will reduce the intensity of UV radiation by two-thirds. This effect
increases with the concentration of solutes and suspended material so
that in milk 90% of the incident energy will be absorbed by a layer only
0.1 mm thick. This low penetrability limits application of UV radiation
in the food industry to disinfection of air and surfaces.
Low-pressure mercury vapour discharge lamps are used: 80% of their
UV emission is at a wavelength of 254 nm which has 85% of the
biological activity of 260 nm. Wavelengths below 200 nm are screened
out by surrounding the lamp with an absorbent glass since these wave-
lengths are absorbed by oxygen in the air producing ozone which is
harmful. The output of these lamps falls off over time and they need to be
monitored regularly.
Air disinfection is only useful when the organisms suspended in air can
make a significant contribution to the product’s microflora and are likely
to harm the product; for example, in the control of mould spores in
bakeries. UV lamps have also been mounted in the head space of tanks
storing concentrates, the stability of which depends on their lowaw.
Fluctuations in temperature can cause condensation to form inside the
tank. If this contacts the product, then areas of locally highawcan form
where previously dormant organisms can grow, spoiling the product.
Process water can be disinfected by UV; this avoids the risk of tainting
sometimes associated with chlorination, although the treated water will
not have the residual antimicrobial properties of chlorinated water. UV
radiation is commonly used in the depuration of shellfish to disinfect the
water recirculated through the depuration tanks. Chlorination would not
be suitable in this situation since residual chlorine would cause the
shellfish to stop feeding thus stopping the depuration process.
Surfaces can be disinfected by UV, although protection of micro-
organisms by organic material such as fat can reduce its efficacy. Food
containers are sometimes treated in this way and some meat chill store
rooms have UV lamps to retard surface growth. UV can however induce
spoilage of products containing unsaturated fatty acids where it accel-
erates the development of rancidity. Process workers must also be
protected from UV since the wavelengths used can cause burning of
the skin and eye disorders.
4.2.3 Ionizing Radiation
Ionizing radiation has frequencies greater than 10^18 Hz and carries
sufficient energy to eject electrons from molecules it encounters. In
practice three different types are used.
(1)High-energy electrons. in the form of bparticles produced by
radioactive decay or machine generated electrons. StrictlyChapter 4 85