The use of redox potential to estimate free chlorine in fresh produce washing operations: possibilities and limitations

Authors: VAN HAUTE, SAM - US Department Of Agriculture (USDA); Zhou, Bin; Luo, Yaguang - Sunny; SAMPERS, IMCA - Ghent University; VAN HAVERBEKE, MARTIJN - Ghent University; Millner, Patricia

Submitted to: Postharvest Biology and Technology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/11/2019
Publication Date: 7/17/2019
Citation: Van Haute, S., Zhou, B., Luo, Y., Sampers, I., Van Haverbeke, M., Millner, P.D. 2019. The use of redox potential to estimate free chlorine in fresh produce washing operations: possibilities and limitations. Postharvest Biology and Technology. https://doi.org/10.1016/j.postharvbio.2019.110957.
DOI: https://doi.org/10.1016/j.postharvbio.2019.110957

Interpretive Summary: Chlorine is used in industrial ready-to-eat fresh-cut produce washing to avoid spread of harmful bacteria. In commercial processing of fresh fruits and vegetables, very large amounts of produce are continuously cut and washed in correspondingly large amounts of chlorinated water. However, the soil, plant debris, and juices from the harvested fruits or vegetables react with the chlorine in the wash water so much that the concentration of free chlorine (FC) needed to prevent the spread of bacteria in the water becomes to low to be effective. Thus, chlorine needs to be added back to the water during the produce washing process. However, to rapidly determine how much chlorine to add to maintain the concentration needed to avoid the spread of harmful bacteria during fast-paced commercial produce wash operations is very challenging. The fresh produce industry often uses oxidation-reduction potential (ORP) to estimate the FC concentration in the water; ORP sensors are immersible and easily configured to signal the process control equipment and pumps to inject chlorine into the water. Even though FC can be the dominant contributor to the ORP signal, other chemical constituents in the water matrix can influence ORP, but their influences during fresh produce washing are poorly understood. In this study, we show that differences in pH, temperature, and FC led to changes in ORP that could be accurately predicted by modeling. Using fresh-cut carrot, onion, romaine and iceberg lettuce, as well as whole tomatoes, as model food products, we showed that ORP decreases when the amount of produce washed increased, even when FC was kept. constant. This effect is complex and could not be explained by common water quality parameters such as turbidity, chemical oxygen demand, and chlorine demand. In addition, the type of acid used to maintain pH during the washing process also influenced the ORP. This study shows that the relationship between ORP and FC depends on the specific industrial washing process, including the particular operational configuration and conditions, water source used, and commodity, and should be determined individually by the processor. Fresh-cut produce washing operators will find the guidance provided in this report useful in determining the predictive relationship between ORP and FC for their specific washing process conditions and commodities, since no single target ORP value is applicable to all commodities and produce washing operations.

Technical Abstract: Maintaining free chlorine (FC) residual at appropriate pH values is a control approach used to prevent pathogen cross-contamination during tomato dump tank handling and fresh-cut produce washing operations. Oxidation reduction potential (ORP) measurements often are used to easily and rapidly estimate FC residual, but ORP is influenced by all redox reactions, not exclusively by those with chlorine. This study examined the relationship between ORP and FC under ideal conditions and during fresh produce washing. An equation predictive of FC was developed in the form logFC = f(ORP, ORP2, ORP.pH). A good correlation between ORP and logFC was maintained when other variables changed, but the resulting ORP-logFC curve changed (slope, intercept). A decrease in pH or temperature led to an increase in ORP. Using tap water to wash the produce instead of distilled water significantly changed the ORP. For different types of tested produce, i.e., fresh-cut carrot, onion, romaine and iceberg lettuce, and for whole tomatoes, increasing the product-to-water ratio (i.e., increasing the organics transferred into the water) led to a decrease in ORP for a specific FC residual. The choice of acidulant during washing also influenced ORP. The large number of factors influencing the ORP-logFC relationship precludes reliance on a single ORP target value to meet conditions for control of pathogen cross-contamination among multiple commodities and situations. Instead, process operators will benefit by following guidance on how to determine the ORP-logFC relationship given specific washing process conditions and commodities. In conclusion: i) the correlation of ORP with logFC is more reliable at the lower end (5 mg/L FC) than at the higher end (100 mg/L FC) of the FC range used in fresh produce washing, ii) an ORP-logFC relationship can be predictive of FC when determined for a specific fresh produce operational system and specific produce type.