Effect of attachment time followed by chlorine washing on the survival of inoculated listeria monocytogenes on tomatoes and spinach

Abstract

The effect of attachment time (30 min, 24, 48 & 72 h) followed by chlorine washing (200 ppm) on the survival of inoculated Listeria monocytogenes on the surface and sub-surface of tomatoes and spinach were studied. The work was done to determine the efficacy of chlorine to decontaminate surface and sub-surface pathogens that may have come into contact with produce during pre-harvest. Tomatoes and spinach leaves were inoculated with a 6 log cfu/ml 18 h culture of L. monocytogenes ATCC 7644 (LM) on the surface and sub-surface and incubated at 20 oC for either, 30 min, 24, 48 or 72 h. LM attached and survived on the surface and sub-surface structures of both control and chlorine washed vegetables after each attachment time, up to 72 h. Higher levels of LM attachment and survival was however noticed on the sub-surface structures. Chlorine had a greater effect on the LM on the surface structures compared to those in the sub-surface structures, possibly because chlorine was not able to access the sub-surface structures where the pathogens were located. Chlorine was not effective in totally inactivating the surface LM on spinach and tomato. This research indicated that LM could attach to both surface and sub-surface structures of both tomatoes and spinach, within 30 min, and that even after 72 h, it still remained viable.