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

Survival of Generic Escherichia coli on In-Field Mature and Immature Gala and Golden Delicious Apples With or Without Overhead Evaporative Cooling Treatment

The application of overhead evaporative cooling (EC) with untreated surface water is common in Washington State to decrease sunburn on apples. While this practice reduces crop and economic losses, applying untreated surface water to produce introduces potential risks of foodborne pathogen contamination. This study examined EC and the survival of inoculated generic Escherichia coli for two apple varieties over three growing seasons. Factors examined included EC treatment (conventional, untreated), canopy location (high, low), and apple variety (Gala, Golden Delicious). Fruit maturity (mature, immature) was also examined for one year. A rifampicin-resistant generic E. coli cocktail was applied to apples at 7.3 ± 0.4 log CFU/apple, and apples were enumerated for E. coli levels at 0, 2, 10, 18, 34, 42, 58, 82, 106, and 154 h postinoculation. Log-linear, Weibull, and Biphasic models were utilized to characterize the die-off pattern of E. coli. For most treatments, inoculated E. coli decreased by almost or over 6 log CFU/apple over the study duration; however, E. coli remained detectable at low levels on apples at 154 h postinoculation. EC treatment and canopy location did not significantly impact daily linear E. coli die-off rates. Apple variety and maturity demonstrated small but statistically significant impacts on daily linear die-off rates. Nonlinear models (i.e., biphasic and Weibull) best captured E. coli die-off on apples, showing a rapid initial decrease followed by a slower decline over time. Overall, results demonstrate that EC, a useful fruit quality practice, did not impact the survival of inoculated generic E. coli on apple surfaces. The study provides valuable insights for the apple industry regarding E. coli die-off rates on in-field apples, guiding practical decisions to mitigate risk.

Irrigation Method Matters: Contamination and Die-off Rates of Escherichia coli on Dry Bulb Onions After Overhead and Drip Irrigation in Washington State (2022-2023)

Two U.S. outbreaks of salmonellosis in 2020 and 2021 were epidemiologically linked to red onions. The 2020 outbreak investigation implicated the production of agricultural water as a likely contamination source. Field trials were designed to investigate the prevalence and survival of Escherichia coli (surrogate for Salmonella) on dry bulb onions after the application of contaminated irrigation water at the end of the growing period. Irrigation water was inoculated at 3 log most probable number (MPN)/100 mL (2022 and 2023) or 5 log MPN/100 mL (2023, drip only) with a cocktail of rifampin-resistant E. coli and applied with the final irrigation (0.4 acre-inch/0.4 ha-cm) to onions. Onion bulbs (40 or 80) were sampled immediately after irrigation and throughout field curing (4 weeks) and E. coli was enumerated using an MPN method. For drip irrigation, at 3 log MPN/100 mL E. coli was detected on 13% of onions at 24 h but not detected at 0 h; at 5 log MPN/100 mL for drip irrigation applied to saturated soil, E. coli was detected in 63% of onions at 0 h. Prevalence significantly (P < 0.05), decreased after 7 d of curing with cell densities of 1-1,400 MPN/onion. At the end of field curing in 2023, 1/80 of onions had detectable E. coli (2.04 MPN/onion). E. coli was detected in a significantly smaller percentage of onions (2022: 13%; 2023: 68%) after a contaminated drip irrigation event compared to overhead irrigation (98-100%; P < 0.05). After overhead irrigation, E. coli was detected in onions (1-1,000 MPN/onion) on day 0. Prevalence decreased significantly (P < 0.05) after 7 d of field curing in both years (2022: 15%; 2023: 7%). E. coli was not detected on Calibra onions (80/year) at the end of field curing in either year but was detected at <12 MPN/onion in 2.5-3.75% of onions (n = 80) for other cultivars. These data confirm limited contamination risk associated with drip irrigation water quality and begin to quantify contamination risks associated with overhead irrigation of dry bulb onions.

