Death of a population of UV-irradiated cells demonstrates log-linear
kinetics similar to thermal death and, in an analogous way, D values can
be determined. These give the dose required to produce a tenfold
reduction in surviving numbers where the dose, expressed in ergs or
mWs, is the product of the intensity of the radiation and the time for
which it is applied. Some published D values are presented in Table 4.7.
Determination of UV D values is not usually a straightforward affair
since the incident radiation can be absorbed by other medium compo-
nents and has very low penetration. Passage through 5 cm of clear water
Figure 4.9 The photochemical dimerization of thymine
Table 4.7 UV resistance of some selected food-
borne micro-organisms
Species D (ergs 102 )
E. coli 3–4
Proteus vulgaris 3–4
Serratia marcescens 3–4
Shigella flexneri 3–4
Pseudomonas fluorescens 3–4
Bacillus subtilis(vegetative cells) 6–8
Bacillus subtilis(spores) 8–10
Micrococcus luteus 10–20
Staph. aureus 3–4
Aspergillus flavus 50–100
Mucor racemosus 20–50
Penicillium roquefortii 20–50
Rhizopus nigrificans 4200
Saccharomyces cerevisiae 3–10
Data from ‘Microbial Ecology of Foods’. Vol. 1.ICMSF.84 The Microbiology of Food Preservation