phase. Scraped surface heat exchangers have been used to process
products containing particles up to 25 mm in diameter but at the cost
of overprocessing the liquid phase. To avoid this, one system processes
the liquid and solid phase separately. A promising alternative is the use
of ohmic heating in which a food stream is passed down a tube which
contains a series of electrodes. An alternating voltage is applied across
the electrodes and the food’s resistance causes it to heat up rapidly. Most
of the energy supplied is transformed into heat and the rate at which
different components heat up is determined by their conductivities rather
than heat transfer.
A common packing system used in conjunction with UHT processing
is a form/fill/seal operation in which the container is formed in the
packaging machine from a reel of plastic or laminate material, although
some systems use preformed containers. Packaging material is generally
refractory to microbial growth and the level of contamination on it is
usually very low. Nevertheless to obtain commercial sterility it is given
a bactericidal treatment, usually with hydrogen peroxide, sometimes
coupled with UV irradiation.
4.2 Irradiation
Electromagnetic (e.m.) radiation is a way in which energy can be prop-
agated through space. It is characterized in terms of its wavelengthl,or
its frequencyn, and the product of these two properties gives the speed,c,
at which it travels (3 108 m sec^1 in a vacuum).
ln¼c ð 4 : 13 ÞThe range of frequencies (or wavelengths) that e.m. radiation can have
is known as the electromagnetic spectrum and is grouped into a number
of regions, visible light being only one small region (Figure 4.8).
The energy carried by e.m. radiation is not continuous but is trans-
mitted in discrete packets or quanta; the energy,E, contained in each
quantum being given by the expression:
E¼hn ð 4 : 14 Þwherehis a constant (6.6 10 ^27 ergs sec^1 ) known as Planck’s con-
stant. Thus, the higher the frequency of the radiation the higher its
quantum energy.
As far as food microbiology is concerned, only three areas of the e.m.
spectrum concern us; microwaves, the UV region and gamma rays. We
will now consider each of these in turn.
Chapter 4 81