Evaluation of the Effectiveness of Aeration and Chlorination during Washing to Reduce E. coli O157:H7, Salmonella enterica, and L. innocua on Cucumbers and Bell Peppers

The attachment strength of bacteria to surfaces can affect the efficacy of sanitizers during washing. This study aimed to determine the effectiveness of chlorination and aeration in the removal of pathogens from the surface of produce. Cucumbers and bell peppers were inoculated with Listeria innocua, Escherichia coli O157:H7, or Salmonella enterica; afterwards, the produce was washed with or without chlorinated water (100 ppm) for 3 min in combination with or without aeration. Cucumbers washed with chlorinated water, with or without aeration, presented significant reductions of L. innocua (3.65 log CFU/cm2 and 1.13 log CFU/cm2, respectively) (p < 0.05). Similarly, bell peppers washed in chlorinated water with aeration (1.91 log CFU/g) and without aeration (2.49 log CFU/g) presented significant reductions of L. innocua. A significant reduction of L. innocua was observed on bell peppers washed with non-chlorinated water with aeration (2.49 log CFU/g) (p < 0.05). Non-chlorinated water was also effective in significantly reducing the level of Salmonella enterica (p < 0.05) on cucumbers and bell peppers. Washing with chlorinated water with aeration reduced Salmonella enterica levels from 4.45 log CFU/cm2 on cucumbers to below the detectable limit (0.16 log CFU/cm2). The highest reduction of Salmonella enterica from bell peppers occurred after washing with chlorinated water with aeration (2.48 log CFU/g). E. coli O157:H7, L. innocua, and Salmonella enterica levels present in non-chlorinated water after washing contaminated produce with or without aeration were significantly greater than those in chlorinated water (p < 0.05). After treatment, the population levels of all pathogens in chlorinated water with or without aeration were below the detectable limit for bell peppers (<1.10 log CFU/mL) and cucumbers (<1.20 log CFU/mL). Using chlorine in combination with forced aeration during washing minimizes cross-contamination of bacterial pathogens.

Effect of Type of Mulch on Microbial Food Safety Risk on Cucumbers Irrigated with Contaminated Water

Mulches are used to block light and retain soil moisture which may affect the survival of bacterial pathogens on soil. This study examined the effectiveness of different types of mulches to minimize microbial risk from contaminated water used for irrigation of cucumbers. A production bed of 120 ft2 with 18 beds (30 ft long) covered with five different types of mulch (paper, paper with fertilizer incorporated (PF), maize-based mulch, biodegradable plastic covering, and conventional plastic) including three beds with no cover was planted with Dasher 2 Variety cucumber. Soil samples from each bed were collected for the first five weeks to examine natural E. coli and coliforms. Well water contaminated with or without nalidixic acid-resistant mutant of E. coli (8 Log CFU/mL) was used for drip irrigation for 7 days before harvesting. Prior to irrigation with contaminated water, naturally present E. coli and coliform in the soil samples with or without mulch were in the range of 3.45-3.78 Log CFU/g and 4.18-5.31 Log CFU/g, respectively. E. coli levels on cucumbers harvested from mulched beds and irrigated with contaminated irrigation water had significantly higher (P < 0.05) levels of E. coli as compared with samples from similar beds irrigated with noncontaminated water. However, Cucumber, harvested within each irrigation water quality were not significantly different regardless of the type of mulch with E. coli levels from 1.72 to 3.30 Log CFU/cm2 (contaminated water) and 0.28-1.86 Log CFU/cm2 (noncontaminated water). A significant die-off of inoculated E. coli was observed on cucumber within 3 days (>1.17 Log CFU/cm2) and >1.38 Log CFU/cm2 after 4 days. Beds with maize mulch were effective on minimizing E. coli contamination on cucumber from contaminated irrigation water.

