Impact of Various Washing Protocols on the Mitigation of Escherichia coli Contamination in Raw Salad Vegetables

Abstract

Vegetables are an essential component of a balanced diet. The consumption of ready-to-eat foods may lead to the risk of infections and illnesses due to microbial contamination. To mitigate the potential of microbial contamination risks, it is critical to promote safe handling practices among consumers. In this study, our research evaluated the efficacy of different vegetable washing methods, specifically with lettuce, tomato, and cucumber, to establish optimal practices for reducing microbial contamination. This study consisted of two phases. Initially, a survey was distributed to 150 volunteers using snowball sampling to assess everyday vegetable handling and washing methods. The survey's results identified four predominant methods: washing with a 5% vinegar solution for 3 min followed by tap water rinse (37.3% of participants), rinsing with tap water for 1 min (29.3%), washing with a 5% salt solution (vegetable soap) for 3 min followed by a tap water rinse (16.6%), and a 3 min tap water rinse (14%). A minor segment (3.33%) reported not washing their vegetables at all. The survey's findings guided the second phase, which tested the aforementioned washing protocols' effectiveness in reducing Escherichia coli (E. coli) levels on spiked contaminated salad vegetables. The tested vegetables were sterilized using UV light, inoculated with 0.5 McFarland E. coli, and then washed using the four identified methods. After that, E. coli enumeration after washing was performed using 3M™ Petrifilm and the comparison was analyzed via one-way ANOVA. During this study, it was revealed that the cucumbers had the highest E. coli contamination levels in comparison to the lettuce and tomato after washing. Interestingly, by comparing the three washing methods, it was found that washing the vegetables with vinegar proved to be the most effective solution for reducing microbial presence on both lettuce and cucumbers. Notably, the natural smoothness of tomato skin led to no significant differences in contamination levels across washing methods. In summary, vinegar washing effectively reduces microbial contamination from salad vegetables, highlighting the need for informed consumer practices to prevent foodborne outbreaks. This study emphasizes the importance of monitoring contamination sources and using safe washing techniques.

Keywords: E. coli; coliform; consumer practices; food safety; foodborne pathogens; infection; microbial contamination; outbreak; ready-to-eat food; salad vegetables; vegetable washing methods; vinegar solution.

Figure 1

(A) A visual representation of the relative microbial load of E. coli strains observed in lettuce, cucumber, and tomato. Bars are means ± SEM. Data were analyzed using one-way ANOVA (* p < 0.05, ** p < 0.01). (B) A figure representing a comparison of microbial growth between the spiked and blank sterilized samples. The p-values for the tested vegetables are less than 0.0001.

Figure 2

(A) A bar graph shows the effectiveness of three washing methods on the cucumber surface. Bars are means ± SEM. Data were analyzed using one-way ANOVA (* p < 0.05). (B) A bar graph demonstrating E. coli growth in lettuce after using the washing methods. Bars are means ± SEM. Data were analyzed using one-way ANOVA (* p < 0.05). (C) A bar graph demonstrating E. coli growth in tomatoes after using the washing methods. Bars are means ± SEM. Data were analyzed using one-way ANOVA.

Figure 2

(A) A bar graph shows the effectiveness of three washing methods on the cucumber surface. Bars are means ± SEM. Data were analyzed using one-way ANOVA (* p < 0.05). (B) A bar graph demonstrating E. coli growth in lettuce after using the washing methods. Bars are means ± SEM. Data were analyzed using one-way ANOVA (* p < 0.05). (C) A bar graph demonstrating E. coli growth in tomatoes after using the washing methods. Bars are means ± SEM. Data were analyzed using one-way ANOVA.

Figure 3

Effect of (A) water, (B) vinegar, (C) and vegetable soap on vegetable surfaces. No effects of cleaning methods were found on cucumber. Bars are means ± SEM. Data were analyzed using the Friedman test (* p < 0.05, ** p < 0.01, *** p < 0.001).