Water Disinfection
A Practical Approach to Calculating Dose Values for Preharvest
and Postharvest Applications
Trevor V. Suslow, Postharvest Specialist, University of California, Davis,
Department of Vegetable Crops, Mann Lab
University of California
Agriculture and Natural Resources
Publication 7256
WHY WATER DISINFECTION IS NEEDED
Clean, disinfected water is necessary to minimize the
potential transmission of pathogens from water to pro-
duce, from healthy to infected produce within a lot,
and from one lot to another over time. Waterborne
microorganisms, including postharvest plant
pathogens and agents of human illness, can be rapidly
acquired and taken up on plant surfaces. Natural plant
surface contours, natural openings, harvest and trim-
ming wounds, and scuffing can be points of entry as
well as safe harbor for microbes. In these protected
sites, microbes are largely unaffected by common or
permitted doses of postharvest water treatments, such
as chlorine, chlorine dioxide, ozone, peroxide, and per-
oxyacetic acid. Therefore, it is essential that enough
sanitizer is maintained in water to kill microbes before
they attach or become internalized in produce. This is
important in some preharvest water uses and in all
postharvest procedures involving water, including
washing, cooling, water-mediated transport (flumes),
and postharvest drenching.
MINIMUM EFFECTIVE DOSES
Standards for the microbial quality of water should
increase closer to harvest maturity and as produce
moves from the field to final processing. However,
excessive treatment, particularly hyperchlorination
(use of high levels of chlorine), has several known and
potential negative effects on product sensory quality,
the environment, and human health. Water treatment
should be managed with the goal of minimizing the
effective dose of sanitizer used for microbial disinfec-
tion. Minimum effective doses are typically represent-
ed as the product of Concentration ( C ) and Time of
exposure (t), or C t . Following the same principles, the
term disinfection hurdle ( Dh) can be used to help guide
water quality management. The disinfection hurdle is
the minimum point at which there is enough free
active disinfectant available to neutralize microbial
activity to an acceptable level.
CHLORINE AND HYPOCHLORITE (BLEACH)
TREATMENT
Ease of use and relative low cost make hypochlorite
(usually liquid sodium hypochlorite) a very common
water disinfectant in the produce industry. The antimi-
crobial activity of chlorine compounds depends largely
on the amount of hypochlorous acid (HOCl) present in
the water after the treatment is applied. This, in turn,
depends on the pH of the water, the amount of organic
material in the water, and, to a more limited extent, the
temperature of the water. Above pH 7.5, very little
(
becomes inactive hypochlorite (OCl-). With very long
contact time, OCl- does have some antimicrobial activi-
ty but would not be expected to result in beneficial con-
trol in typical postharvest handling systems. Below pH
6.0, noxious chlorine gas (Cl2) is formed and does not
serve as an effective water disinfectant. Of the many
possible forms of chlorine, HOCl is the most readily
transferred across a microbial cell wall to begin the
killing process. Thus, in the management of chlorine, it
is important to maximize HOCl concentrations and
minimize all other forms of chlorine. It is highly desir-
able to keep the pH of the water between 6.0 and 7.5 to
ensure adequate HOCl activity without the formation
of chlorine gas, which can lead to health problems for
workers and more corrosion on equipment.
The amount of HOCl needed to maintain the most
active antimicrobial action depends on several dynamic
factors. Chlorine is very reactive, combining with
almost any oxidizable material to form secondary com-
pounds. The amount of chlorine needed for disinfection
of water depends not only on the pH but also on the
amounts and kinds of inorganic (particularly ammonia,
nitrites, iron, and manganese) and organic (particularly
amino acids and simple proteins) substances present in
the water. Because chlorine is rapidly used up by
organic and inorganic molecules in wash water, a mini-
mum level of total chlorine, the chlorine demand, ( g e n e r-
ally influenced by soil, plant “trash,” and exudates
from cut surfaces) must first be satisfied in the water
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before sufficient amounts of free available chlorine can kill
microorganisms. The treatment times of fruits and veg-
etables are usually very short. To minimize the poten-
tial for excessive chlorination at peak chlorine demand,
it is important to periodically replace or filter the water
and blend it with potable water.
