Similarities between RF and microwave heating
Governed by Maxwell’s Equations and producing only thermal effects
Both RF and microwave waves are EM waves and transfer to treated products by radiation. The quantity of heat absorbed by the products depends on the dielectric properties of the material while the temperature changes inside the material depend on the thermal properties of the material. RF and microwave heating take place due to the polarization effect of the EM field radiation, and the governing equations for RF and microwave heating are same.
Rapid and volumetric heating
Both RF and microwave heating are rapid and exhibit volumetric heating. Dielectric properties play a very important role in both RF and microwave heating. RF and microwave are known as dielectric heating, and are usually applied to dielectric materials. Most food materials belong to this category and can be treated by RF or microwave heating.
High energy efficiency
RF and microwave systems are capable of instantaneously applying or removing the heat source, and the system heating efficiency is much higher than conventional hot air or hot water heating. RF and microwave systems have been recognized to have 50–70% heating efficient in comparison to 10% efficiency with conventional ovens (Memelstein, 1997).
The main challenge in RF and microwave heating is reducing the temperature difference between the hot and cold spots in the food allowing for a more uniform heating. The cold and hot spots during dielectric heating exist due to the non-uniformity of the electrical field distribution and product properties. Generally, the dielectric loss factor would keep increasing with the temperature of the material; the already hot area will receive more energy than the cold area. This phenomenon is known as thermal runway which would give rise to significant non-uniform heating.
Differences between RF and microwave heating
Different frequency/wavelength and penetration depth
The frequency ranges for RF and microwave are different. RF uses much lower frequencies than microwaves. Therefore, RF waves have longer penetration depth than microwaves. Penetration depth limits the size and shape of the treated food. Thus, RF heating can be used to treat bulk material with relatively larger dimensions than microwave heating. Another difference is the limitation in the shape of the product. Microwave treatment has no limitation in shape while products need to be of a regular, simple shape when using RF heating.
Different system design/cost
The source and system design for RF and microwave heating are different. In RF heating, the electrical field is generated in a directional manner between a pair of electrode plates, but the electrical field could approach the product from all directions in microwave heating depending on different mode design; the microwaves are transmitted by waveguides which the RF system does not have. The system configuration is also totally different for the RF and microwave systems. RF systems are generally simpler to construct than microwave systems, and the cost is less than that of microwave systems.
Different heating uniformity
With concern to heating pattern, RF heating can provide relatively uniform field distribution as compared to the complex non-uniform standing wave patterns in a microwave heating system. As a result, a better heating pattern for RF processing could be obtained compared to microwave processing. RF heating system efficiency is a little higher than microwave system (~70% vs. ~50%).