Introduction:
Fresh fruit and vegetable harvesting, postharvest handling and cooling, packing and processing activities that involve the use of water have a higher potential to amplify the extent of contamination by plant pathogens and microbes of food safety concern. Small errors in contamination prevention and water disinfection procedures can have severe consequences due to the ease of spread of microbes, particularly in recirculated water systems.

Accurate monitoring and recording of disinfection procedures is an important component of a sound postharvest quality and safety program. Oxidation-Reduction Potential (ORP), measured in millivolts (mV), has recently been introduced to fresh produce packers and shippers as an easily standardized approach to water disinfection for harvest and postharvest handling. Operationally much like a digital thermometer or pH probe, ORP sensors allow the easy monitoring, tracking, and automated maintenance of critical disinfectant levels in water systems.

The purpose of this article is to provide a brief overview of the application of ORP monitoring to postharvest sanitation processes and describe the relationship of mV values to traditional standards relying on estimates of ppm (parts per million) of active disinfectant.

Overview:
Disinfection of water is a critical step to minimize the potential transmission of pathogens from a water source to produce, among produce within a lot, and between lots over time. Water-borne microorganisms whether postharvest plant pathogens or agents of human illness can rapidly move from a limited point source to non-contaminated produce. Natural plant surface contours, natural openings, harvest and trimming wounds, and handling injuries can serve as points of entry for microbes. Within these protected sites, microbes are unaffected by common postharvest water treatments such as chlorine, chlorine dioxide, ozone, peroxide, peroxyacetic acid, UV-irradiation and other approved treatments. It is essential, therefore, that the water used for washing, cooling, transporting, postharvest drenches, or other procedures be maintained in a condition suitable for the application. The standards for microbial quality of the water increase as product moves from the field to final packaging. This is particularly true for recirculating water systems, such as hydrocoolers or ice-injection systems. Some specific applications, such as water sprays onto the surface of a field packed commodity (example: cauliflower harvest operations often included a dilute chlorinated water spray and protective film overwrap of the trimmed heads) require the maintenance of high water quality at the moment of harvest.

Monitoring is an essential control point procedure to ensure the disinfection potential of water that is used for cleaning surfaces or is intended for intimate produce contact. Traditionally, the most widely used water treatment, chlorine or hypochlorite, has been monitored by qualitative assessments of ppm (parts per million) total and/or free available chlorine (see DANR publication #8003). Titration kits, or more commonly chemical impregnated paper strips, estimate the range of antimicrobial forms of chlorine (the most effective is hypochlorous acid or HOCl) in the water solution. There is no test kit that differentiates the more active HOCL from the far less active ionic form, hypochlorite (OCl-) [See discussion of pH effects on HOCL to OCl- balance below].

Similar colorimetric test kits are also available for ozone monitoring in water.

Recordkeeping to document effective antimicrobial conditions for any harvest or postharvest process may be a log sheet or checklist. Periodic sampling schedules based on experience with the specific commodity and dynamic conditions (soil, plant debris, fluids or solids from damaged product, or other factors) can be effective if trained personnel adhere to established protocols.

Practical experience provides compelling concerns that proper process control and protocols are not always followed. Accurate chlorine estimation generally requires more detailed and time consuming procedures than many operators will commit. Since chlorine tests do not distinguish HOCl and OCl-, it is also important to monitor and control the pH of the water system. The dynamic balance of the two forms of hypochlorite in water changes dramatically between pH 6.5 and 8.0. The faster acting antimicrobial form, HOCl, exists as 95 to 80% of the “free chlorine” detected with the paper test strips at pH 6.5 to 7.0. This level drops to less than 20% at pH higher than 8.0. Therefore, although a strong color reaction on the test paper or colorimetric kit is observed during monitoring, the effectiveness of the disinfectant is far less at high pH. This is particularly problematic for applications with short contact times. “Rules of thumb” based on odor or visual cues are rarely predictive of microbial disinfection. Continuous flow systems employed without monitoring may apply unnecessary, undesirable, potentially unhealthy, or unlawful levels of disinfectant to water systems. Even when monitoring is practical, too often no record of disinfection potential of the water is kept.

Advantages of ORP
Oxidation-Reduction Potential (ORP) offers many advantages to “real time” monitoring and recording of water disinfection potential, a critical water quality parameter. Improvements in probe design and continuous analog recording (paper strip or revolving chart) or computer-linked data input are available. Probes have been integrated to audible, visual and remote alarm systems to notify the operator of out-of-range operation. ORP is ideal for automated injection systems and can be combined with pH control injections to optimize performance. Hand-held devices are affordable and essential back up to cross-reference the operation of an in-line probe.

