Postharvest
Chlorination
Basic Properties and Key Points for
Effective Disinfection
TREVOR SUSLOW
Extension Specialist, Department of Vegetable Crops, University of California, Davis
Postharvest handling of many vegetables and fruits usually involves the use offlumes, water dump tanks, spray washers, or hydrocoolers. Most postharvest
processes recirculate used water (process water) to conserve water and energy. Dirt,
organic matter, and disease-causing pathogens can accumulate in process water dur-
ing bin dumping, hydrocooling, and flume recirculation.
Disinfection is the treatment of process water to inactivate or destroy pathogen-
ic bacteria, fungi, viruses, cysts, and other microorganisms. The goal of disinfection
is to prevent the transfer of these organisms from process water to produce and from
one produce item to another during postharvest handling, increasing the likelihood
that the produce is microbiologically safe for human consumption. Disinfection may
employ chemicals such as chlorine, iodine, ozone, or peroxide, or it may use phys-
ical processes such as microfiltration or ultraviolet illumination.
Disinfection is part of an overall sanitation and safety management program.
Chlorination of process water is one of the primary elements of a properly managed
postharvest sanitation program. In conjunction with an overall safety management
program, chlorination is generally effective, comparatively inexpensive, and may be
implemented in operations of any size.
Chlorination alone is not a sanitation program—it is best viewed as a way
to minimize the transmission of pathogens from infested produce or debris to
noninfested surfaces such as harvest or process cuts, wounds, or natural plant
surface openings.
F O R M S O F C H L O R I N E I N W AT E R
Chlorine (Cl) is a very potent disinfectant with powerful oxidizing properties. It is
soluble in water, either by injection of chlorine gas or by the addition of hypochlo-
rite salts (see fig. 1). This solution, called chlorine (or chlorinated) water, consists
of a mixture of chlorine gas (Cl2),
hypochlorous acid (HOCl), and
hypochlorite ions (OCl–) in amounts
that vary with the water pH. The terms
free chlorine, reactive chlorine, and
(more correctly) available chlorine are
used to describe the amount of chlorine
in any form available for oxidative reac-
tion and disinfection. Available chlorine
P U B L I C A T I O N 8 0 0 3
UNIVERSITY OF
CALIFORNIA
Division of Agriculture
and Natural Resources
http://danrcs.ucdavis.edu
NaOCl + H2O ↔ NaOH + HOCl
HOCl ↔ H+ + OCl–
HOCl + HCl ↔ H2O + Cl2
Figure 1. Forms of chlorine in water.
http://danrcs.ucdavis.edu
does not include chlorine combined with ammonia or other less readily available
forms of chlorine with weak antimicrobial activity such as chloramines.
Total chlorine refers to the total available and combined chlorine that is present
in water and still available for disinfection and oxidation of organic matter. Although
combined chlorine compounds are more stable than available chlorine forms, they
are slower in disinfectant action. In process water, the desired form of chlorine is
hypochlorous acid, which is a much more effective bactericide than the hypochlo-
rite ion.
The degree of acidity or alkalinity of a solution as measured on a scale of 0 to 14
is known as pH. The midpoint of 7.0 on the pH scale represents neutrality; that is,
a neutral solution is neither acid nor alkaline. Values below 7.0 indicate acidity; val-
ues greater than 7.0 indicate alkalinity.
Although hypochlorous acid concentra-
tion is highest at pH 6.0 (table 1), the
best compromise of activity and stability
is achieved by maintaining a water pH
between 6.5 and 7.5. At low pH, chlorine
gas is released from water.
Chlorine may incompletely oxidize
organic materials to produce undesirable
byproducts in process water, such as
chloroform (CHCl3) or other tri-
halomethanes, that have known or sus-
pected carcinogenic potential. At high
pH, chlorine reacts with organic nitro-
gen-based materials to produce chlo-
ramines. From a U.S. government regula-
tory perspective, the benefits of proper
chlorination as a primary tool for sanita-
tion outweigh concern for the potential presence of these byproducts. The use of
chlorination for produce washing (chlorinated water in direct contact with produce)
has been banned in a few countries other than the United States and may affect the
export of chlorinated produce. Assistance and information on export regulations
may be obtained from the FDA Center for Food Safety and Applied Nutrition (200
C St. SW, Washington, D.C. 20204).
Concern for the potential hazards associated with chlorine reaction byproducts
and wastewater disposal have heightened efforts to evaluate and register alternative
water disinfection and surface sanitizer treatments for produce and postharvest han-
dling. These are discussed briefly under “Other Disinfectants,” below.
