Effect of surrounding vegetation on microbial survival or die‐off on watermelon surface in an agriculture setting

Effectiveness of Aqueous Chlorine Dioxide in Minimizing Food Safety Risk Associated with Salmonella, E. coli O157:H7, and Listeria monocytogenes on Sweet Potatoes

Sodium hypochlorite (NaOCl) is a commonly used sanitizer in the produce industry despite its limited effectiveness against contaminated human pathogens in fresh produce. Aqueous chlorine dioxide (ClO₂) is an alternative sanitizer offering a greater oxidizing potency with greater efficacy in reducing a large number of microorganisms. We investigated the effect of aqueous chlorine dioxide treatment against human pathogens, Salmonella, Escherichia coli O157:H7, and Listeria monocytogenes seeded on sweet potatoes. Sweet potatoes were spot inoculated (4.2 to 5.7 log CFU/cm²) with multi-strain cocktails of Salmonella spp., E. coli O157:H7, and L. monocytogenes and treated for 10–30 min with 5 ppm aqueous ClO₂ or water. Aqueous ClO₂ treatment was significantly (p < 0.05) effective in reducing Salmonella with a reduction of 2.14 log CFU/cm² within 20 min compared to 1.44 log CFU/cm² for water treatment. Similar results were observed for L. monocytogenes with a 1.98 log CFU/cm² reduction compared to 0.49 log CFU/cm² reduction observed after 30 min treatment with aqueous ClO₂ the water respectively. The maximum reduction in E. coli O157: H7 reached 2.1 Log CFU/cm² after 20 min of treatment with aqueous ClO₂. The level of the pathogens in ClO₂ wash solutions, after the treatment, was below the detectable limit. While in the water wash solutions, the pathogens’ populations ranged from 3.47 to 4.63 log CFU/mL. Our study indicates that aqueous ClO₂ is highly effective in controlling cross-contamination during postharvest washing of sweet potatoes.

Evaluation of Microbiological Quality of Agricultural Water and Effect of Water Source and Holding Temperature on the Stability of Indicator Organisms' Levels by Seven U.S. Environmental Protection Agency-Approved Methods

ABSTRACT

The Food Safety Modernization Act Produce Safety Rule requires covered produce growers to monitor the quality of their agricultural water on a regular basis by some U.S. Environmental Protection Agency (EPA)-approved methods recognized by the U.S. Food and Drug Administration. In this study, we evaluated the changes in the population of indicator organisms in surface water up to 6 months, and the effects of water source and holding temperature on the survival of indicator organisms by seven EPA-approved methods (five methods for Escherichia coli and two methods for Enterococcus). The levels of E. coli and Enterococcus in the surface water were variable with sampling month, ranging from 1.61 ± 0.04 to 2.68 ± 0.15 log most probable number (MPN)/100 mL and from undetectable level to 1.19 ± 0.29 log MPN/100 mL, respectively. At 25°C (holding temperature), there were significant reductions (P < 0.05) in E. coli and Enterococcus populations in surface water after 48 and 24 h, respectively, whereas at 4°C, no significant changes in the bacterial populations were observed up to 48 h. Methods 1603, 1604, 1103.1, 10029, and Colilert showed a comparable sensitivity in quantifying E. coli, whereas method 1600 and Enterolert showed a variable sensitivity with the type of water. The results indicated that regular monitoring of agricultural water is essential to examine whether the microbial quality of water is appropriate for its intended use. Water samples should be maintained at 4°C to minimize the changes in microbial populations between sampling and testing. The comparison of the sensitivity of EPA methods for quantifying indicator organisms could provide growers with useful information for choosing the method for their water quality analysis.