CALCULATING REQUIRED HOCl ADDITIONS
In clean water, very low levels of HOCl kill most bacte-
ria and some viruses. Approximately 1 minute with 1 to
2 parts per million HOCl should be sufficient contact
time. As water quality decreases and complexity
increases, contact time or concentration must increase
to maintain adequate microbial kill. Because contact
times during postharvest handling are usually deter-
mined by product flow requirements, it is the concen-
tration of the added disinfectant that is adjusted.
Effective concentrations of HOCl (or other forms of
chlorine) should ideally be determined by microbial
testing within each system. Water quality management
is often perceived as a time-consuming and costly activ-
ity; however, it is strongly recommended that it receive
a high level of attention.
As a starting point, the calculations below can be
made to determine the minimum concentration of free
available chlorine as HOCl in wash or cooling water
that is needed to kill free-floating pathogenic bacteria
and viruses. The calculations are based on an adapta-
tion of tables that were developed to achieve potable
water quality standards in treated water. Higher levels
of HOCl or other treatments are needed for B a c i l l u s o r
C l o s t r i d i u m spores and for parasites like G i a r d i a a n d
C r y p t o s p o r i d i u m.
Use table 1 to calculate the target measured concen-
tration of HOCl and match it with the contact time of
the system to establish the effective dose.
Example
Based on the contact time of the system, calculate the
HOCl concentration necessary for an effective dose. For
washing and cooling, the system has a 5-minute resi-
dence time ( t ) , and the water has a pH that is constant
around 7.5 to 7.8 without adjustment when product is
running through the system. The water temperature is
maintained at 34° to 38°F (1.1° to 3.3°C). The table
shows that the disinfection hurdle, Dh, is 20. Using
these known values in the following equation, solve for
C , the minimum HOCl concentration.
Using this result, free available chlorine can be mea-
sured using a titration kit or colorimeter specific for free
available chlorine. At pH 7.8 and 34° to 38°F (1.1° to
3.3°C), only 50 percent of the measured free available
chlorine, by the commonly used methods, is in the
desired HOCl form. Therefore, a minimum reading of 8
parts per million is needed to hit the targeted disinfec-
tion hurdle.
It is easy to see the impact of water pH on C. In the
same example, adding citric acid to maintain pH 7.0,
C = 2.75 parts per million (HOCl is 87% of free chlorine
at pH 7.0). It is common for pH to increase to 8.5 during
hypochlorite treatment, resulting in C = 41 parts per
million (HOCl is 17.5% of free chlorine at pH 8.5).
It is important to remember that test kits for measur-
ing free available chlorine are only suitable for concen-
trations of up to 4 parts per million. It is necessary to
dilute any treated water with distilled water to bring it
into a measurable range and then to multiply the result
by the dilution factor. Typical dilutions are 1:10
although a dilution of 1:100 may be necessary where
concentrations of disinfectant are high due to the
increased importance of controlling fungal spores.
Always follow the instructions provided by the test kit
s u p p l i e r .
It is easiest to adjust and standardize water pH to 6.5
to 7.0, causing the majority of free available chlorine to
convert to the HOCl form. Consulting a second table of
pH and temperature then becomes unnecessary.
Measurements of HOCl in water may also be adequate-
ly determined by using a calibrated ORP (oxidation
reduction potential) sensor. As in the first example, 4 to
6 parts per million HOCl typically gives a sensor read-
ing of 725 to 750 millivolts.
2 • Water Disinfection: A Practical Approach to Calculating Dose
Values for Preharvest and Postharvest Applications
C = Dh ÷ t or 20 ÷ 5 = 4 mg /L (4 ppm)
Remember, the calculated amount may be very
different from the actual dose of total hypochlo-
rite solution added to the system at peak
demand. The target Dh is determined by the sen-
sitivity of the most resistant microbe being man-
aged. (Erwinia soft rot bacteria and E. coli are rela-
tively sensitive, Geotrichum sour rot and
R h i z o p u s are much more resistant.) Microbe sensi-
tivity must be determined by direct testing
because sensitivity charts calibrated to this sys-
tem of calculating effective doses are not yet
a v a i l a b l e .
