Purpose: This study examined the effect of UV-C dose and fruit surface characteristics on the inactivation kinetics of Listeria monocytogenes on the surface of organic apple, pear, strawberry and cantaloupe.
Methods: Fruit surfaces spot inoculated with a cocktail of L. monocytogenes (NRRL B-33006; NRRL B-33069 and NRRL B-33385) were exposed to UV-C light (0 to 11.9 ± 0.32 KJ/m2) at 23°C. Weibull model was used to describe the inactivation kinetics.
Results: L. monocytogenes reduction was higher on apples (1.61 log CFU at 3.75KJ/m2) and pears (1.54 log CFU at 11.9KJ/m2) surface compare to cantaloupes (0.95 log CFU at 11.9KJ/m2) and strawberries surface (0.95 log CFU at 11.9KJ/m2). For all fruits, surface reduction achieved within the first minute of treatment (1.10 ± 0.16 KJ/m2) were significantly (P < 0.05) higher as compared to the remaining 2 to 13 min of the treatment. The shape parameter (p) obtained from Weibull model were between 0.12-0.28 for all fruits indicating that the remaining microbes were resistant to inactivation or had the ability to adapt to the applied stress. The time for first decimal reduction was lower in apples (63 s) and pears surface (141 s) compare to cantaloupes (1320 s) and strawberries (3025 s) surface. The root mean square surface roughness as observed from a Stylus surface profilometer was higher on strawberry (296 µm) and cantaloupe (55.8 µm) than apple (30.3 µm) and pear (40.2 µm). The rough surface of strawberry and cantaloupe may have shadowed the microbial cells, impairing the germicidal effect of UV-C light.
Significance: UV-C light is effective in reducing L. monocytogenes from fruit surfaces but its efficacy is influenced by their surface characteristics.