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Food vacuum drying apparatus and Progress

Datetime:2015-2-17 14:11:05   Hits:1309

  Vacuum drying a wide range of important applications in the food industry, into the past few years, vacuum technology and microwave heating technology and other drying technology, there are some new vacuum drying device types, proposed that China should speed up the research and development of vacuum microwave drying new ideas means.

0.

  Preface Currently, an important trend in food processing technology is to maximize their nutrition and food color, flavor, and the choice of the drying process and equipment has a great influence on nutrition, color, flavor and food products. There are many dried food and "food" linked to the characteristics, it is different from the drying chemical products, the former need to consider food hygiene, nutrition loss, changes in color, flavor, etc., there are strict limits on the drying temperature and time; dried food is different from drying of pharmaceutical products, because food is often low value-added products, and drugs are generally high value-added products, the former must consider the economic drying process. Around the "quality and economy" in recent years, food drying technology and equipment still made a lot of progress, wherein the vacuum drying method or other heating technology, gives new meaning to vacuum drying device and vitality.

1.

  Food vacuum drying characteristics of vacuum drying is based on a fundamental principle: saturated water vapor pressure and temperature are closely related, in a vacuum state, the boiling point of water decreases, that is, that is, at a low temperature operating under vacuum operation, avoid high temperatures destroying the vitamins and other nutrients, while improving the drying rate. In addition, in the vacuum system, the unit volume of air content of less than atmospheric content, in this relatively hypoxic environment food drying can reduce or even avoid foods oxidized fat chance, browning pigment or other oxidative deterioration, etc., so vacuum dried to obtain a better quality of food.

2.

  Conventional vacuum drying apparatus vacuum drying in the food, pharmaceutical, chemical and other industries have a wide range of applications, the country will also develop and introduce a variety of vacuum drying equipment, the structure of a variety of forms. Commonly used in the food industry mainly in the form box, double cone vacuum dryer, vacuum belt dryer or the like. These conventional vacuum drying device mainly hot air, steam or electricity heating by thermal conduction, convection or radiation heat to the principles of the internal materials from the outside.

  2.1 box-type vacuum dryer box-type vacuum drying is the oldest and most simple of a vacuum dryer, a number of hollow heating plate in a vacuum oven, heating plate through which ships steam heating, can also be used electric heating or other radiant heating. Material in a metal tray placed on a hot plate, heat conduction through the heat reaches the inside of the material, so that the water is heated to evaporate. Box-type vacuum dryer is currently used in practice continues to be widespread, suitable for drying liquid, paste, powder and granular food materials.

  2.2 Cone Vacuum Dryer double cone vacuum dryer is to make symmetric conical jacketed vessel is rotated by the inclination of the cone itself inside the material constantly being stirred, fed by the side of the joint by the rotation of the rotary shaft steam or heat carrier, and set up with the aid of filters other end of the exhaust pipe shaft center. Double cone vacuum dryer to achieve a higher degree of vacuum, internal structure is simple, easy to clean, discharge all raw material.

  2.3 Vacuum belt dryer Vacuum belt dryer is a continuous stainless steel belt composed of steel bypass the heating drum and cooling drum, it was a multi-layered structure, constituting the main dryer, and then incorporated into a sealed vacuum chamber . Book Book tiled material in the tape along with a hot plate movement, since under vacuum, the material in the hot plate was boiling like foam, so the finished product has a porous; good operating system are all closed, sanitary conditions, the actual operation the degree of vacuum between 100 ~ 10Kpa, the heating temperature is about 150 ℃, its transport conditions (drying temperature and time) between freeze-drying and spray drying, freeze drying product quality and is very close, but the gap is freeze-dried and vacuum operation belt dryer is a continuous operation, particularly suitable for heat-sensitive and vulnerable to oxidation of dried food, liquid or slurry material can even be used in foods used this dried orange juice, tomato juice, instant tea and so on.

  2.4 vacuum drum dryers vacuum drum drier drum sealed in a vacuum chamber, vacuum drum dryer, the feed, discharge and scraper so must be controlled from the dried outdoors, high drying costs, it is only It can be used for very heat sensitive dry food, fruit juice, yeast, and other baby food.

3.

  Progress vacuum drying apparatus conventional vacuum drying device they use heat conduction, convection or thermal radiation heating mode, the heating is slow and not uniform, in recent years, researchers vacuum technology and microwave heating technology and other drying technology, there are some new types of vacuum drying device.

  3.1 vacuum freeze-drying water, there are three state of aggregation, namely liquid, solid and vapor. As the pressure is reduced, little change in the freezing point, while more and more low boiling point side, more and more close to freezing point. When the pressure drops below a certain value, that is the boiling point and freezing point combined with solid ice without passing through the liquid directly into the vapor state. Triple point of water pressure 610.5Pa, triple point temperature of 0.0098 ℃, at a pressure below the triple point pressure, the solid ice can absorb heat directly converted to steam vapor, frozen, dried principle in this. Vacuum freeze-drying, the product placed in a vacuum chamber, located between two heated plates, using a low temperature stimulate far-infrared heating, to ensure uniform drying, the temperature of the heating plate according to the drying process is precisely controlled heating curve. Select the degree of vacuum to be able to ensure that the entire drying process are all water-insoluble form of ice and melt for most vegetables and meat, the choice of 0.5 ~ 1Pa, corresponding to about 25 ℃ sublimation temperature is appropriate. Due to vacuum freeze-drying food is dehydrated at low temperatures, less loss of nutrients in food flavor, you can maximize the retention of the original nutrients and flavor, excellent rehydration, can be completed in a few seconds to recover within a few minutes, its color, taste, shape substantially identical with the fresh product. Vacuum freeze-drying a lot of products: coffee, instant tea, fruit juice, herbs, vegetables onions, garlic, ginger, grinding Ru, meat, shellfish and the like. Vacuum freeze-drying one-time investment is large, so that discouraged many food manufacturers, such as Denmark ATLAS RAY50 type of freeze-drying equipment (freeze-dried area of 45m2

