Abstract
Customer demands for fresh salads are increasing, but leafy green vegetables have also been linked to food‐borne illness due to pathogens such as Escherichia coli O157:H7. As a safety measure, consumers often wash leafy vegetables in water before consumption. In this study, we analyzed the efficiency of household washing to reduce the bacterial content. Romaine lettuce and ready‐to‐eat mixed salad were washed several times in flowing water at different rates and by immersing the leaves in water. Lettuce was also inoculated with E. coli before washing. Only washing in a high flow rate (8 L/min) resulted in statistically significant reductions (p < .05), “Total aerobic count” was reduced by 80%, and Enterobacteriaceae count was reduced by 68% after the first rinse. The number of contaminating E. coli was not significantly reduced. The dominating part of the culturable microbiota of the washed lettuce was identified by rRNA 16S sequencing of randomly picked colonies. The majority belonged to Pseudomonadaceae, but isolates from Enterobacteriaceae and Staphylococcaceaceae were also frequently found. This study shows the inefficiency of tap water washing methods available for the consumer when it comes to removal of bacteria from lettuce. Even after washing, the lettuce contained high levels of bacteria that in a high dose and under certain circumstances may constitute a health risk.
Highlights
The repeated washing steps under the water tap gave further significant reductions of both total aerobic count and Enterobacteriaceae count on both mixed salad and romaine lettuce
This study shows that even though the salad mix is already washed and ready to eat, it contains high amounts of viable bacteria belonging to the Enterobacteriaceae family, a family which includes several pathogenic taxa
Simulated household washing could at best reduce the total aerobic count and the Enterobactreriaceae count with 90%
Summary
Six bags of whole romaine lettuce heads (Lactuca sativa) and 18 bags of ready-to-eat mixed salad containing friseé (Cichorium endivia var. crispum), red salad (Lactuca sativa), and red mangold (Beta vulgaris) were purchased at a supermarket in Lund, Sweden in February 2016 and brought directly to the laboratory. Total aerobic count of ready-to-eat mixed salad rinsing decreased significantly (p ≤ .01) from a median value of 7.2–6.7 log CFU/g after the first wash (Table 1) at the higher water flow of 8 L/min. When washing at a lower water flow (2 L/min), neither the aerobic count nor the Enterobacteriaceae count was reduced significantly on ready-to-eat mixed salad. E. coli inoculated romaine lettuce harbored a median value of 5.0 (4.4–5.4) log CFU E. coli/g, after the first wash 4.2 (4.1–5.1) log CFU/g and after the fifth wash 3.9 (3.5–4.6) log CFU/g (intermedian range 25–75% presented within parenthesis). Members of the families Micrococcacea, Xanthomonadaceae and Pseudomonadaceae were found before rinsing, and Microbacteriaceae and Staphylococcaceae were found after rinsing
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