Vacuum dry heating
- How does it work?
- What can it be used for?
- What can it not be used for?
- Related Facilities
- Further Information
|Key words||vacuum drying, heat, absorbed water, microwave vacuum dehydration|
How does it work?
|Primary objective||Fast and effective drying of products|
|Working principle|| The use of vacuum combined with heat can be an effective method for drying. Lower moisture content can be achieved when using a combination of vacuum and heat compared to heat alone (particularly with products that are porous or have a very high surface area). The use of vacuum drying has shown further improvement in quality of food products. It reduces thermal stress and sustains better colour and texture of dried products compared to those that were air-dried [1, 13].
A vacuum drying system consists of the vacuum chamber and source of the heat. Both oil-sealed and oil-free (dry) mechanical vacuum pumps can be used. A condensate trap (typically cooled to very low temperatures) is also used to pump and trap the liquid(s) to be removed. Base pressures of less than 0.1 Torr (1,333 × 10-4 bar) are often required, depending on the liquid to be removed and the temperatures which we want to achieve. Care should be taken to protect oil-sealed pumps from the liquid and operating the pumps properly to minimize the impact of the liquid vapours being ingested into the pump (often with the use of a gas ballast and/or gas purge) [1, 13].
The vacuum drying process often involves multiple steps of applying heat and vacuum. Reducing the pressure (applying vacuum) at the surface of a liquid (such as water) will allow the liquid to evaporate without elevating the temperature. The evaporation of the liquid will continue until the product is dried or until enough heat is removed (through evaporation) that the remaining liquid freezes. At this point the drying process continues as sublimation which is much slower compared to evaporation. In general, this is not desirable. The purpose of heat is to maximize drying while preventing freezing of the liquid. There are several methods of applying heat to the product being dried. Some products have temperature limitations which may impact the methods that may be employed. These methods include:
Microwave drying is more convenience in compared to conventional drying methods and it offers many kinds of advantages [2, 3, 8].
|Additional effects|| It can prevent oxidation of sensitive products that cannot be dried in
the presence of air. It can also recover the solvent that is removed from the product.
|Important process parameters||Pressure, time, temperature, electrical power|
|Important product parameters||dielectric properties (for microwave heating), water contents , size and number of pores (texture), thermophysical properties|
What can it be used for?
|Products||Solid and liquid food|
|Solutions for short comings||This technology is more efficient and faster with better quality of the final product.|
What can it NOT be used for?
|Products||food with high content of fat|
|Other limitations|| Microwave drying - tThe penetration of microwaves is limited (problem with large diameter products) .
A high temperature can not be used for temperature sensitive products.
|Risks or hazards|
|Maturity||This technology is used in industry.|
|Modularity /Implementation||This technology can be included in production line (batch system).|
|Consumer aspects||Some consumers are convinced about harmfulness of microwave heating.|
|Environmental aspects||Smaller energy consumption|
Facilities that might be interesting for you
|Institutes||Wageningen UR - FBR, University College Dublin, Indian Agricultural Research Institute, Washington State University|
|Companies||Püschner, Weiss-Gallenkamp, LACO Technologies|
|References||  LacoTechnologies, Vacuum drying, Technologies application note, Note#05-07
 Püschner P. and Hoon L. L. S. (2007): MICROWAVE VACUUM DRYING OF FRUITS & VEGETABLES, The 5th Asia-Pacific Drying Conference (ADC07), August 13-15, 2007, Hong Kong
 CLARY C.D., MEJIA-MEZA E., WANG S. AND PETRUCCI V. E. (2007): Improving Grape Quality Using Microwave Vacuum Drying Associated with Temperature Control, JOURNAL OF FOOD SCIENCE, 72 (1): 23 – 28
 Artnaseaw Apichart, Theerakulpisut Somnuk, Benjapiyaporn Chatchai (2010): Drying characteristics of Shiitake mushroom and Jinda chili during vacuum heat pump drying, FOOD AND BIOPRODUCTS PROCESSING, 88 (C2-3): 105-114
 Cui Zheng-Wei, Sun Li-Juan, Chen Wei et al.(2007): Preparation of dry honey by microwave-vacuum drying , JOURNAL OF FOOD ENGINEERING, 84 (4): 582-590
 Litvin S., Mannheim CH., Miltz J. (1998): Dehydration of carrots by a combination of freeze drying, microwave heating and air or vacuum drying , JOURNAL OF FOOD ENGINEERING, 36 (1): 103-111
 Lombrana J. I., Rodriguez R., Ruiz U.(2010): Microwave-drying of sliced mushroom. Analysis of temperature control and pressure, INNOVATIVE FOOD SCIENCE & EMERGING TECHNOLOGIES, 11(4): 652-660
 Wojdylo A, Figiel A. Oszmianski J.(2009): Effect of Drying Methods with the Application of Vacuum Microwaves on the Bioactive Compounds, Color, and Antioxidant Activity of Strawberry Fruits , JOURNAL OF AGRICULTURAL AND FOOD CHEMISTRY, 57 (4): 1337-1343
 Yongsawatdigul J., Gunasekaran S. (1996): Microwave-vacuum drying of cranberries .1. Energy use and efficiency, JOURNAL OF FOOD PROCESSING AND PRESERVATION, 20 (2): 121-143
 Drouzas A.E., Tsami E., Saravacos G.D. (1999): Microwave/vacuum drying of model fruit gels, Journal of Food Engineering, 39: 117-122
 Drouzas A.E. and Schubert H. (1996) Microwave application in vacuum drying of fruits, Journal of Food Engineering , 28: 203-209.
 Zhanga M., Tangb J., Mujumdarc A.S. and Wang S. (2006): Trends in microwaverelated drying of fruits and vegetables- review, Trends in Food Science & Technology 17: 524-534
Pressure, time, temperature, electrical power dielectric properties (for microwave heating), water contents , size and number of pores (texture), thermophysical properties Dryers 2.2.2 physical other other Web of science: vacuum dry heat - 200 articles were found (FOOD SCIENCE TECHNOLOGY), , also sources from internet (Google) were used. WikiSysop :Template:Review document :Template:Review status