Alejandro Tomás-Callejas

Survival and distribution of Escherichia coli on diverse fresh-cut baby leafy greens under preharvest through postharvest conditions

Escherichia coli O157:H7 has been associated in multiple outbreaks linked to the consumption of whole produce and fresh-cut leafy vegetables. However, plant-based foods had not been traditionally recognized as a host for enteric pathogens until the elevated incidence of produce-related outbreaks became apparent. The survival dynamics of two cocktails of generic E. coli (environmental water, plant and soil isolates) and E. coli O157:H7 within the phyllosphere of Mizuna, Red Chard and Tatsoi during their production, harvest, minimal processing, packaging and storage over two greenhouse production cycles were studied. Genotyping of applied generic E. coli strains to evaluate their comparative survival and relative abundance in the phyllosphere by REP-PCR is also reported. The Mizuna, Red Chard and Tatsoi shoots were grown under standard greenhouse conditions and fertility management. Both E. coli cocktails were spray-inoculated separately and determined to result in an initial mean population density of log 4.2 CFU/cm². Leaves were harvested as mini-greens approximating commercial maturity, minimally processed in a model washing system treated with 3 mg/L of ClO₂ and stored for 7 days at 5 °C. Rapid decline of generic E. coli and E. coli O157:H7 populations was observed for all plant types regardless of the leaf age at the time of inoculation and the irrigation type across both seasonal growth cycle trials. The decline rate of the surviving populations for the fall season was slower than for the summer season. The minimal processing with 3 mg/L of ClO₂ was not sufficient to fully disinfect the inoculated leaves prior to packaging and refrigerated storage. Viable populations of E. coli and E. coli O157:H7 were confirmed throughout storage, including the final time point at the end of acceptable visual leaf quality. In this study, the ability of low populations of E. coli to survive during production and postharvest operations in selected mini-greens has been demonstrated. However, further field-based trials are needed to expand understanding of the post-contamination fate of enteric bacterial pathogens on leafy vegetables. In summary, this research work provides baseline data upon which to develop food safety preventive control guidance during the production and minimal processing of these crops.

Evaluation of current operating standards for chlorine dioxide in disinfection of dump tank and flume for fresh tomatoes.

Standard postharvest unit operations that rely on copious water contact, such as fruit unloading and washing, approach the criteria for a true critical control point in fresh tomato production. Performance data for approved sanitizers that reflect commercial systems are needed to set standards for audit compliance. This study was conducted to evaluate the efficacy of chlorine dioxide (ClO(2)) for water disinfection as an objective assessment of recent industry-adopted standards for dump tank and flume management in fresh tomato packing operations. On-site assessments were conducted during eight temporally distinct shifts in two Florida packinghouses and one California packinghouse. Microbiological analyses of incoming and washed fruit and dump and flume system water were evaluated. Water temperature, pH, turbidity, conductivity, and oxidation-reduction potential (ORP) were monitored. Reduction in populations of mesophilic and coliform bacteria on fruit was not significant, and populations were significantly higher (P < 0.05) after washing. Escherichia coli was near the limit of detection in dump tanks but consistently below the detection limit in flumes. Turbidity and conductivity increased with loads of incoming tomatoes. Water temperature varied during daily operations, but pH and ORP mostly remained constant. The industry standard positive temperature differential of 5.5°C between water and fruit pulp was not maintained in tanks during the full daily operation. ORP values were significantly higher in the flume than in the dump tank. A positive correlation was found between ORP and temperature, and negative correlations were found between ORP and turbidity, total mesophilic bacteria, and coliforms. This study provides in-plant data indicating that ClO(2) can be an effective sanitizer in flume and spray-wash systems, but current operational limitations restrict its performance in dump tanks. Under current conditions, ClO(2) alone is unlikely to allow the fresh tomato industry to meet its microbiological quality goals under typical commercial conditions.

