| Abstract vacuum drying in the food industry in a wide range of important applications, into the past few years, vacuum technology and microwave heating technology and other drying technology, there are some new types of vacuum drying device, made in China should speed up the research and development of vacuum microwave Drying Equipment new viewpoints.
0. Foreword
At present, food processing technology is an important development trend to maintain maximum nutrition and food flavor color, and drying process and equipment selection of nutritional food products, the Competition has a great influence. With a lot of dry food, "food" linked to the characteristics, it is different from the dry chemical products, the former need to consider food hygiene, nutrition, loss, color change, etc. smell of drying temperature and time, there are strict restrictions also different from the dry food drying of pharmaceutical products, because food is often low value-added products, and drugs in general is a high-value-added products, the former drying process must take into account the economy. At the level of "quality and economy", in recent years the food drying technology and equipment still made a lot of progress, including vacuum and other drying methods or heating technology, given the vacuum drying installation of new content and vitality.
1. The characteristics of vacuum drying food
Vacuum drying is based on a basic principle: Water vapor pressure and temperature closely related, in a vacuum, the lower the boiling point of water, that is, in the vacuum that is operated under low temperature operation, high temperature can be avoided in nutrients such as vitamin damage, while improving the drying rate. In addition, in the vacuum system, the unit volume of air in less than content in the atmosphere, and in this relatively hypoxic environment for food drying can reduce or even prevent the oxidation of fat food opportunities, or other oxide pigment Browning degenerate, and so on, Therefore, using vacuum drying to obtain better quality of food.
2. Traditional vacuum drying device
Vacuum drying in the food, pharmaceutical, chemical and other industries a wide range of applications, and there are also development and the introduction of a variety of vacuum drying equipment, and the structure of a variety of forms. Commonly used in the food industry in the form of major box-type, double cone vacuum dryers, vacuum dryer, and other belt. These traditional vacuum drying device used mainly hot air, steam or electric heating, use of heat conduction, convection, or radiation principle will be conveyed to the external heat from the internal materials.
2.1 box-type vacuum desiccator
Box-type vacuum drying is the oldest and also the most simple of a vacuum dryer, in a vacuum oven, how hollow block heating, heating plate inside the general-steam heating, electricity can be heated or other radiation heating. Material placed on the metal plate heating plate, heat through thermal conduction within reach materials, heating water evaporation. Box-type vacuum dryer in actual use remains widespread, apply to liquid, paste, powder and granular materials dry food.
2.2 bipyramidal vacuum desiccator
Bipyramidal vacuum dryer is symmetrical so that the conical containers Rotary jacket, the internal cone by the tilt of the material itself was stirring constantly, through rotary connector from the side of the shaft into steam or heating carrier, and use on the other side of a central axis of the belt filter to the exhaust pipe of exhaust. Bipyramidal vacuum dryer can reach a higher vacuum, the internal structure is simple, easy to clean up, and materials can all emit.
2.3 vacuum belt dryer
Vacuum belt dryer is a continuous band composed of stainless steel, strip bypass the heating and cooling roller drum, a multi-storey structure, a main dryer, and then into the enclosed vacuum chamber. Books and materials of a tile heating plate in the ensuing Belt Movement, as in a vacuum conditions, the materials were in the boiling heat-foam board, the finished product is porous; confined to the system-wide operation, health conditions are good, the actual operation of the vacuum between 100 ~ 10 Kpa, heating temperature of 150 ℃, their transport conditions (temperature and drying time) between the freeze-drying and spray-drying between product quality and freeze-drying very close, freeze-drying is the gap operation of vacuum belt dryer is continuous operation, particularly suitable for vulnerable thermosensitive and dry oxidation of food, liquid or paste can be used to absorb material, commonly used in the food drying orange juice, tomato juice, such as instant tea.
2.4 vacuum drum dryer
Vacuum drum dryer drum will be confined in a vacuum indoors, in a vacuum drum dryer, feed, unloading and scraper, and so must be controlled from the outdoor drying, drying high cost, it can only be used in very dry food thermosensitive If juice, yeast, infant food.
3. Vacuum Drying progress of the new devices
Traditional vacuum drying device used most of heat conduction, convection or thermal radiation heating mode, slow and inadequate heating uniformity, in recent years, researchers vacuum technology and microwave heating technology, and other drying technology, there are some new vacuum Drying Equipment types.
3.1 vacuum freeze-drying
There are three water aggregation, that is, liquid, vapor and solid state. With the pressure to decrease steadily, freezing little changed, and more and more low boiling point side, getting closer and closer to the freezing point. When the pressure drops to a certain value, and that the boiling point of combining the freezing point, without solid ice can be directly converted into liquid and vapor states. Triple Point water pressure 610.5 Pa, Triple Point 0.0098 ℃ temperature, pressure, pressure points lower than the three-phase, solid ice can absorb heat directly into the water-vapor steam, freeze-drying on the principle of this. Vacuum freeze-drying, the product on the vacuum chamber, in between two heated plates, using low-temperature stimulate far-infrared heating to ensure uniform drying, heating the temperature drying process is based on the precise control of the heating curve. Vacuum the choice must be able to ensure that the entire drying process to have all the water in the form of ice melting and insoluble, for the majority of vegetables and meat, the choice of 0.5 to 1 Pa, corresponding to about 25 ℃ the sublimation temperature is appropriate.
