1. Microwave heating principle

Absorbing microwave energy is the interaction result that occurs between polar molecules in material and microwave electromagnetic field. Under the action of external alternating electromagnetic field， polar molecules appear polarization phenomenon and change their alternating orientation following with the change of magnetic polarity. So many polar molecules mutually rub and loss energy and they convert electromagnetic energy into heat energy.  

According to Debye theory , the time R which refers to polar molecules in the polarization process is related with polarity changing angular frequency W in external alternating electromagnetic field and it often appears the result of WR≈1 in microwave band.

Calculated by two common operating frequency 915 MHz and 2450 MHz which are often used in microwave heating equipment in China, R’s value is about 10-10s magnitude, so the process of transforming microwave energy into heat energy happened in material’s inside has real-time features.

Utilization of microwave energy

Microwave heating belongs to category of dielectric heating and different materials’ features have different absorption capacity to microwave. This selectivity of absorbing microwave energy provides favorable conditions for the utilization of microwave energy. General speaking, microwave devices cavity is made of metal material, so the microwave loss caused by cavity absorption is only a very small portion of the total power dissipation and most of the microwave energy entered into cavity is absorbed by the medium to form a high heating characteristic of energy efficiency which focuses more energy on the material being heated.

2. Applications for microwave heating 

For some agricultural products with poor thermal conductivity and high viscosity, in order to possibly keep their primary flavor, we use microwave to do heat treatment to play a good result.

When we use microwave to bake the tea from Anhui and process them into refined ones, the needed time is shortened by 1/5 compared to conventional processing time and the water content in tea falls to 3%, this most preserves tea’s original color and flavor. In addition, in 1972 the Nestle company in France used microwave vacuum dryer to concentrate juice and let them moisture falls from 20% to 3%. They successfully produced micro-porous instant juice powder which color, flavor and nutrients remained almost unchanged.

3. Wood drying by microwave

As wood has poor thermal conductivity, in order to avoid appearing fracture and deformation, the traditional drying method we used to drying it not only needs long time but also requests uniform drying. Compared to furnaces drying, microwave drying ahs some features such as uniform heating, short drying time and easy operation and meanwhile it also can achieve continuous operation. Currently, microwave drying equipments for wood materials have being promoted and used in many regions in China.

For deep frozen food, we usually process them after warming. So the microwave heating technology can be widely used in food plants. Adopting microwave to warm frozen food has some characteristics such as short time, no dropping and pollution and low loss rate. In the early 1993 in China, one munitions factory had built one microwave heating production line for frozen pork and its processing capacity each day can reach to 10 tons to 15 tons.