Resistance to ionizing radiation depends on the ability of the organism
to repair the damage caused. Inactivation kinetics are generally loga-
rithmic, although survival curves often appear sigmoidal exhibiting a
shoulder and a tail to the phase of log-linear death. The shoulder is
usually very slight but is more pronounced with bacteria which have
more efficient repair mechanisms where substantially more damage can
be accumulated before death ensues.
D values can be derived from the linear portion of these curves and
Table 4.8 presents 6D values (the dose to produce a millionfold reduc-
tion) reported for a number of foodborne organisms. These are expressed
in terms of the absorbed dose of ionizing radiation which is measured in
Grays (1 Gy¼1 joule kg^1 ). Resistance generally follows the sequence:
Gram-negativeoGram-positivemouldsosporesyeastsoviruses:
Food-associated organisms do not generally display exceptional re-
sistance, although spores of some strains ofClostridium botulinumtype A
have the most radiation resistant spores. Since studies on food irradia-
tion started, a number of bacteria which are highly resistant to radiation
have been isolated and described. Although one of these,Deinococcus
radiodurans, was first isolated from meat, their role in foods is not
significant in the normal course of events.
Although patents describing the use of ionizing radiation in the
treatment of food appeared soon after the discovery of radioactivity at
the turn of the 20thcentury, it was not until after the Second World War
that food irradiation assumed commercial potential. This was largely due
to technological advances during the development of nuclear weapons,
Table 4.8 Radiation resistance of some foodborne
micro-organisms
Species 6D dose (KGy)
E. coli 1.5–3.0
SalmonellaEnteritidis 3–5
S. Typhimurium 3–5
Vibrio parahaemolyticus o0.5–1
Pseudomonas fluorescens 0.5–1
Bacillus cereus 20–30
B. stearothermophilus 10–20
C. botulinumtype A 20–30
Lactobacillusspp. 2–7.5
Micrococcusspp. 3–5
Deinococcus radiodurans 430
Aspergillus flavus 2–3
Penicillium notatum 1.5–2
S. cerevisiae 7.5–10
Viruses 430
Data from ‘Microbial Ecology of Foods’. Vol. 1. ICMSF.Chapter 4 87