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Using Oxidation Reduction Potential (ORP) for Water Disinfection Monitoring, Control and Documentation

Large volumes of water are primary approach to standardizing water commonly used during the postharvest disinfection parameters. Operationally much handling and processing of minimally like a digital thermometer or pH probe, ORP processed fruits and vegetables. Economic sensors allow the easy monitoring and considerations and wastewater discharge tracking of critical disinfectant levels in regulations make water recirculation a water systems. Coupled with pH sensors, common practice in the industry. Few more sophisticated systems use ORP sensors practices have the capacity of water to provide automated “demand-based” recirculation to increase the potential risk injection of hypochlorite (or other approved of food-borne illness by readily oxidizing disinfectant) and acid, typically distributing a point source contaminant citric acid (See Cautions below). In this (one lot, one bin, or even one plant) to section we provide a brief overview of the non-contaminated produce. Disinfection application of ORP monitoring to of water is a critical step to minimize the postharvest sanitation processes and potential transmission of pathogens from a describe the relationship of mV values to water source to produce, among produce traditional standards relying on estimates of within a lot, and between lots over time. ppm (parts per million) of active Water-borne microorganisms whether disinfectant. postharvest plant pathogens or agents of Benefits of ORP Systems human illness can be rapidly acquired and taken up on plant surfaces. Natural plant Oxidation-Reduction Potential surface contours, natural openings, harvest (ORP) offers many advantages to “real and trimming wounds, and handling injuries time” monitoring and recording of water can serve as points of entry for microbes. disinfection potential, a critical water quality Within these protected sites, microbes are parameter. Improvements in probe design unaffected by common postharvest water and continuous analog recording (paper strip treatments such as chlorine, chlorine or revolving chart) or computer-linked data dioxide, ozone, peroxide, peroxyacetic acid, input are available. Record-keeping can UV-irradiation and other approved become a largely automated activity. treatments. It is essential, therefore, that the Evaluation of process control by fluctuating water used for washing, cooling, water quality, product, and season, for transporting, postharvest drenches, or other example, are easier with the graphic outputs procedures be maintained in a condition of available systems. Probes have been suitable for the application. The standards integrated to audible, visual and remote for microbial quality of the water increase alarm systems to notify the operator of out- as product moves from the field to final of-range operation. Hand-held devices are processing. affordable and essential back up to cross- reference the operation of an in-line probe. Accurate monitoring and recording of disinfection procedures is an important A primary advantage is that using component of a sound postharvest quality ORP for water system monitoring provides and safety program during product cooling the operator with a rapid and single-value and processing. Many fresh cut processors assessment of the disinfection potential of have adopted Oxidation-Reduction Potential water in a postharvest system. Research has (ORP), measured in millivolts (mV), as a shown that at an ORP value of 650 to 700 Suslow 10-00 2

mV, spoilage bacteria and bacteria such as ORP set point. This makes sense from a E. coli and Salmonella are killed within a cost, sensory quality, safety, and few seconds. Spoilage yeast and the more environmental responsibility perspective. sensitive types of spore-forming fungi are also killed at this level after a contact time Disadvantages of ORP Systems of a few minutes or less. Unfortunately, resistant spore-forming decay pathogens and Potential disadvantages of ORP- human parasites, such as Cryptosporidium, based systems are largely operational issues are highly tolerant of chlorine, bromine, related to the routine of equipment iodine and other weak oxidizers or maintenance, calibration, and crosschecking metabolic poisons used for water of fixed position sensors. In practical disinfection. If hazard analysis identifies the terms, always have a back-up system of potential for the presence of these parasites, calibrated hand-held ORP probes and treatment with peroxyacetic acid or standard ppm kits. Sensors become fouled ozonation of source water would be a and need periodic cleaning and calibration. suitable control measure. A combination of ORP probes may become temporarily ORP and chemical indicator monitoring for saturated by over-injection of disinfectant. It ozone concentrations would be necessary. can take several minutes or longer for the sensor to come back to equilibrium with the A practical benefit in postharvest surrounding water, which can limit the uses (such as transport flumes, bin- response time. drenchers, cooling flumes, hydrocoolers, water-spray vacuum cooling, ice production, It is important to use ORP as a and ice injection) is that the measured ORP “window” of operation rather than a fixed values accurately define the antimicrobial point. Sensors rarely establish a fixed point potential of the water for free-floating in a real system. The ‘bounce” observed in microbes. More conventional systems of the sensor readout may be as much as 25 measuring parts per million (ppm) with mV, especially in hand held units, titration kits or paper test strips can give the depending on whether the probe is stationary same information but these must be or in movement. The size of the sensing combined with a measurement of water pH surface will also influence the fluctuations in and reference to a table of hypochlorous readings. Better sensors have a larger acid (HOCl) availability. The water pH detection area. The best approach is to becomes an essential variable since the standardize a uniform method for taking color-based test kits and paper strips detect measurements and set thresholds for a hypochlorous acid and hypochlorite ion window of operation that achieves the equally. Recent research in commercial and microbial reduction objectives appropriate model postharvest water systems has shown for the operation. that, if necessary, ORP limits can be relied on to determine microbial kill potential ORP and Ozone across a broad range of water quality. In a clean water system, using ORP It is important to point out that to measure the dissolved ozone status works monitoring the build up of inorganic and well. In our experience, however, the strong organic particles remains important to oxidizing power of ozone in complex (even prevent the excessive application of chlorine moderate turbidity) systems can result in or other disinfectants to maintain a constant ORP values far below expected and even negative (reducing) values. In general, Suslow 10-00 3

