Establishment of operating standards for produce wash systems through the identification of specific metrics and test methods

The main objective of this proposal is helping producers to maintain the quality of the process water in commercial washing systems through control of water quality variables and the selection of adequate test methods for monitoring the process. Water disinfection is one of the most critical processing steps in fruit and vegetable production aimed at preventing cross-contamination. In the packinghouses and processing facilities, it is difficult to treat and maintain properly the quality of the process water because of the variability in the demand of disinfectant, the lack of operational limits and test methods to monitor the process and the different commercial operations. This project will investigate the most common disinfection agents used in packinghouses and processing facilities. Four scenarios have been selected based on different water characteristics including fresh-cut onions (excessive cell exudates, very high organic matter and turbidity), chopped lettuce (high organic matter and low turbidity), baby leaves (low organic matter and low turbidity) and peppers and tomatoes (low organic matter and high turbidity). Operational limits will be established commercial facilities and lab-scale experiments using inoculated foodborne pathogens. Results obtained should inform producers about the realistic expectations for controlling selected water quality variables in produce washing systems.

The establishment of scientifically-based critical operating standards for specific washing systems can be successfully performed when based on a sound experimental design, which usually require carrying out testing in the producer's facilities. Critical factors suitable to establish monitoring standards need to be identified and controlled for each specific washing system under current industrial conditions. Additionally, a greater understanding of in-practice dose measurement in relation to water quality and commodity-specific parameters are needed to select the most adequate test methods for operational monitoring standards. Therefore, collaboration with industrial partners aware of the need of evidence-based standards for produce wash systems is mandatory to successfully identify critical factors. This proposal is based on this type of collaboration with up to seven industry cooperators. The main objective of this proposal is the establishment of critical monitoring standards for different produce wash systems and the adequate test methods of the selected metrics. Disinfection agents commonly used in packinghouses and processing facilities including sodium hypochlorite, calcium hypochlorite, T-128 (2nd generation process aid), stabilized pure chlorine dioxide and peroxyacetic acid (PAA) will be evaluated in four different produce wash waters with very different water characteristics including fresh-cut onions (excessive cell exudates conferring very high organic matter and turbidity), chopped lettuce (high organic matter and low turbidity), baby leaves (low organic matter and low turbidity) and whole peppers and tomatoes (low organic matter and high turbidity). Each case scenario of produce wash system, which includes a combination of disinfection agent/fresh commodity, will allow us to understand the relevance of different water quality variables for the identification of specific metrics and test methods. Selection of water quality variables (e.g. sanitizer residual, oxidation reduction potential (ORP), chemical oxygen demand (COD), UV Absorbance, turbidity, pH, conductivity) able to monitor the efficiency of the disinfection system are still needed for these produce wash systems that represent a whole range of different situations. Selected water quality variables will be proposed as critical parameters to control disinfection of produce wash under commercial industrial practices. Validation of critical limits for specific produce wash water will be performed under lab scale conditions to demonstrate that the proposed variables are able to control cross-contamination mimicking industrial conditions. Different fed rate of product in relation to the absolute volume of water in the stage as well as the rate of make-up water will be determining considering the wash system as a reactor to be modeled. Within this project, a greater understanding of on-line measurements of commercially available sensors in relation to water quality characteristics and commodity-specific parameters will be also determined. The proposed objective will be performed in collaboration with six Industry Cooperators. A close collaboration between fresh produce producers and producers of wash systems has been scheduled in order to evaluate specific case studies. The main beneficiaries of the project will be the growers who will be guided using evidenced-based standards for washing of specific commodities.

1. Identification of specific water quality variables suitable as critical parameters and their critical and optimal operational limits for water disinfection under commercial conditions. Monitoring will be performed in commercial facilities covering 98% of the most commonly used disinfection treatments (e.g. chlorine-based sanitizers and peroxyacetic acid) in multiple commodity types representing the best and worse scenarios of process wash water. 

2. Validation of critical limits of the selected critical parameters. Validation of critical parameters will be performed in lab-scale experiments using artificially inoculated foodborne pathogens. 

3. Performance of commercial sensors and test kits for the on-line monitoring of the critical parameters that can function adequately because of the accuracy, precision and low range of error. 

4. Verification of critical and operational limits. Verification of the selected critical and operational limits will be carried out on-line to confirm the performance of the monitoring standards used to control the microbiological quality of the process wash water. 

5. Elaboration of evidence-based standards for the most common disinfectants based on specific metrics and test methods for microbiological quality of produce wash systems, to meet the immediate need in the process control for produce industry operators.

The critical parameters that affect the performance of different wash systems (defined by factors like the disinfectant used and the product washed) were determined. The operational limits for the different parameters in different wash systems were established. Suitable methods for the measurement of the selected parameters were assessed and selected. The efficacy of the established operational limits for the inactivation of pathogenic microorganisms was verified. Finally, evidence-based standards for different wash systems were proposed. In the case of chlorine, residual FC and total chlorine, pH, and organic matter would be the most critical parameters. The organic matter, on-line monitoring of UV254, which correlates with the COD, could be used for many types of produce wash water. In the case of PAA, the level of disinfectant is the only critical factor to be controlled, as organic matter and pH are not related to the disinfection efficacy. Water replenishment is necessary to reduce the volume of sanitizer added and dilute the organic matter accumulated in the wash water, which directly affects the formation and accumulation of DPBs. The lower operational limit for FC varies between different types of produce wash water. It ranges from 10 mg/L (for onion wash water), to >30 mg/L for wash water of baby leaves. The lower operational limit for PAA would be higher than 200 mg/L for peeled garlic, and higher than 400 mg/L for bell peppers. The commercial electrochemical sensor PAASense (Palintest, Gateshead, UK) is suitable for measuring PAA in a wide range of concentrations in produce wash water with high organic content. The electrochemical method Chlorosense (Palintest), and a DPD-based method (Spectroquant, Merck, Darmstadt, Germany) are appropriate sensors for FC and total chlorine.