How Microwaves and Radio Frequencies Generate Heat
Heating with microwave and radio frequency involves primarily two mechanisms: dielectric and ionic. Water in the food is often the primary component responsible for dielectric heating. Due to their dipolar nature, water molecules try to follow the electric field associated with electromagnetic radiation as it oscillates at the very high frequencies listed in Table 126.96.36.199. Such oscillations of the water molecules produce heat. The second major mechanism of heating with microwaves and radio frequency is through the oscillatory migration of ions in the food that generates heat under the influence of the oscillating electric field (US FDA, 2015).
The rate of heat generation per unit volume, Q, at a particular location in the food during microwave and radio frequency heating can be characterized by the formula (Buffler 1993; Datta 2000):
Q is the rate of heat generation per unit volume where E is the strength of electric field of the wave at that location, f is the frequency (Table 188.8.131.52) of the microwaves or the radio frequency waves, e0 the permittivity of free space (a physical constant), and e" is the dielectric loss factor (a material property called dielectric property) representing the material's ability to absorb the wave. Not apparent from the above equation, there is another dielectric property called the dielectric constant that affects the strength of the electric field inside the food. The dielectric properties depend on the composition (or formulation) of the food, moisture and salt being the two primary determinants of interest (Mudgett 1994; Datta et al., 1994). The subsequent temperature rise in the food depends on the duration of heating, the location in the food, convective heat transfer at the surface, and the extent of evaporation of water inside the food and at its surface (US FDA, 2015).