By Tatiana Koutchma Ph.D., Research Scientist, Agriculture and Agri-Food Canada
In conventional thermal processing systems, heat energy is transferred through conduction and convection from a hot medium to a cooler product that may result in large temperature gradients. Heat exchangers typically utilize pressurized steam from petroleum-fired boilers with less than 25 - 30 percent of the energy conversion. Five innovative heating techniques, including infrared (IR), microwave (MW), dielectric or radio-frequency (RF), ohmic (OH), and magnetic induction heating (MI) utilize electro-magnetic energy and can heat foods faster and more efficiently. Except for magnetic induction heating, heat is generated within the product as a result of the transfer of electro-magnetic energy directly into the product. This initiates volumetric heating due to frictional interaction between water molecules and charged ions. These methods offer a considerable speed advantage, particularly in solid foods and high efficiency of energy conversion ranging from 60 percent, up to almost 100 percent.
Infrared (IR) radiation encompasses the portion of the electro-magnetic spectrum bordering on visible light and microwaves. IR can be classified into three regions, namely, near infrared (NIR), mid-infrared (MIR), and far-infrared (FIR), corresponding to the spectral ranges of 0.75 - 1.4, 1.4 - 3, and 3 - 1000 μm, respectively. The amount of the IR radiation on any surface has a spectral dependence because energy coming out of an emitter is composed of different wavelengths and the fraction of the radiation in each band, dependent upon the temperature and emissivity of the emitter.