R E G I S T E R E D P R O D U C T S
Chlorine is commercially available in three forms that have been approved for use
(registered) by the U.S. Environmental Protection Agency (EPA) and, for California,
by the California Department of Pesticide Registration (DPR). For a discussion of
other disinfectants, including some that may not be registered, see “Other
Disinfectants,” below.
Chlorine Gas (Cl2)
Chlorine gas is the least expensive but most demanding source of chlorine from
a safety and monitoring standpoint. Generally restricted to use in very large operations,
POSTHARVEST CHLORINATION: Basic Properties and Key Points for Effective Disinfection 2
WARNING
Never combine chlorine
with ammonia or acety-
lene, as poisonous chlorine
gas can be produced.
pH of Approx. % Approx. %
process of chlorine of chlorine
water as HOCl as OCl-
3.5 90 0
4.0 95 0
4.5 100 trace
5.0 100 trace
5.5 100 trace
6.0 98 2
6.5 95 5
7.0 78 22
7.5 50 50
8.0 22 78
8.5 15 85
9.0 4 96
9.5 2 98
10.0 0 100
Table 1. Activity of
chlorine forms in water
of varying pH
the use of chlorine gas requires automated, controlled injection systems with in-line
pH monitoring. Chlorine gas reduces the pH of water to below 6.5.
Calcium Hypochlorite (CaCl2O2)
Calcium hypochlorite is the most common source of chlorine used for disinfecting
produce and produce process water. Registered formulations are 65 percent or 68
percent active ingredient (a.i.). It is available as a granulated powder, compressed
tablets, or large slow-release tablets. In dry storage, calcium hypochlorite is more
stable than liquid sodium hypochlorite. Phytotoxicity (bleaching or burning) of pro-
duce can occur if calcium hypochlorite granules fail to dissolve in cool wash tank
water or in a hydrocooler system. Always dissolve granules in a small volume of
warm water before adding them to cooling or wash water. Calcium hypochlorite
increases water pH to slightly above 7.5.
Sodium Hypochlorite (NaOCl)
Sodium hypochlorite is the source of chlorine commonly used in small-scale opera-
tions. It is generally used in concentrations of 5.25 percent or 12.75 percent a.i. in
liquid form, because the solid forms readily absorb water from air and release chlo-
rine gas. Only registered formulations are approved for use on produce (household
bleach is not a registered material for produce). Sodium hypochlorite is generally
more expensive than other forms of chlorine due to the added shipping cost of the
water-based formulations. Excess sodium buildup from repeated applications of
sodium hypochlorite to recirculating water may damage sensitive produce. Sodium
hypochlorite increases water pH to above 7.5.
K E Y P O I N T S F O R P R O P E R C H L O R I N E D I S I N F E C T I O N
Water Source
Potable water should be used for all postharvest washing, grading, and cooling oper-
ations. Contaminated water used during postharvest operations can transmit dis-
eases that decay the produce or adversely affect human health. Water taken and used
directly from rivers or holding ponds should not be used for postharvest washing or
cooling. Because some pathogens of concern to human safety are not easily killed by
chlorination, even under optimal conditions, beginning with clean potable water is
the best preventive step available. The effectiveness of other disinfectant options,
such as ozonation and UV treatment of process water, is currently being evaluated
against these chlorine-resistant microorganisms. When using a nondomestic water
source, water quality evaluations should be performed by a certified analytical lab.
For further information, see A. E. Greenberg, ed., Standard Methods for the
Examination of Water and Wastewater, 19th ed. (Washington, D.C.: American Public
Health Assn., 1995).
Temperature
Although chlorine activity slightly increases with temperature, some chlorine gas is
lost to the atmosphere as warmer temperature increases the rate of volatilization.
Low temperature and improper pH values in hydrocooling, for example, can great-
ly reduce disinfection efficiency. In general, the need for rapid cooling to optimize
postharvest quality makes temperature adjustment of chlorinated process water for
optimizing disinfection activity unavailable as a management option.
Organic Matter
Chlorine is highly reactive with leaves, soil, and any plant or vegetable matter when-
ever oxygen is present. Each chemical reaction reduces the amount of active chlorine
POSTHARVEST CHLORINATION: Basic Properties and Key Points for Effective Disinfection 3
in the water. Changing chlorinated water frequently or filtering out organic matter
and debris is essential for effective sanitation. Also, prewashing very dirty produce
can prolong the useful life of chlorinated cooling water.
Concentration and Length of Exposure
Disinfection is best accomplished by deriving contact (exposure) times and concen-
trations through direct experience for each type of produce and local conditions.