Assessment of root uptake and systemic vine-transport of Salmonella enterica sv. Typhimurium by melon (Cucumis melo) during field production

Among melons, cantaloupes are most frequently implicated in outbreaks and surveillance-based recalls due to Salmonella enterica. There is limited but compelling evidence that associates irrigation water quality as a significant risk of preharvest contamination of melons. However, the potential for root uptake from water and soil and subsequent systemic transport of Salmonella into melon fruit is uncharacterized. The aim of this work was to determine whether root uptake of S. enterica results in systemic transport to fruit at high doses of applied inoculum through sub-surface drip and furrow irrigation during field production of melons. Cantaloupe and honeydew were grown under field conditions, in a silt clay loam soil using standard agronomic practices for California. An attenuated S. enterica sv. Typhimurium strain was applied during furrow irrigation and, in separate plots, buried drip-emitter lines delivered the inoculum directly into the established root zone. Contamination of the water resulted in soil contamination within furrows however Salmonella was not detected on top of the beds or around melon roots of furrow-irrigated rows demonstrating absence of detectable lateral transfer across the soil profile. In contrast, positive detection of the applied isolate occurred in soil and the rhizosphere in drip injected plots; survival of Salmonella was at least 41 days. Despite high populations of the applied bacteria in the rhizosphere, after surface disinfection, internalized Salmonella was not detected in mature melon fruit (n=485). Contamination of the applied Salmonella was detected on the rind surface of melons if fruit developed in contact with soil on the sides of the inoculated furrows. Following an unusual and heavy rain event during fruit maturation, melons collected from the central area of the beds, were shown to harbor the furrow-applied Salmonella. Delivery of Salmonella directly into the peduncle, after minor puncture wounding, resulted in detection of applied Salmonella in the sub-rind tissue below the fruit abscission zone. Results indicate that Salmonella internalization from soil and vascular systemic transport to fruit is unlikely to occur from irrigation water in CA production regions, even if substantially above normal presumptive levels of contamination. Although contaminated irrigation water and subsequently soil in contact with fruit remains a concern for contamination of the external rind, results suggest an acceptable microbial indicator threshold and critical limit for the presence of Salmonella in applied water may be possible by defining appropriate microbiological standards for melon irrigation in California and regions with similar climate, soil texture, and crop management practices.

Acidified sodium chlorite optimisation assessment to improve quality of fresh-cut tatsoi baby leaves.

BACKGROUND The fresh-cut industry commonly uses sodium hypochlorite (NaClO) for disinfection. However, there are certain problems related to its use, and acidified sodium chlorite (ASC) could be an alternative sanitiser to replace it. There is limited research evaluating the effect of ASC on the overall quality of fresh-cut produce, especially sensory quality. In this study the decontamination efficacy and quality attribute effects of ASC on fresh-cut tatsoi after application and during storage were investigated. RESULTS Tatsoi baby leaves were minimally processed at 8 °C and stored under passive modified atmosphere packaging for up to 11 days at 5 and 10 °C. Low to moderate doses of ASC (100-500 mg L⁻¹) showed an initial antimicrobial efficacy on natural microflora and Escherichia coli as effective as that of NaClO. Regarding contact time, ASC was effective in reducting the E. coli population during the first 30 s of washing, and an increase in contact time did not improve the antimicrobial effect. Sensory quality attributes were well kept for up to 11 days at 5 °C but for only 5 days at the abusive temperature of 10 °C. CONCLUSION ASC provides an alternative sanitising technique to NaClO for maintaining the quality and safety of fresh-cut tatsoi baby leaves for up to 11 days at 5 °C.

Growth of Salmonella enterica in foliar pesticide solutions and its survival during field production and postharvest handling of fresh market tomato

To evaluate in vitro the growth kinetics of Salmonella enterica in pesticide solutions labelled for fresh market tomato and the effect of ag‐chemical application with contaminated water to tomatoes during field production.