Dielectric heating
Molecular rotation occurs in materials containing polar molecules having an electrical dipole moment, with the consequence that they will align themselves in an electromagnetic field.
[1] Dipole rotation is the mechanism normally referred to as dielectric heating, and is most widely observable in the microwave oven where it operates most effectively on liquid water, and also, but much less so, on fats and sugars.
In this case, although the heating is accomplished by changing the electric field inside the capacitive cavity at radio-frequency (RF) frequencies, no actual radio waves are generated or absorbed.
[citation needed] Dielectric heating at low frequencies, as a near-field effect, requires a distance from electromagnetic radiator to absorber of less than 1/2π ≈ 1/6 of a wavelength.
Because lower frequency electrical fields penetrate non-conductive materials far more deeply than do microwaves, heating pockets of water and organisms deep inside dry materials like wood, it can be used to rapidly heat and prepare many non-electrically conducting food and agricultural items, so long as they fit between the capacitor plates.
Microwave frequencies penetrate conductive materials, including semi-solid substances like meat and living tissue.
If they were, then the piece of food or liquid in question would absorb all microwave radiation in its outer layer, leading to a cool, unheated centre and a superheated surface.
In agriculture, RF dielectric heating has been widely tested and is increasingly used as a way to kill pests in certain food crops after harvest, such as walnuts still in the shell.
RF heating is used in the wood industry to cure glues used in plywood manufacturing, fingerjointing, and furniture construction.
Modern microwave ovens make use of electromagnetic waves with electric fields of much higher frequency and shorter wavelength than RF heaters.
By raising the temperature of the feed quickly and causing moisture to move to the surface, it can decrease the overall drying time.
Dielectric heating may be applied part-way through the drying cycle, when the food enters the falling rate period.
The objectives in baking biscuits are to produce a product of the right size, shape, color, and moisture content.
In a conventional oven, reducing the moisture content to the desired level can take up a large part of the total baking time.
Conventional heating results in large disparity in temperature and longer processing times which can cause overprocessing on the food surface and impairment of the overall quality of the product.
[13] Electromagnetic energy can achieve higher processing temperatures in shorter times, therefore, more nutritional and sensory properties are conserved.
