Reassessing postharvest water management for pathogen infiltration/internalization

Ensuring the safety of fresh produce is essential, and postharvest water management plays a key role in preventing pathogens from infiltration and internalization during produce washing. Current industry guidelines require that wash water be warmer than the produce to reduce the risk of bacterial infiltration, such as Salmonella enterica in tomatoes and Listeria monocytogenes in netted melons. However, maintaining strict temperature differentials is challenging due to seasonal and daily variations in produce temperature.
This project will evaluate whether these temperature requirements are necessary when alternative safety measures such as antimicrobial treatment and reduced immersion time, are in place. Specifically, it will test whether maintaining 150 ppm of free chlorine at pH 6.5-7.5 and limiting immersion time to under 30 seconds can prevent bacterial internalization, even when temperature differentials exist. Additionally, the research will examine commercial washing methods for tomatoes and cantaloupes, including spray systems that eliminate full submersion, to assess their effectiveness in reducing infiltration risk. By reevaluating current standards, metrics and operational limits, this project aims to provide the industry with more flexible, cost-effective, and practical food safety measures, helping to reduce operational challenges while maintaining consumer health protection.

Ensuring the safety of fresh produce requires effective postharvest water management strategies that minimize the risk of pathogen infiltration. Current industry guidelines recommend that wash water for product immersion be at least 5°C (or 10°F) warmer than the product to prevent bacterial internalization. However, maintaining these temperature differentials poses operational challenges, increases costs, and complicates audit compliance due to the highly variable pulp temperature of incoming produce across different seasons and even within daily operations.
This project aims to provide science-based evidence to determine whether strict temperature differential management is necessary or if other risk-reduction strategies such as maintaining adequate antimicrobial concentrations and reducing immersion time, can effectively prevent contamination. By offering packers a more practical approach to process control, this research seeks to improve the management of infiltration risks. The project setup will combine controlled laboratory experiments with commercial-scale research in collaboration with six industry partners. Laboratory experiments will assess Salmonella enterica infiltration in tomatoes under 20 different conditions, varying tomato temperatures (10°C, 20°C, 30°C, and 40°C) and water temperature differentials (-10°C, -5°C, 0°C, +5°C, and +10°C) at two immersion times: 2 minutes (Objective 1) and < 30 seconds (Objective 2). Five experimental trials, conducted monthly with different tomato types and harvest time characteristics, will follow industry guidelines (150 ppm free chlorine at pH 6.5-7.5). Wash water collected from a commercial tomato packing line will be inoculated with a low-level (1 CFU/mL) four-strain cocktail of Salmonella enterica. Physicochemical analyses of water samples will include pH, temperature, oxidation-reduction potential (ORP), electrical conductivity (EC), total dissolved solids (TDS), and chemical oxygen demand (COD). After submersion, tomatoes will be surface-sterilized with 70% ethanol, and core tissue from beneath the stem scar will be analyzed for Salmonella detection following enrichment. The project will determine whether maintaining proper antimicrobial concentrations and limiting immersion time to less than 30 seconds can prevent bacterial infiltration, even when wash water is cooler than the tomato pulp. In parallel, commercial-scale trials (Objective 3) will evaluate microbial infiltration risks in greenhouse-grown tomatoes and netted melons under real-world packinghouse conditions. The commercial practices will compare water-based handling systems, such as dump tanks, to spray-wash systems, which eliminate full immersion. Microbial analyses will detect Salmonella, L. monocytogenes, E. coli, and Listeria spp in both wash water and core
tissue. The hypothesis is that properly maintained spray-washing systems will significantly eliminate the internalization of any microorganisms present in the water. 
This project will provide the fresh produce industry, including tomato and melon growers and packers, with critical data to support more practical and cost-effective food safety protocols. The anticipated outcomes may lead to revised industry guidelines, shifting the focus from temperature control to maintaining adequate antimicrobial levels and minimizing immersion time. Additionally, this research will offer regulators and auditors science-based justification for refining compliance requirements, ensuring that food safety practices remain both practical and effective while achieving public health protection.

Objective 1: Reassessing temperature differential standards

Objective 2: Assessing the role of immersion time in infiltration

Objective 3: Evaluating infiltration risks in commercial packinghouses

This project is ongoing. A final report will be provided when the project is finished.