Evaluating the potential for contamination of leafy greens with Listeria when using retrofitted washing machines

A large portion of foodborne outbreaks are associated with leafy greens, including by the pathogen Listeria monocytogenes. Small and medium-sized leafy green growers commonly retrofit washing machines to dry triple-washed leaves, utilizing their spin cycle. The potential for this practice to cause microbial contamination has not been explored. This work investigated the risk of cross-contamination of Listeria associated with this practice. Listeria innocua was inoculated and dried onto spinach prior to washing, after which leaves were dried in a retrofitted washing machine. After drying, three contact surface layers of the machine were sampled. Nearly complete recovery of 98% and 99% from some contact points were observed, with the highest level of recovery observed in the bottom water-collecting contact points of the machine. Further, L. innocua from contaminated loading baskets were transferred to uninoculated spinach during the spin-drying process with a microbial recovery of 10¹-10² CFU/g, suggesting a risk of cross-contamination.