) Offer up to $ 1.05 million, domestic lyophilizer just started, and there is a great quality gap with foreign products, in addition to vacuum freeze-drying production costs are higher because of the need to maintain a high vacuum (-25 ℃) low temperature, long drying time, high energy consumption, these factors make the freeze-dried in the food industry to use a great deal of resistance.

  3.2 jet, also known as continuous vacuum dryer (Filtermat spray dryer)

  Jet, also known as continuous vacuum dryer Filtermat spray dryer, the equivalent combination with a vacuum dryer and a spray dryer. Niro

  Hudson company developed the kind of dryer has successfully solved the sticky food ---- such as high sugar, high fat containing dried problem or high acid content of food materials, viscous material will use a conventional spray dryer sticky wall phenomenon, drying difficult. Drying material sprayed vertically downward pressure through a nozzle spray drying chamber, the hot air is sprayed down, semi-dry powder material gathered on a moving mesh belt, is also discharged from the exhaust fan, the dried material on the belt further movement, cooled and collected. Since the spray Ta Neiwei holding medium degree of vacuum, air temperature of only about 100 ℃, and general spray drying hot air temperature of about 150 ℃, and thus less loss of heat-sensitive materials, while reducing the height of the spray tower.

  3.4 microwave vacuum drying microwave wavelength is 1.0 ~ 0.001m, a frequency of 300 ~ 300000MHz, with a penetrating electromagnetic radiation, microwave drying principle is: a microwave generator microwave radiation to the dried material, when the microwave injection materials when inside, through the water and other polar molecules with the microwave frequency for synchronous rotation, such as dried vegetable products using microwave 915MHz, the polar water molecules and other vegetables inside rotating 915 million times per second, water and other polar molecules are the results of such high-speed rotation of the material instantaneously generated friction heat, causing the material surface and the internal temperature was raised, a large number of water molecules to escape from the material, to achieve the effect of drying. Traditional methods such as steam heating, hot air, electric heating, etc. are by heat conduction, convection, radiation heat from the principle of internal and external transfer material, from outside to inside take some time, the time of thermal conductivity of the material required for the worse the more long, slow and uneven heating heating, high energy consumption. Microwave heating is the heated object itself becomes body heat, so called internal heating method, microwave food from all directions through the food inside and outside at the same time heating, does not require a heat transfer medium, nor the use of convection, the food inside and outside temperature rises at the same time, fast heating and uniform, only a fraction of one or a few tenths of conventional heating methods, and can better retain the vitamins in food and food original color, flavor. Experiments show that the dried fresh vegetables whose nutrients chlorophyll, vitamins and other remaining 3%, 17% retained can be dried, hot air rapid drying may retain 40% of microwave drying may retain 60% to 90% vacuum freeze-drying can be {2} 97% retention. Microwave vacuum drying and vacuum microwave technology organic combination, give full play to the microwave heating fast and evenly, vacuum vaporization point lower water features, is a promising drying technology. Microwave vacuum drying technology in France, Japan, the United States in recent years has been the laboratory to industrial production, this technique is well suited for deep processing of heat-sensitive foods.

University of California, in collaboration with a company, the use of microwave vacuum dried seedless raisins, maintaining the original shape and color of the grapes, avoiding the traditional process in the past (65 ℃ hot air drying 24 hours) product color, shape, flavor and nutrition changes disadvantages, product quality greatly improved. French International Microwave Corp. microwave vacuum dryer (2450MHz, 48Kw) microwave vacuum drying chamber diameter 1.5m, length 12m, instant orange powder processing, not only to maintain the original color, flavor, vitamin retention is much higher than spray dry. In recent years, domestic and foreign demand for large high-grade dehydrated vegetables, dehydrated vegetable production, the drying process is the key determinant of the quality of products, dehydrated vegetables and dried preparation is good quality, but expensive vacuum freeze-drying equipment vacuum freeze production high costs; since the 1980s have been abroad vacuum microwave - hot air (45% to 55%) production of dehydrated vegetables, freeze-dried in quality and technology products considerably, and less one-time investment, the total cost of a relatively large decline.

4. Conclusion (1)

  Vacuum drying with a drying temperature is low, the drying chamber relative hypoxia, can prevent fat oxidation, browning pigment series of advantages, suitable for drying heat-sensitive food material, in addition to equipment costs, the drying cost is relatively low, and dried in vacuum food drying midpoint has an important position. (2)

  Vacuum drying and microwave drying technology or other methods combined, there have been many new vacuum drying device, vacuum drying gives new meaning and vitality. (3)

  Vacuum microwave drying absorption of microwave heating and drying of both the advantages of vacuum, is a promising technology for drying, it is recommended to speed up our research and development of vacuum microwave drying apparatus.

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