monitoring ozone with ORP at the generator ORP of 3 ppm NaOCl from pH 6 - 9 source works well but measuring wash water becomes unreliable. Ozone detection kits are 800 available and work well at the limit of approved concentrations for produce cooling 7.0 6.5 and washing operations 700 7.5 6.0

8.0 ORP, pH, and ppm 600

The routine measurement of ORP in ORP (mV) 8.5 millivolts is not a linear relation at typical 500 use rates in the produce industry. In general, 250A a ten-fold increase in total or free chlorine 9.0 Quikchek concentrations will not result in a 400 corresponding proportional increase in 0123 millivolts. This is predominantly a [HOCl] (ppm) familiarity and comfort issue rather than one that impacts safety standards. For clean water, 3 to 5 ppm free chlorine will provide more than adequate microbial control for Relation Between ORP and Free Chlorine @pH 7.0 free floating bacteria in a very short contact in Filtered Well Water time. This water quality will likely result in 6 measurements of 650-700 mV ORP if the water pH is 6.5 to 7. Lowering the pH to 6.0 5 will raise the ORP as more hypochlorous 4 acid becomes available. Raising the pH to 3 8.0 will lower the ORP value as more 2 hypochlorite ion is present. Maintaining Free Cl (ppm) 1 constant pH but adding more chlorine will 0 raise the ORP to a plateau of 900 to 950 0 200 400 600 800 mV, generally around 25 ppm free chlorine. ORP (mV) Doubling the free chlorine will not result in a sizeable gain in ORP and may result in undesirable disinfection-by-products, product damage, and flavor tainting. Relation Between ORP (mV) and Free Chlorine @ pH 7.0 in Filtered Well Water Excessive chlorination, especially at pH below 6.8, will often create an 250 uncomfortable and potentially unhealthy 200 environment for workers. For most 150 postharvest systems, it is unnecessary to 100 operate above 800 mV, a set point used in 50 Free Cl (ppm) primary wash and cooling systems where 0 high concentrations of inorganic and organic 0 200 400 600 800 1000 matter, or harvest and processing wound ORP (mV) exudates are released to the water.

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Program (USDA CSREES Agreement # 99-41560- 0821)

Summary of Results from Various Lab Simulation and Commercial Hydrocooler Survey Studies

Survival at ORP (mV) Pathogen/Indicator < 485 550 < x < 620 > 665 E. coli O157:H7 > 300 s < 60 s < 10 s

Salmonella spp. >300 s > 300 s < 20 s

L. monocytogenes >300 s > 300 s < 20 s

Thermotolerant > 48h > 48h < 30 s coliform

Prepared by Trevor V. Suslow, Ph.D. Extension Specialist Postharvest Quality and Safety from Seed to Shelf Dept. of Vegetable Crops University of California [email protected] &http://vric.ucdavis.edu

Research support from the California Asparagus CAUTION: Recent evidence from Commission, California Lettuce Research Board, USDA/ARS strongly suggests that citric California Melon Research Board, California Tree Fruit Agreement, and California Tomato acid may interfere with the lethal action Commission and industry collaborators that of hypochlorous acid at levels likely to contributed to the development of baseline result in ORP ≤ 650-700 mV. Consider information summarized in this leaflet is gratefully using inorganic food grade acids (ex. acknowledged. Partial support for the production and distribution of this brochure is contributed by muriatic or phosphoric) the National Good Agricultural Practices Training

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ORP 675 mV pH Strong Oxidizers pull electrons away from platinum probe creating a small voltage differential to the reference probe.

Platinum Reference Electrode Electrode

Embedding matrix