The most common way to heat a product is from the outside in -- heat penetrates, molecule by molecule, into the interior of the product. While heating a product in this manner may take some time -- and large equipment -- it has certain advantages. That is why as a rule, when a product's moisture content is high (50 to 95%), conventional heating methods such as convection and infrared often are the most economical technology. But, heating products with low moisture content (below 20%) is another matter.
Microwave energy has the unique ability to heat certain molecules while not heating others. Because of this unique characteristic, microwave energy often is employed to draw out the last few percent of moisture in a range of products. This ability can be especially helpful in applications such as dry chemical processing, where any moisture not removed can have damaging results.
Microwaves pass through a product like light through glass. Materials without a dipolar electrical charge (some plastics, for example) do not react to the waves. However, asymmetrically charged materials -- especially water, which is by far the most common material with a dipolar charge -- react to the microwaves by trying to align themselves to the electromagnetic charge. Because the microwaves' electromagnetic field reverses as much as a couple billion times per second, the dipolar molecules continually change alignment, producing energy that is converted into heat. In effect, the water molecules heat themselves until the product becomes so hot that the water evaporates.
Combination Curing With MicrowaveFar better than other heating methods, microwaves can remove the last few percent of moisture from a product's interior, and this process can be completed quickly without overheating the already dried material. In fact, drying materials to levels as low as 3% moisture content within a relatively short time is possible with microwaves.
In many applications where microwave technology is considered, though, a combination of micro-wave and conventional heating methods often is best. Conductive or convective processing steps can be used before, during and after the microwave stage. The choice depends on the product and the process. If microwaves are used first, the result is more effective initial heating. If conventional and microwave heating are applied at the same time, there is a synergistic effect: Microwave heat pumps the product's moisture to the surface, where it is evaporated by the convective airflow -- producing more effective drying than convection-only systems.
For many products, a microwave zone near the end of the process, with or without hot air convection, produces the best results. With conduction only, once the product's moisture content drops below 25%, there is a danger of overheating the surface of the product, causing a "skin" to form. To prevent this, the product must be allowed to rest, or equilibrate, while the water vapor works itself out of the product. If continuous airflow were used, the product's exterior could overdry.
Combining the two methods of heating and drying (convection and microwave) avoids the skinning effect while providing quick processing. In the first stage, a high percentage of the moisture content is quickly removed using convection heat. Because the moisture content is high, equilibration periods are not required, yet the product's exterior does not "skin over." Once the moisture content level drops below 25%, the product can convey into a microwave zone, where the moisture that remains, deep in the product's interior, is quickly driven to the product's surface. This synergistic combination of heating methods can cut overall heating time by up to 80%.
5 Ways to Use MicrowaveWhether used in combination with conventional heating or alone, microwaves have a place in industrial processing. Here's five potential applications:
Baking. Microwave's ability to speed internal heating can accelerate the baking process. Microwave heating can be combined with convection or infrared heating to create a crispy crust and pleasing appearance in less time.
Curing. Because it leads to rapid heating throughout the product, microwaves may be used for bulk polymerization reactions that are heat initiated.
Blanching. When combined with a humid or steam atmosphere, rapid uniform heating with microwaves can be effective for blanching fruits and vegetables. Microwave processing does not incur the leaching losses associated with hot water or steam blanching.
Removing Solvent. Many solvents are efficiently vaporized with microwaves, permitting solvent removal at relatively low temperatures. Microwaves also can be used to cure glue and other adhesives.
Moisture Leveling. Because their heating effect is roughly proportional to the moisture content, microwaves can be used to equalize the moisture content within a product that has a nonuniform moisture level.