to ORP as the Standard of Postharvest Water Disinfection Monitoring
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.
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
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.
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].
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.