Microwave Heating

Use of MW energy started during the World War II as by product of radar technology.

These electromagnetic waves lie in the frequency ranges of 300 MHz to 300 GHz with

corresponding wavelengths from 1 to 0.001 m. Worldwide, MW frequency of 915 and 2450

MHz are used for domestic and industrial purpose (Vadivambal & Jayas, 2008). MW heating

system generally consists of MW generator or magnetron, wave guide and applicator. MW

heating, however depends on the dielectric property of the food material, which in turn depends

on the frequency of the microwaves, food temperature, moisture content, salt content or ionic

conductivity, and other constituents.

Food materials are poor insulator, have the capability of store and dissipate electrical

energy when subjected to rapidly alternating electromagnetic field. The electromagnetic field

polarizes the bound water molecules and causes ionization. As the electromagnetic field polarity

changes very fast (at frequency of 2450 MHz), the polarized molecules and ions oscillate

between the electromagnetic field. Collision between the polarized molecules, ionized atoms and

polar molecules thus generate enough heat energy to evaporate the water molecule, by freeing it

from its present state. The volumetric heat generation is instantaneous, throughout the food

material, compared to conventional conduction or convection heating process (Tang et al., 2002).

In MW heating system, both the dielectric constants and the loss factor due to polarization of

bound water in foods would increase with temperature. On the other hand, these two properties

of free water would decrease when temperature increases (Tang et al., 2002). There are certain

limitations with MW heating though i.e. hot spot generation due to non-uniform heating and

limitation of penetration depth (Vadivambal & Jayas, 2007). Microwaves at 915 MHz have more

penetration depth as compared to 2450 MHz but heating rate is slower in the latter case.

Literatures suggests that MW pretreatment prior to osmotic dehydration and convective air

drying, increases mass transfer rate, reduces drying time considerably by increasing the effective

diffusivity (Moreno et al., 2000; Severini, et al., 2005; Contreras et al., 2008).