The vacuum freeze-drying food is at a very low temperature dehydration, the nutritional content of food flavor material losses small, can maximize retain the original flavor and nutrition, rehabilitation Waterborne excellent, in a matter of seconds to a few minutes to complete recovery , and its color, taste, shape and the same basic fresh goods. Many vacuum freeze-drying products: coffee, instant tea, juices, herbs, vegetables are onions, garlic, ginger and grinding-ru, meat, such as Xianbei. Vacuum freeze-drying one-time investment, deterred many food manufacturers, such as Denmark's ATLAS RAY50 type of equipment freeze-dried (lyophilized area of 45 m2) Price reached 1.05 million US dollars, domestic Donganji has just begun, and the quality of foreign products still a big gap between the addition of vacuum freeze-drying production costs are also higher, because of the need to maintain high vacuum and (-25 ℃), low temperature, long drying time, higher energy consumption, these factors make freeze-drying in the food industry the use of the great resistance.
3.2 Continuous Spray Dryer also known as vacuum (Filtermat spray dryer)
Spray Dryer also known as Continuous vacuum Filtermat spray dryers, vacuum belt is equivalent to dryer and spray dryer combination. Niro Hudson Corporation developed the kind of dryer successfully resolved the sticky foods, such as sugar, fatty acid content of high or high dry food materials, the viscous materials using traditional spray Nien will happen fog dryer wall phenomenon, dry difficulties. Dry material through the process of vertical downward pressure nozzle spray drying room Fenxiang, hot air is sprayed down, the semi-dry powder materials gathered in the mobile network band, exhaust gas discharged from fans, good drying materials in the mobile network into further cooling to collect. As of the middle of the spray tower vacuum, air temperature only about 100 ℃, and general spray drying air temperature of 150 ℃, thus less thermosensitive material losses, while reducing the height of the spray tower.
3.4 microwave vacuum drying
Microwave wavelength is 1.0 ~ 0.001 m, and the frequency of 300 MHz to 300,000, with penetrating electromagnetic wave radiation, microwave drying principle is: microwave generator to microwave radiation drying materials, while the microwave into the internal materials, thoroughly polarity of the water molecules, such as the frequency of the microwave for synchronous rotation, such as dry vegetable products using 915 MHz microwave, the vegetables, such as polar water molecules rotate 915 million times per second, water and other polar molecules such as high-speed rotation The results create friction materials instantaneous heat, result in the material surface and the internal temperature at the same time, a large number of water molecules escape from the material to the effect of dry materials. Traditional methods such as steam heating, hot air and electric heating is the use of heat conduction, convection, radiation heat from the principle of external-internal materials, Yaobiaojili need a certain amount of time, materials, heat conduction properties of the time required for the worse the more long, slow heating and uneven heating energy consumption and higher. Microwave heating is the object itself that is heated in a fever, so called internal heating method, and microwave food from all directions through the food inside and outside at the same time heating, it does not require heat transfer media, nor the use of convection increases in temperature inside and outside food, heating fast and uniform, just the traditional method of heating a fraction of the 10% or more, and better able to retain vitamins in foods and food color the original flavor. Experiments show that there are, dried vegetables chlorophyll, vitamins and other nutrients only 3%, can be dried to retain 17 per cent, hot air drying quickly to retain 40 per cent, microwave drying may retain 60% to 90%, can be vacuum freeze-drying retain 97% (2).
Microwave vacuum drying and microwave technology is organically integrated vacuum technology, and give full play fast and uniform microwave heating, vacuum conditions at low water vaporization characteristics, 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 technology is suitable for the deep thermosensitive food processing.
And the University of California, a company co-operate by using microwave vacuum drying seedless raisins, grapes maintain the original shape and color to avoid the traditional process (65 ℃ hot air drying 24 hours) Product color, shape, taste and nutrition changes defects, product quality has been greatly increased. France International Microwave Microwave Vacuum Dryer Manufacturing Company (2450 MHz, 48Kw) microwave vacuum drying chamber diameter of 1.5 m, 12 m length, processing instant orange powder, the product will not only maintain the original claim, the reservation is much higher than vitamin spray Fog dry.
In recent years both at home and abroad for luxury dehydrated vegetables in great demand, and dehydrated vegetables production, the drying process is to determine the quality of key products, vacuum freeze-drying of dehydrated vegetables Although good quality, but the vacuum freeze-drying equipment is expensive, the cost of production high; abroad since the 1980s has been using vacuum microwave - hot air (45% ~ 55%) production of dehydrated vegetables, and the quality of freeze-drying technology, the product is, and one-time small investment, the total cost more |
