lead to cold spots in some microwaved foods and concern over the risks
associated with consumption of inadequately heated meals has led to
more explicit instructions on microwaveable foods. These often specify a
tempering period after heating to allow the temperature to equilibrate.
Microwaves have been slow to find industrial applications in food
processing, although they are used in a number of areas. Microwaves
have been used to defrost frozen blocks of meat prior to their processing
into products such as burgers and pies thus reducing wear and tear on
machinery. There has also been a limited application of microwaves in
the blanching of fruits and vegetables and in the pasteurization of soft
bakery goods and moist (30% H 2 O) pasta to destroy yeasts and moulds.
In Japan, microwaves have been used to pasteurize high-acid foods, such
as fruits in syrup, intended for distribution at ambient temperature.
These are packed before processing and have an indefinite microbiologi-
cal shelf-life because of the heat process and their low pH. However, the
modest oxygen barrier properties of the pack has meant that their
biochemical shelf-life is limited to a few months.
4.2.2 UV Radiation
UV radiation has wavelengths below 450 nm (nC 1015 Hz) and a quan-
tum energy of 3–5 eV (10^12 ergs). The quanta contain energy sufficient
to excite electrons in molecules from their ground state into higher
energy orbitals making the molecules more reactive. Chemical reactions
thus induced in micro-organisms can cause the failure of critical meta-
bolic processes leading to injury or death.
Only quanta providing energy sufficient to induce these photochemical
reactions will inhibit micro-organisms, so those wavelengths that are
most effective give us an indication of the sensitive chemical targets
within the cell. The greatest lethality is shown by wavelengths around
260 nm which correspond to a strong absorption by nucleic acid bases.
The pyrimidine bases appear particularly sensitive, and UV light at this
wavelength will, among other things, induce the formation of covalently
linked dimers between adjacent thymine bases in DNA (Figure 4.9). If
left intact these will prevent transcription and DNA replication in
affected cells.
The resistance of micro-organisms to UV is largely determined by
their ability to repair such damage, although some organisms such as
micrococci also synthesize protective pigments. Generally, the resistance
to UV irradiation follows the pattern:
Gram-negativesoGram-positivesyeastsobacterial spores
omould sporesooviruses:Chapter 4 83