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Microwave vacuum(freezing)drying

time:2008-08-18 01:32:48  View:        

Vacuum freezing drying(FD dehydration)originated in 1909 that was used in the researches of biologic materials and tissues. Microwave vacuum freezing sublimation drying (MD dehydration) stems from World War Two. During that time a lot of fresh blood products were needed while many tough problems werent able to be overcomed, in addition, production of antibiotics needs freeze-drying technology. In order to meet the need of the war, scientists proposed a new research project called "plasma invariance freezing vacuum microwave sublimation dehydration research project", "plasma project" in short. Freeze the plasma phlebotomized newly to ice cube, then, on the condition of vacuum, apply microwave sublimation drying and dehydration, it  turns out that only dried plasma powder is left(about 1%water contained), Aluminum-foil multiplex bags are used to seal it and keep it avoiding the light, the quality-guarantee period can reach up to two years. If it is transported to the front line and mix with a little quantity of sterile distilled water so as to dilute it, herein, it can be injected the same as fresh plasma. This project originated the utility of microwave sublimation dehydration drying.

Since 1987,our country began to research microwave vacuum drying and microwave vacuum freezing and drying technology. In 1993,big-size microwave vacuum freezing equipments have been developed to freeze and dry vegetables. During recent years, the technology is innovated continuously that overcome the difficult point of arc discharge under low vacuum condition .Now, the practical microwave vacuum freezing and drying equipments have entered into the market and been produced in batches. Microwave vacuum(freezing)drying ways have many superiorities than other drying ways:

1.keep the original color, smell ,taste and nutritious components well; Because the freeze-drying foods are produced under the condition of low temperature and vacuum(lack of oxygen),enzymes and bacteria will not appear, the food never transform and oxidate, the nutritious components lose less, especially the vitamin which is easy to decompose at high temperature, when it is unfreezed, less water-solubility nutritious components will lose along with water. In accordance with the research result done by the food container institute: freeze-drying method doesnt harm the protein in the meat, egg, bean, greengrocery, sweet corn,only 5% of Vc、β carrot element and other water-solubility vitamins will lose, it never lose any fat-solubility vitamin (Va,D,E,K).Microwave sublimation dehydration drying way can keeps the original nutritious components. For example,only 3% of the vitamin and chlorophyll can be kept in insolation way,17% by drying in the shade,40% in hot-wind drying way, above 70% by FD vacuum freezing drying way;97% with

microwave. Therefore,it is called invariance dehydration.

2. fast drying speed, no matter the shape and thickness: Microwave penetrability heating way and selective heating way can dry the materials fast and homogeneously. When process 510cm thick materials,the drying time can be saved 30% than before; if the thickness is increased to be 2030cm,the time can be saved up to 60%.For example, 1821 hours is needed to freeze and dry shallots in general heat transfer way while only 89 hours is needed with microwave.

3. can keep the original shape, taste and flavor well, good rehydration.

4. save energy and reduce the consumption: high electrothermal efficiency, it can be up to 94% accordance to the theory.

5.good quality: When adopt General heat transfer to sublimate and dehydrate the materials, sometimes heat the water by sealing it and increasing the pressure, and make it circulate in the heating boards,so that transmit the heat energy to the trays and the materials, the heat-sensitive materials will change the features, color, taste, even be burnt, even the imported equipments ,the eligibility rate is no more than 90%, the domestic equipments is only about 60%.But the unique heating way of microwave makes the temperature of materials inside and outside rise simultaneously, after being dehydrated, the qualities of products are the same up and down, the eligibility rate is up to 100%,the preserving time can be up to 3 to 5 years, to the utmost, it can be preserved for ten years. However, the general method only can keep it only for several months, no more than one year.

6.low drying temperature, it is fit for the high grade and heat-sensitive products: At present, It is widely used on the temperature-sensitive drugsdrinkjuicematerials rich in vitamin. Microwave vacuum drying equipment for instant orange juice powder, with vacuum chamber of 1.5 m diameter  , 12m long, 48KW frequency, use polyteliafluoroethylene conveying belt to transmit the materials continuously,that has expansion effect. At first paste the original orange juice on the conveying belt and make it be 37mm thick, under the 10.67kPa13.38kPa(80100mmHg) pressure,input the microwave energy and heat it for 40min, it can be expanded to be  80100mm thick,and turns to be instant orange powder containing 20% water,the process capability is 50kg/h.The products not only keep the original color,smell and taste,but also the vitamin amount kept is far more than the amount produced by spray drying method.


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