A primary advantage is that using ORP for water system monitoring provides the operator with a rapid and single-value assessment of the disinfection potential of water in a postharvest system. Research has shown that at an ORP value of 650 to 700 mV, spoilage bacteria and bacteria such as E. coli and Salmonella are killed within a few seconds. Spoilage yeast's and the more sensitive type of spore-forming fungi are also killed at this level after a contact time of a few minutes or less. Expanded studies of ORP:Contact Time for a range of postharvest pathogens are in progress.

Frequently asked questions
How does ORP relate to ppm?
ORP does not relate directly to ppm because it measures the oxidizing activity of the water and not the concentration of the oxidizer (chlorine, ozone, other oxidizing disinfectants). As shown in the figures below, as the concentration of chlorine increases the ORP values increase but at a slower rate of change. In a similar manner, as the pH is altered to increase the relative proportion of the antimicrobial HOCL concentration (low pH, more acid) the ORP value increases. An ORP value of 650mV measured at pH 6.5 or 8.5 provides the same killing potential, although the ppm “free chlorine” needed is greater at the higher pH value.

A ten-fold increase in ppm chlorine (10ppm to 100ppm of added NaOCl) does not result in a linear increase in mV; the ORP probes approach their saturation capacity and reach a plateau. The higher oxidation potential is reflected in higher mV values, up to the maximum capacity of the specific probe.

What are the effective ORP ranges?
Detailed determinations of effective ORP values for microorganisms of concern to postharvest quality, shelf life, and food safety are not yet available in a scientifically reviewed form. Studies completed to date strongly support the establishment of 650 mV as the minimum threshold value for typical anti-bacterial activity. This value of 650 mV is consistent with the standards that were developed and have been used in parts of Europe since mid-1980 for municipal drinking water quality. Maintaining this ORP will provide rapid inactivation of soft-rot Erwinia and seudomonas bacteria as well as other non-spore-forming microorganisms. Resistant fungal spores or parasitic oocysts will require higher ORP values and/or longer contact times.

How do contact times get involved?
Disinfectants often have several mechanisms of action that will be lethal to microbes at different rates. One of the fastest acting mechanisms is oxidation. A strong oxidizer, such as ozone, or a strongly oxidizing condition, such as concentrated hypochlorite or chlorine dioxide, will rapidly steal electrons from the microbial membrane resulting in the loss of its vital functions. Under milder oxidizing conditions, between 500 and 600 mV, bacterial inactivation will occur but only after much longer contact exposure.

Does pH affect ORP?
The effect of pH is on the activity of the specific disinfectant being used for water treatment. ORP expresses the measure of this activity under the variously interacting water constituents. In this way, it is easier to define and maintain a required disinfection potential using ORP than by using ppm and pH. Chlorine is strongly pH dependent (see above), ozone is moderately sensitive to pH, and chlorine dioxide is least sensitive.

Should I still measure ppm?
With any process control, but especially a critical control point for food safety programs, it is important to develop systems to crosscheck disinfectant levels. Standard paper strips or colorimetric test kits or, if using a panel-mounted ORP probe, a recently calibrated hand-held probe should be used periodically to assure that the primary ORP readings are accurately monitoring the desired conditions.

How does ORP work?
ORP meters measure the very small voltages generated when the measuring probe is placed in water in the presence of an oxidizing agent. The electrode is made of platinum or gold, which reversibly loses its electrons to the oxidizer. A voltage is generated which is compared to a silver electrode in a silver salt solution, similar to a pH probe. The more oxidizer available, the greater the comparative voltage generated between the two probes.

Does ORP behave differently for chlorine vs. ozone?
Any specific ORP value describes the oxidation potential of the water, irrespective of the source or nature of the disinfectant in use. Our experience with model systems thus far, however, is that ORP measurement is more straightforward in chlorinated water than ozonated water. Chlorinated water maintains a relatively constant ORP until the “chlorine demand” of suspended organics and inorganics exceeds the capacity to maintain free chlorine in the water. In contrast, in laboratory studies, ozonated water stabilized at 800mVfell rapidly to 250mV following the introduction of bacterial contaminants or organic material to the water. Bacteria were not recoverable (nonviable) within the few seconds necessary to conduct the first sampling. A high oxidation potential from ozone was clearly available in the water and ozone injection continued throughout the study. Surprisingly, the ORP probe could not measure the rate of reaction. As available oxidation was completed the ORP values climbed slowly back to the original 800 mV level.