Exposure times of 3 to 5 minutes at concentrations of 50 to 75 parts per million
(ppm) or less (1 to 1.5 ounces of calcium hypochlorite at 65 percent a.i. per 100 gal-
lons of water provides 50 to 75 ppm) maintained at pH 6.5 is generally adequate for
controlling most postharvest pathogens suspended in water.
Microorganisms differ in their sensitivity to chlorine: bacteria are most sensitive,
many fungal spores are less sensitive, and some spore-forming animal parasites are
highly insensitive. In practice, total chlorine concentrations may need to exceed 300
ppm to sustain sufficient available chlorine activity in process water throughout the
daily use cycle. The practical duration of contact exposure is generally 10 to 15 min-
utes. Caution must be used as some produce is sensitive to surface bleaching or pit-
ting at high concentrations. For example, bell peppers are not affected by 250 ppm
available chlorine but carrots may lose orange color intensity, and celery and aspara-
gus may develop light-brown surface pits when exposed to chlorine concentrations
exceeding 250 ppm.
Performance Enhancers
Chlorine kills only what it directly contacts. Water films that form on very small
contours on plant surfaces may prevent the chlorinated water from directly contact-
ing target microorganisms. Adding approved surfactants to process water reduces
water surface tension and may increase the effectiveness of chlorination. Consult a
postharvest chemical supplies dealer for available and approved materials. Examples
of these materials are polysorbate 80, other sorbitan esters, and Chlorine Potentiator
(Bonagra Technologies, Inc.).
Monitoring
The chlorine concentration and pH of chlorinated process water should be checked
frequently using test paper strips, colorimetric kits, or electronic sensors. The opti-
mal frequency of testing is best determined through on-site experience. In general,
monitoring should increase as the concentration of suspended materials in the water
increases. Different tests measure different forms of chlorine; some are accurate only
at very low concentrations. Dilution of most process water with distilled or deion-
ized water is required to obtain useful results from these tests. Select a chlorine test
kit that is based on DPD (N, N diethyl-p-phenylenediamine) that specifically tests
for available (reactive) chlorine. Become familiar with what is being measured and
how water quality affects the results. Muriatic (hydrochloric) acid (HCl) or citric
acid (C6H8O7) is commonly used to maintain wash or cooling water at a pH of 6.5
to 7.5. Consult a postharvest management service for designing an effective and safe
acid-injection system.
Some automated cooling systems monitor the oxidation reduction potential
(ORP) of process water using probes that measure activity in millivolts (mV). The
relationship between ORP, contact time, and microbial inactivation for chlorine-
based oxidizers in laboratory tests and field confirmation tests are used to establish
the setting for the system. For example, an ORP setpoint of 600 to 650 mV is com-
monly used in hydrocooling systems.
POSTHARVEST CHLORINATION: Basic Properties and Key Points for Effective Disinfection 4
POSTHARVEST CHLORINATION: Basic Properties and Key Points for Effective Disinfection 5
Table 2. Chlorine concentrations generally used on selected vegetables
Commodity Treatment type Available chlorine
(ppm)
Artichokes Sprayer over continuous belt 100–150
Asparagus Sprayer over continuous belt 100–150
Hydrocooler* 125–150
Bell peppers Sprayer over continuous belt 150–200
Dump tank 300–400
Broccoli Sprayer over continuous belt 100–150
Brussels sprouts Sprayer over continuous belt 100–150
Cabbage (shredded)† Sprayer over continuous belt 100–150
Carrots Sprayer over continuous belt 100–150
Flume 150–200
Cauliflower Sprayer over continuous belt 100–150
Celery Hydrocooler* 100
Sprayer over continuous belt 100–150
Corn Sprayer over continuous belt 75–100
Cucumbers Sprayer over continuous belt 100–150
Garlic (peeled)† Sprayer over continuous belt 75–150
Greens, chopped leafy Sprayer over continuous belt 100–150
Lettuce, butterhead Sprayer over continuous belt 100–150
Lettuce, iceberg
whole, shredded† Sprayer over continuous belt 100–150
Hydrovac cooler*
Lettuce, romaine Sprayer over continuous belt 100–150
Melons, all types Sprayer over continuous belt 100–150
Dump tank 100–150
Mushrooms‡ Sprayer over continuous belt 100–150
Onions, green Sprayer over continuous belt 100–150
Peas, pod-type Sprayer over continuous belt 50–100
Peppers, chili Sprayer over continuous belt 300–400
Potatoes, brown or red Flume 200–300
Dump tank (prewashed) 30–100
Sprayer over continuous belt 100–200
Potatoes, white Dump tank (for bleaching) 500–600
Pumpkins Sprayer over continuous belt 100–200
Radishes Sprayer over continuous belt 100–150
Dump tank 25–50
Spinach Sprayer over continuous belt 75–150
Sweet potatoes Dump tank (prewashed) 100–150
Squash, all types Sprayer over continuous belt 75–100
Tomatoes Flume 200–350
Dump tank 200–350
Turnips Dump tank 100–200
Yams Dump tank 100–200
Note: This table represents the combined range of concentrations from the product labels and technical
information of formulations currently registered in California. These concentrations are guidelines reflect-
ing industry practice; always follow directions, use rates, and tolerances listed on approved product
labels. Determine cultivar sensitivity within a given concentration range.
