy of the drying process.Around the "quality and economy", in recent years, food drying technology and equipment has made a lot of progress, and the vacuum and other drying methods or heating technology combined, gave the vacuum drying device new connotation and vitality.1.
The characteristics of vacuum drying of food: Vacuum drying is based on the fundamental principle that the saturated vapor pressure of water is closely related to temperature. Under vacuum conditions, the boiling point of water decreases. That is, operating under vacuum, that is, operating at low temperatures, can avoid the destruction of nutrients such as vitamins at high temperatures, and at the same time increase the drying speed. In addition, in a vacuum system, the content of air per unit volume is lower than that in the atmosphere. Drying food in this relatively oxygen-deficient environment can reduce or even avoid the oxidation of fat, Browning of pigments or other oxidative deterioration in food. Therefore, vacuum drying can achieve better food quality.
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Traditional vacuum drying equipment is widely used in the food, pharmaceutical, chemical and other industries. Various vacuum drying devices have also been developed and introduced in China, and their structural forms are diverse. The commonly used forms in the food industry mainly include box-type, double-cone vacuum dryers, belt vacuum dryers, etc. These traditional vacuum drying devices mainly use hot air, steam or electricity for heating, and transfer heat from the outside to the inside of the material by the principles of heat conduction, convection or radiation.
2.1 Box-type vacuum dryer Box-type vacuum drying is the oldest and simplest type of vacuum dryer. Inside the vacuum drying chamber, there are multiple hollow heating plates. Generally, steam is passed through the heating plates for heating, but electric heating or other radiation heating can also be used. The materials are placed in a metal tray on the heating plate. Heat is conducted to the interior of the materials, causing the moisture to evaporate. Box-type vacuum dryers are still widely used in practice at present and are suitable for the drying of liquid, slurry, powder and bulk food materials.
2.2 Double-cone Vacuum Dryer: The double-cone vacuum dryer rotates a conical container with a symmetrical jacket. The material is continuously stirred by the inherent inclination of the internal cone. Steam or heating carriers are fed into one side of the rotating shaft through a rotary joint, and the material is discharged through an exhaust pipe with a filter screen at the center of the other end of the shaft. The double-cone vacuum dryer can achieve a high vacuum degree, has a simple internal structure, is easy to clean, and all materials can be discharged.
2.3 Vacuum Belt Dryer The vacuum belt dryer is composed of a continuous stainless steel belt, which is wound around the heating drum and the cooling drum. The structure is multi-layered, forming the main body of the dryer, and then it is placed in a closed vacuum chamber. The materials are spread thinly on the belt heating plate and move along with it. Due to the vacuum conditions, the materials are in a boiling state and foaming on the heating plate, so the finished product has porosity. The entire system operates in a closed manner with good hygiene conditions. The actual operating vacuum degree is between 100 and 10Kpa, and the heating temperature is around 150℃. Its operating conditions (drying temperature and time) lie between freeze-drying and spray drying. The quality of the finished product is very close to that of freeze-drying, but freeze-drying is intermittent operation while the vacuum belt dryer operates continuously. It is particularly suitable for drying heat-sensitive and highly oxidized foods. It can be used for liquid or paste materials. It is commonly used in food to dry orange juice, tomato juice, instant tea, etc.
2.4 Vacuum Drum Dryer The vacuum drum dryer seals the drum in a vacuum chamber. In the vacuum drum dryer, feeding, discharging and scraping must all be controlled from outside the drying chamber, so the drying cost is very high. Therefore, it can only be used for drying very heat-sensitive foods, such as fruit juice, yeast, baby food, etc.
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New Progress in vacuum Drying equipment: Traditional vacuum drying equipment mostly uses heat conduction, convection or radiation for heating, which is slow and not uniform. In recent years, researchers have combined vacuum technology with microwave heating technology and other drying technologies, giving rise to some new types of vacuum drying equipment.
3.1 Vacuum freeze-dried water has three aggregated states, namely liquid, solid and vapor. As the pressure keeps decreasing, the freezing point changes little, while the boiling point side gets lower and lower, getting closer and closer to the freezing point. When the pressure drops to a certain value, the boiling point combines with the freezing point, and solid ice can directly transform into vapor without passing through the liquid state. The triple point pressure of water is 610.5Pa, and the triple point temperature is 0.0098℃. When the pressure is lower than the triple point pressure, solid ice can absorb heat and directly transform into gaseous water vapor. The principle of freeze-drying lies precisely here. During vacuum freeze-drying, the product is placed in a vacuum chamber between two heating plates. Low-temperature far-infrared heating is utilized to ensure uniform drying. The temperature of the heating plates is precisely controlled according to the heating curve of the drying process. The selection of vacuum degree should ensure that all moisture remains in the form of ice throughout the drying process and does not dissolve. For most vegetables and meats, a vacuum degree of 0.5 to 1Pa, corresponding to a sublimation temperature of around 25℃, is appropriate. Because vacuum freeze-dried food is dehydrated at a very low temperature, the loss of nutrients and flavor substances in the food is minimal, which can retain the original nutrition and flavor to the greatest extent. It has excellent rehydration properties and can be restored within seconds to minutes. Its color, taste and shape are basically the same as those of fresh products. There are many vacuum freeze-dried products: coffee, instant tea, fruit juice, herbs, etc. For vegetables, there are scallions, garlic, ginger, mushrooms, meat, scallops, etc. Vacuum freeze-drying requires a large one-time investment, which departs many food manufacturers. For instance, the RAY50 freeze-drying equipment from ATLAS in Denmark (with a freeze-drying area of 45 square meters)
The quoted price is as high as 1.05 million US dollars. Domestic freeze dryers are just getting started, and there is still a considerable gap in quality compared with foreign products. In addition, the production cost of vacuum freeze-drying is also relatively high. This is because it requires maintaining a high vacuum and a low temperature of (-25℃), with a long drying time and high energy consumption. These factors bring great resistance to the application of freeze-drying in the food industry.
