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Microwave heating refers to the use of electromagnetic
waves of certain frequencies to generate heat in a material. Industrial food
microwaves use only 2450 and 915 MHz
Principles
In
microwave heating, continuous electromagnetic waves are produced in the
magnetron and transmitted through a hollow metallic tube into a resonant cavity
where the food is processed. Foods are heated because of molecular friction
caused by alternating polarization of molecules. Foods absorb microwave energy
in the form of orientational and ionic polarization. Orientational polarization
results from dipolar molecules, such as water, which tend to align according to
the applied electric field. The electric field oscillates at 2450 or 915
million times per second (MHz), making the dipolar molecules rotate, thus
promoting molecular friction, which in turn results in heat dissipation. Ionic
polarization occurs when dissolved salts are present. Positive and negative
ions tend to migrate to opposite-charged regions, colliding with other ions and
converting kinetic energy into heat. Dipole rotation is more important than
ionic polarization as a microwave heating mechanism.
Effects of Microwaves
on Microbial Inactivation
Heat sensitive nutrients such as
vitamins and flavor constituents can be retained better through rapid heating
than by conventional heat treatments. However, little conclusive evidence
exists for any real flavor differences between many conventionally and
microwave-heated foods microwave energy inactivates microbes via conventional
thermal mechanisms, including thermal irreversible denaturation of enzymes,
proteins, and nucleic acids.
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