A look at Microwave Processing
The first thing which comes to our mind while on the topic of microwave processing is what microwaves are? Microwaves are basically electromagnetic waves that have a frequency between 300 MHz and 300 GHz. These two frequencies correspond to wavelengths of 1 m and 1 mm, respectively. All domestic microwave ovens and laboratory microwave processors operate at 2.45 GHz (corresponding to a wavelength of 12.2 cm, or just over 4-3/4"). Microwave energy is not suited for every application. But, in those applications where it fits, it can provide many process benefits. Let us next look at the microwave process.
The most common way to heat a product is from the outside in - heat penetrates, molecule by molecule, into the interior of the product. As a rule, when a product's moisture content is high (50 to 95%), conventional heating methods are the most economical technology. But, heating products with low moisture content (below 20%) is difficult. 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 drying, where the residual moisture can have damaging results.
Microwaves pass through a product like light passes through glass. Materials without a dipolar electrical charge (some plastics, for example) do not react with 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 of the microwaves' electromagnetic field reverses as much as a couple of billions times per second, the dipolar molecules continually change alignment, producing energy that is converted into heat. The amount of energy converted is dependent on the electric field strength, the microwave frequency and the dielectric properties of the food. In effect, the water molecules heat themselves until the product becomes so hot that the water present in the product evaporates. Combination curing with microwave is better than any other
heating methods. Microwaves can remove the last few percent of moisture from a product's interior and this process can be completed rapidly 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, a combination of microwave and conventional heating methods often is the best. Conductive or convective processing steps can be used before, during and after the microwave stage. The choice depends on product and the process. If microwaves are used first, the result is more effective for 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 convective airflow - producing more effective drying than convection-only systems.
The heating of materials by microwave is affected by a number of properties of the equipment and the material being heated. The important factors influencing the heating patterns are:
- Microwave frequency
- Microwave power and speed of heating
- Mass of the material
- Moisture content of the product
- Product density
- Product temperature