4.2.1 Microwave Radiation
The microwave region of the e.m. spectrum occupies frequencies between
109 Hz up to 10^12 Hz and so has a relatively low quantum energy. For the
two frequencies used in food processing, 2450 MHz and 915 MHz, this is
around 10^18 ergs or 10^6 eV. Domestic microwave ovens use 2450 MHz
which is less penetrating than the lower frequency.
Unlike the other forms of radiation we will discuss, microwaves act
indirectly on micro-organisms through the generation of heat. When a
food containing water is placed in a microwave field, the dipolar water
molecules align themselves with the field. As the field reverses its polarity
about 2.5 109 times each second the water molecules are continually
oscillating. This kinetic energy is transmitted to neighbouring molecules
leading to a rapid rise in temperature throughout the product. In foods
with a high salt content, surface heating due to ions acquiring kinetic
energy from the microwave field can also contribute, but this is generally
of minor importance.
Microwaves are generated using a magnetron, a device first developed
in the UK during research into radar during the Second World War.
Magnetrons are used both commercially and domestically, but their
biggest impact has been in the domestic microwave oven and in catering
where their speed and convenience have enormous advantages. The
principal problem associated with the domestic use of microwaves is
non-uniform heating of foods, due to the presence of cold spots in the
oven, and the non-uniform dielectric properties of the food. These can
Figure 4.8 The electromagnetic spectrum
82 The Microbiology of Food Preservation