Bio-based antimicrobial compositions and sensing technologies to improve food safety

Microbial contamination of food products is a significant challenge that impacts food safety and quality. This review focuses on bio-based technologies for enhancing the decontamination of raw foods during postharvest processing, preventing cross-contamination, and rapidly detecting microbial risks. The bio-based antimicrobial compositions include bio-based antimicrobial delivery systems and coatings. The antimicrobial delivery systems are developed using cell-based carriers, microbubbles, and lipid-based colloidal particles. The antimicrobial coatings are engineered by incorporating biopolymers with conventional antimicrobials or cell-based antimicrobial carriers. The bio-based sensing approaches focus on replacing antibodies with more stable and cost-effective bio-receptors, including antimicrobial peptides, bacteriophages, DNAzymes, and engineered liposomes. Together, these approaches can reduce microbial contamination risks and enhance the in-situ detection of microbes.

Understanding inactivation of Listeria monocytogenes and Escherichia coli O157:H7 inoculated on romaine lettuce by emulsified thyme essential oil

Effects of thyme essential oil (TEO) emulsion (TEE) with cationic charge formulated using cetylpyridinium chloride (CPC) on attachment strength and inactivation of Listeria monocytogenes and Escherichia coli O157:H7 on romaine lettuce surface were examined in this study. Regardless of the inoculation time (2 h and 24 h), pathogen attachment was stronger on the adaxial surface of the romaine lettuce than on the abaxial surface because of the lower roughness of the former. Moreover, attachment strength increased with increasing inoculation time. TEE washing had the strongest inhibitory effect on pathogen attachment at 2 h when compared with that of TEO, CPC, and sodium hypochlorite (SH), demonstrating a 3.32 and 2.53 log-reduction in the size of the L. monocytogenes and E. coli O157:H7 populations, respectively, compared to the control samples. Additionally, the TEE washing effects were maintained even after inoculation for 24 h, and it decreased attachment to adaxial surface of the samples. These results indicate that TEE could be a good alternative to SH in improving the microbiological safety of romaine lettuce.

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.

Effectiveness of daily rinsing of alfalfa sprouts with aqueous chlorine dioxide and ozonated water on the growth of Listeria monocytogenes during sprouting

Alfalfa sprouts have been implicated in multiple foodborne disease outbreaks. This study evaluated the growth of Listeria monocytogenes during sprouting of alfalfa seeds and the effectiveness of daily chlorine dioxide & ozone rinsing in controlling the growth. Alfalfa seeds inoculated with L. monocytogenes were sprouted for 5 days (25°C) with a daily aqueous ClO₂ (3 ppm, 10 min) or ozone water (2 ppm, 5 min) rinse. Neither treatment significantly reduced the growth of L. monocytogenes on sprouting alfalfa seeds. The initial level of L. monocytogenes was 3·44 ± 0·27, which increased to c. 7·0 log CFU per g following 3 days of sprouting. There was no significant difference in the bacterial population between the treatment schemes. Bacterial distribution in roots (7·63 ± 0·511 log CFU per g), stems (7·51 ± 0·511 log CFU per g) and leaves (7·41 ± 0·511 log CFU per g) were similar after 5 days. Spent sanitizers had significantly lower levels of bacterial populations compared to the spent distilled water control. The results indicated that sprouting process provides a favourable condition for the growth of L. monocytogenes and the sanitizer treatment alone may not be able to reduce food safety risks. SIGNIFICANCE AND IMPACT OF THE STUDY: Sprouts are high-risk foods. Consumption of raw sprouts is frequently associated with foodborne disease outbreaks. Optimum sprouting procedure involves soaking seeds in water followed by daily water rinsing to maintain a moist environment that is also favourable for the growth of pathogenic micro-organisms. The present study emphasized the potential food safety risks during sprouting and the effect of applying daily sanitizer rinsing in the place of water rinsing to reduce those risks. The finding of this study may be useful in the development of pre-harvest and post-harvest risk management strategies.