3.2 The jet continuous vacuum dryer is also known as (Filtermat spray dryer)
The jet continuous vacuum dryer, also known as the Filtermat spray dryer, is essentially a combination of a belt vacuum dryer and a spray dryer. Niro
The dryer developed by Hudson Company has successfully solved the drying problem of sticky foods ----, such as food materials with high sugar content, high fat content or high acid content. Materials with high viscosity will stick to the wall when using traditional spray dryers, making drying difficult. During the drying process, the material is sprayed vertically downward into the spray drying chamber through pressure nozzles, and hot air is also sprayed downward. The semi-dry powder material accumulates on the moving mesh belt, and the exhaust gas is discharged by the fan. The dried material further moves, cools and is collected on the mesh belt. Due to the maintenance of a moderate vacuum degree inside the spray tower, the hot air temperature only needs to be around 100℃, while the general hot air temperature for spray drying is about 150℃. Therefore, the loss of heat-sensitive materials is less, and at the same time, the height inside the spray tower is reduced.
3.4 Microwave Vacuum Drying Microwave is an electromagnetic radiation wave with a wavelength of 1.0 to 0.001 meters and a frequency of 300 to 300,000 MHZ, which is penetrating. The principle of microwave drying is: The microwave generator radiates microwaves onto the drying materials. When the microwaves enter the interior of the materials, they cause polar molecules such as water to rotate synchronously with the frequency of the microwaves. For example, when drying vegetable products with 915MHz microwaves, the polar water molecules inside the vegetables rotate 915 million times per second. The result of such high-speed rotation of polar molecules like water is that the materials generate frictional heat instantaneously. This causes the surface and interior of the material to heat up simultaneously, allowing a large number of water molecules to escape from the material, achieving the effect of material drying. Traditional heating methods such as steam, hot air and electricity transfer heat from the outside to the inside of the material by the principles of heat conduction, convection and radiation. It takes a certain amount of time for the heat to spread from the outside to the inside. The poorer the heat conduction performance of the material, the longer the time required. Therefore, the heating speed is slow, the heating is uneven and the energy consumption is high. Microwave heating makes the object to be heated itself the heat source, thus it is called an internal heating method. Microwaves pass through the food from all directions, heating both the inside and outside of the food simultaneously. It neither requires a heat transfer medium nor utilizes convection. The temperature of both the inside and outside of the food rises at the same time. The heating speed is fast and uniform, requiring only a fraction or a few tens of the temperature of traditional heating methods. It can also better preserve the vitamins in food and the original color, aroma and taste of food. Experiments have shown that the chlorophyll, vitamins and other nutrients in sun-dried fresh vegetables remain at only 3%, while those in air-drying can retain 17%, those in rapid hot air drying can retain 40%, those in microwave drying can retain 60% to 90%, and those in vacuum freeze-drying can retain 97%. Microwave vacuum drying is an organic combination of microwave technology and vacuum technology, fully leveraging the characteristics of rapid and uniform microwave heating and low water vaporization point under vacuum conditions. It is a very promising drying technology. Microwave vacuum drying technology has been advanced 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 foods.
The University of California in the United States has collaborated with a certain company to use microwave vacuum drying for seedless raisins, which has maintained the original shape and color of the grapes and avoided the shortcomings of the traditional process (drying in 65℃ hot air for 24 hours), such as changes in product color, shape, flavor and nutritional components. The product quality has been greatly improved. The microwave vacuum dryer (2450MHz, 48Kw) manufactured by the French International Microwave Company has a diameter of 1.5 meters and a length of 12 meters in the microwave vacuum drying chamber. It is used to process instant orange powder. The product not only retains the original color, aroma and taste, but also has a much higher retention of vitamins than spray drying. In recent years, there has been a huge demand for high-end dehydrated vegetables both at home and abroad. The drying process is the key to determining the quality of dehydrated vegetables. Although vacuum freeze-drying is used to prepare dehydrated vegetables with good quality, the vacuum freeze-drying equipment is expensive and the production cost is high. Since the 1980s, vacuum microwave-hot air (45% to 55%) has been used abroad to produce dehydrated vegetables. In terms of quality, it is comparable to the products made by freeze-drying process, but with less one-time investment and a significant reduction in total cost.
4 Closing Remarks (1
Vacuum drying has a series of advantages such as low drying temperature, relatively anoxia in the drying room, avoiding fat oxidation and pigment Browning, which is suitable for the drying of heat-sensitive food materials. In addition, the equipment cost and drying cost are relatively low, and vacuum drying plays an important role in food drying. (2
Vacuum drying combined with microwave heating technology or other drying methods, there are many new vacuum drying devices, giving vacuum drying new connotation and vitality. (3
Vacuum microwave drying to absorb the advantages of both microwave and vacuum drying, the drying technology is a promising, suggested to speed up the development, the development of vacuum microwave drying device in our country.