3.3. Scanning electron microscopy of E. coli O157:H7 on apricot surface
Trichomes with different lengths, stomata and cuticle waxes were
observed on apricot surfaces (Fig. 3). Wax deposition on the surface of
apricots formed ridging and cracks (gaps). E. coli O157:H7 was not
found on the trichomes. Instead, the bacteria were attached on the epidermic
cuticle around trichome bases. Some bacteria were found inside
the stomata, and many were located in the cracks/gaps between the
ridging of cuticle wax. The results suggest that UV-C light may be
blocked by the surface structures on the apricot surface, preventing
UV-C exposure of all bacteria. Our results suggested that some bacteria
may be internalized in stomata and other protective areas, which may
render UV-C less effective as UV-C with low penetration ability cannot
reach to the bacteria. Previously studies on other types of fresh produce
have suggested that pathogens may be internalized through various
portals of entry including stomata, lenticels, trichomes, wounds, area
of decay and stem scars (Ryser et al., 2009). The internalization minimizes
pathogen exposure to chemical sanitizers and reduces the effectiveness
of the sanitizers.
Our results show that all bacteria tested exhibited exponential decay
curves in response to increasing UV-C doses (i.e. rapid decreases in population
at lower UV-C range [0–74 mJ/cm2
]), and slow decreases at
higher UV-C dose ranges. Murakami et al. (2006) found that the E. coli
K12 inactivation in malate buffer containing 5 g/mL of suspended solids
could be interpreted as being made up of two linear plots; the slope for
the first linear plot (lower dose range, 0–13 mJ/cm2
) was 0.24 log reduction
per each mJ/cm2
, while the slope of the second linear plot
(higher than 13 mJ/cm2
) was 0.06. This phenomenon may be explained
as the exposure of two groups of the same bacteria to the UV-C dose,
where the first group of bacteria was located at a more exposed attachment
site and received a higher dose of UV-C. The second group of bacteria,
located in more protected sites such as the interior of a biofilm,
experienced a shadowing effect, resulting in their receiving lower
doses of radiation. This shielding could also occur due to fruit surface
structures, as there were many trichomes and stomata capable of