HIGHLIGHTS

Evaluation of ultraviolet (UV-C) light treatment for microbial inactivation in agricultural waters with different levels of turbidity

Produce growers using surface or well water to irrigate their crops may require an appropriate water treatment system in place to meet the water quality standard imposed by FSMA Produce Safety Rule. This study evaluated the potential of using ultraviolet (UV-C) treatment in reducing the microbial population in agricultural water. Waters with turbidity levels ranging from 10.93 to 23.32 Nephelometric Turbidity Units (NTU) were prepared by mixing pond water and well water. The waters were inoculated with a cocktail of generic Escherichia coli (ATCC 23716, 25922, and 11775) and then treated with UV-C light (20-60 mJ/cm2). All tested doses of the UV-C treatment reduced the E. coli levels significantly (p < .05) in the water samples with the turbidity levels up to 23.32 NTU. The decrease in the turbidity from 23.32 to 10.93 NTU increased the level of reduction by more than 2.15 log most probable number (MPN)/100 ml). UV-C treatment effectively reduces microbial load in agriculture water; however, turbidity of water may significantly affect the disinfection efficacy. The study also demonstrated that sprinkler system resulted in a higher level of contamination of cantaloupes compared with drip irrigation. The results indicated that UV-C treatment could be a promising strategy in reducing the produce safety risks associated with irrigation water.

Meeting Report: Key Outcomes from a Collaborative Summit on Agricultural Water Standards for Fresh Produce

On February 27 to 28, 2018, the Produce Safety Alliance convened a national water summit in Covington, KY to discuss the requirements of the United States Food and Drug Administration's (FDA) Food Safety Modernization Act Standards for the Growing, Harvesting, Packing, and Holding of Produce for Human Consumption (Produce Safety Rule [PSR]). The goals of the meeting were to better understand the challenges growers face in implementing the requirements in Subpart E-Agricultural Water and work collaboratively to develop practical solutions to meet fruit and vegetable production needs while protecting public health. To meet these goals, the summit engaged a diverse group of stakeholders including growers, researchers, extension educators, produce industry members, and regulatory personnel. Key outcomes included defining implementation barriers due to diversity in water sources, distribution systems, commodity types, climates, farm size, and production activities. There was an articulated need for science-based solutions, such as the use of agricultural water system assessments and sharing of federal, state, and regional water quality data, to ensure qualitative and quantitative standards reduce microbial risks. These identified challenges and needs resulted in significant debate about whether reopening the PSR-Subpart E for modification or attempting to address concerns through guidance would provide the best mechanism for alleviating concerns. In addition, training, outreach, and technical assistance were identified as vital priorities once the concerns are formally addressed by FDA. The water summit highlighted the critical need for transparency of FDA's progress on reevaluating the Subpart E requirements to help guide growers' decisions regarding the use of agricultural water.

Attachment strength and on-farm die-off rate of Escherichia coli on watermelon surfaces

Pre-harvest contamination of produce has been a major food safety focus. Insight into the behavior of enteric pathogens on produce in pre-harvest conditions will aid in developing pre-harvest and post-harvest risk management strategies. In this study, the attachment strength (S_R) and die-off rate of E. coli on the surface of watermelon fruits and the efficacy of aqueous chlorine treatment against strongly attached E. coli population were investigated. Watermelon seedlings were transplanted into eighteen plots. Prior to harvesting, a cocktail of generic E. coli (ATCC 23716, 25922 and 11775) was inoculated on the surface of the watermelon fruits (n = 162) and the attachment strength (S_R) values and the daily die-off rates were examined up to 6 days by attachment assay. After 120 h, watermelon samples were treated with aqueous chlorine (150 ppm free chlorine for 3 min). The S_R value of the E. coli cells on watermelon surfaces significantly increased (P<0.05) from 0.04 to 0.99 in the first 24 h, which was primarily due to the decrease in loosely attached population, given that the population of strongly attached cells was constant. Thereafter, there was no significant change in S_R values, up to 120 h. The daily die-off rate of E. coli ranged from -0.12 to 1.3 log CFU/cm². The chlorine treatment reduced the E. coli level by 4.2 log CFU/cm² (initial level 5.6 log CFU/cm²) and 0.62 log CFU/cm² (initial level 1.8 log CFU/cm²), on the watermelons that had an attachment time of 30 min and 120 h respectively. Overall, our findings revealed that the population of E. coli on watermelon surfaces declined over time in an agricultural environment. Microbial contamination during pre-harvest stages may promote the formation of strongly attached cells on the produce surfaces, which could influence the efficacy of post-harvest washing and sanitation techniques.