3. Lack of photoreactivation of injured Salmonella post-UV-C
treatments
Statistically, there was no relationship between the UV exposure
and the ability of injured Salmonella to recover after exposure to
visible light. Therefore, significant differences were analyzed only
Fig. 2. Survival of Salmonella on tomato surfaces after UV light treatment at an intensity of 743.6 mW/cm2 for 0e300 s. Values with different letters were significantly different
(p < 0.05).
W. Lim, M.A. Harrison / Food Control 66 (2016) 166e173 169
among UV treatment and among visible treatment. Salmonella
counts after UV radiation were significantly lower than those of
controls (p < 0.05) (Fig. 4). Since Salmonella population levels on
tomato surfaces exposed to visible light for 3 or 5 h were less than
those on tomatoes not exposed to visible light, no significant injury
repair by photoreactivation by visible light treatments was
observed. The lower Salmonella population on those exposed to
visible light might be due to the fact that there was limited nutrient
availability on the tomato surfaces or the surfaces were too dry to
support recovery of Salmonella. Lang, Beuchat, and Harris (2004)
found that air drying reduced recovery of E. coli O157:H7, Salmonella,
and Listeria monocytogenes on the surface of tomatoes by
more than 1 log, suggesting that bacterial cells may be injured or
stressed during desiccation.
To determine if recovery under darkness occurred, post-UV
treated Salmonella contaminated tomatoes were stored in closed
stainless steel containers. The results were similar to those of the
visible light treatment with Salmonella levels reduced significantly
(p > 0.05) after UV treatment (Fig. 5). The treatments of 3 h and 5 h
dark exposure did not result in injury recovery.
Kuo et al., (1997) reported similar results on the lack of repair of
UV damage. Their study indicated that neither 1 h of light exposure
nor 1 h of dark treatment post-UV treatment significantly influenced
the photoreactivation or dark repair ability of S. Typhimurium
on shell eggs. However, photoreactivation of S. dysenteriae and
S. Typhimurium was detected in a study done by Hu et al., (2012).
After UV light treatment, contaminated surface water samples were
exposed to a fluorescent lamp with an intensity of 0.066 mW/cm2
at a wavelength of 360 nm for 3 h. They noticed that photoreactivation
decreased significantly with higher UV doses.
Various factors may affect photoreactivation. Lindenauer and
Darby (1994) noted that the number of pyrimidine dimers
formed, the amount of photolyase present, the temperature during
the formation of the photoreactivation enzyme and dimer complex,
and the dose of photoreactivation radiation could affect photoreactivation.
Photoreactivation is dependent on the genetic and
physiological state of the exposed cell (Harm, 1980).