*For more information on hydrocooler chlorination see UC Perishables Handling Newsletter no. 84
(Nov. 1995) (special issue on hydrocooling), also available on the Internet at
http://postharvest.ucdavis.edu.
† Residual water must be removed by centrifugation or some other dewatering process following treatment.
‡ Not a common treatment. When used, follow with an antioxidant to prevent browning. Ascorbic acid or
erythorbic acid in combination with citric acid are examples of these antioxidants.
http://pom44.ucdavis.edu/postharv.html
Wastewater Disposal
A disposal plan for chlorinated process water must be in place before any chlorina-
tion system is used. Although land application has been allowed, always determine
if a local permit or other restrictions apply. The EPA Office of Wastewater
Management sets federal policy and standards for disposal of chlorinated process
water and the impacts of chlorinated byproducts in environmental water systems.
State and local water resource management agencies, air quality management
boards, and wastewater resource boards are responsible for regulatory oversight of
disposal issues.
Worker Safety
As a concentrate, chlorine gas is extremely dangerous and should be used only in
properly designed containment systems that are isolated from the processing plant
area. Chlorine fumes released from treated water will cause worker discomfort and
eye irritation. If a chlorine odor is even barely detectable by a worker just entering
an area where a chlorinated processing and cooling water system is operating, it is
likely that the maximum safe chlorine concentration has been reached. In addition
to being a health hazard, excessive chlorine reaction odor (odors from interaction
with organic amine compounds) or chlorine gas may also indicate improper pH
adjustment. The Occupational Safety and Health Administration (OSHA) (see web-
site http://www.osha-slc.gov) establishes and publishes the Threshold Limit Value
(TLV) and Short-Term Exposure Limit (STEL) for worker exposure to chlorine,
chlorine dioxide, ozone, and other hazardous materials.
O T H E R D I S I N F E C TA N T S
In the search for effective disinfectant treatments for process water sanitation,
the postharvest handling industry often operates within areas of regulatory uncer-
tainty. Some produce handlers and processors use chlorine dioxide and ozone for
sanitation and other postharvest applications, in part because of their characteristics
in relation to registered materials
(see fig. 2). For applications to whole
or peeled produce, handlers and
processors may be assuming that
these materials have an approved
Generally Regarded as Safe (GRAS)
status. Recent expert advisory panel
recommendations have made this
determination, but, to date, the
U.S. Food and Drug Administration
(FDA) has not released an official
determination on these materials.
Unlike chlorine gas, calcium hypochlorite, and sodium hypochlorite, no postharvest
uses of chlorine dioxide or ozone in contact with produce are currently registered
by the U.S. EPA or the California DPR.
Chlorine Dioxide (ClO2)
A yellow to red gas with 2.5 times the oxidizing potential of chlorine gas, chlorine
dioxide is explosive at concentrations above 10 percent a.i. or at temperatures above
130oC (266oF). It is normally diluted to less than 10 percent a.i. and shipped frozen
for many industrial uses, including water treatment and sanitation of food process-
ing contact surfaces. On-site generation of chlorine dioxide is also available by
combining either chlorine gas and sodium chlorite or sodium hypochlorite,
POSTHARVEST CHLORINATION: Basic Properties and Key Points for Effective Disinfection 6
OCl
Relative antimicrobial disinfection efficency
HOCl O3 ClO2
Strong Moderate Low
Efficacy impacted by pH
Figure 2. Disinfection
efficiency and efficacy
for chlorine forms.
http://www.osha-slc.gov
hydrochloric acid, and sodium chlorite. As with chlorine gas, the safety hazards
associated with the use of chlorine dioxide demand detailed attention to proper
engineering controls to prevent or reduce exposure. Violent explosions can occur
when chlorine dioxide comes into contact with ammonia compounds.
The disinfecting power of chlorine dioxide is relatively constant within a pH of
6 to 10. It is effective against most microbes at concentrations
