speaking they are particles rather than electromagnetic radiation,
although in some of their behaviour they do exhibit the proper-
ties of waves. Because of their mass and charge, electrons tend to
be less penetrating than ionizing e.m. radiation; for example,
5 MeVbparticles will normally penetrate food materials to a
depth of about 2.5 cm.
(2)X-raysgenerated by impinging high energy electrons on a suit-
able target.
(3)Gammagraysproduced by the decay of radioactive isotopes. The
most commonly used isotope cobalt 60,^60 Co, is produced by
bombarding non-radioactive cobalt,^59 Co, with neutrons in a
nuclear reactor. It emits high-energyg-rays (1.1 MeV) which can
penetrate food up to a depth of 20 cm (cf.bparticles). An isotope
of caesium,^137 Cs, which is extracted from spent nuclear fuel rods,
has also been used but is less favoured for a number of reasons.Ionizing radiation can affect micro-organisms directly by interacting
with key molecules within the microbial cell, or indirectly through the
inhibitory effects of free radicals produced by the radiolysis of water
(Figure 4.10). These indirect effects play the more important role since in
the absence of water, doses 2–3 times higher are required to obtain the
same lethality. Removal of oxygen also increases microbial resistance
2–4 fold and it is thought that this may be due to the ability of oxygen to
participate in free radical reactions and prevent the repair of radiation
induced lesions. As with UV irradiation, the main site of damage in cells
is the chromosome. Hydroxyl radicals cause single- and double-strand
breaks in the DNA molecule as a result of hydrogen abstraction from
deoxyribose followed byb-elimination of phosphate which cleaves the
molecule. They can also hydroxylate purine and pyrimidine bases.
Figure 4.10 The radiolysis of water.JDenotes main reactive radicals
86 The Microbiology of Food Preservation