Table 1. Guidelines for meeting the disinfection hurdle
in postharvest water treatment*
Value of Dh in C x t = Dh
Water pH range 32° to 41°F 50°F
7.0–7.5 12 8
7.5–8.0 20 15
8.0–8.5 30 20
8.5–9.0 35 22
*Values given are the product of concentration of HOCl
and time of exposure of a diversity of microbes in water to
achieve greater than 99 percent kill. The value t is deter-
mined by the specific process or operation and assumes
adequate mixing to accomplish uniform exposure.
Source: Modified from White 1992 and reflect results from lab-
oratory and field research data.
Table 2. Current projected value of Dh in postharvest
water at pH 7.0
T a r g e t Typical contact
m i c r o o r g a n i s m 32° to 41°F time (minutes)
N o n - s p o r e - f o r m i n g
b a c t e r i a 3 – 6 1–5
Many viruses 3 – 1 0 1–5
Many yeasts 7 5 – 1 0 0 1 0 – 3 0
S p o r e - f o r m i n g
b a c t e r i a 1 5 0 – 2 5 0 1 5 – 6 0
Fungal spores 1 5 0 – 5 0 0 1 5 – 6 0
Parasite spores
G i a r d i a 3 0 – 1 0 0 5 – 1 0
Cryptosporidium highly tolerant use UV or ozone
Source: Modified from White 1992 and reflect results from
laboratory and field research data.
GLOSSARY OF TECHNICAL TERMS
Antimicrobial activity. The effectiveness of a sanitizer
or disinfectant in killing microorganisms.
C o r r o s i v e. The capacity of an element to weaken or eat
away at equipment, especially metal.
D i s i n f e c t i o n. The act of adding or applying a sanitizer
to kill microorganisms that cause decay in produce or
illness in humans.
Disinfection hurdle. A descriptive concept term that
symbolizes the minimum effective exposure to achieve
microbial kill. Disinfection is one of several hurdles in a
prevention, reduction, and contamination control pro-
Water Disinfection: A Practical Approach to Calculating Dose • 3
Values for Preharvest and Postharvest Applications
gram. The disinfection hurdle is different for different
types and classes of microorganisms.
P a t h o g e n s. Microorganisms such as bacteria, fungi,
parasites, and viruses that can cause disease in humans
or plants.
Peak chlorine demand. The maximum amount of chlo-
rine in a batch of water that is occupied, or “used up,”
by inorganic and organic material. After the peak chlo-
rine demand is known, it can be better established how
much more chlorine or more clean water should be
added to maintain the target disinfection hurdle.
Additional steps, such as minimizing adhering soil,
prewashing, or filtration may be necessary to reduce
the peak chlorine demand.
Potable water. Water that is clean enough to be consid-
ered drinkable.
Product sensory. Characteristics of a product, in this
case fresh produce, related to smell, taste, appearance,
and texture.
R e a c t i v e. A chemical that is especially reactive is one
that does not stay in one form for very long. In the case
of water disinfection, it is important that chlorine stay
in a particular form (HOCl) in order to be effective,
making the reactivity of chlorine of particular interest.
S a n i t i z e r. A chemical that is added or applied, in this
case to water, in order to kill pathogens. A surface or
water can be sanitized and free of pathogens, but sani-
tizing does not make the material or the water sterile.
S e n s i t i v i t y. The sensitivity of a system or test refers to
the lowest concentration that the system or test can
detect or respond to. For example, if a chlorine test can
only detect concentrations of chlorine at or higher than
1 ppm, the system’s sensitivity is said to be at 1 ppm.
ADDITIONAL INFORMATION
Below are some of the many articles, research papers,
and reviews available about this broad topic.
Suslow, T. 1997. Postharvest chlorination: Basic
properties and key points for effective sanita-
tion. Oakland: University of California Division
of Agriculture and Natural Resources,
Publication 8003. http://anrcatalog.ucdavis.edu
Suslow, T. 1998. Introduction to ORP as the standard
of postharvest water disinfection monitoring.
http://vric.ucdavis.edu (Go to Vegetable
Information and click on Topics: Food Safety.)
Suslow, T. 1998. Prevention of postharvest water
infiltration into fresh market tomatoes: Food
safety and spoilage control practices.
http://vric.ucdavis.edu (Go to Vegetable
Information and click on Topics: Food Safety.)
White, G. C. 1992. Handbook of chlorination and
alternative disinfectants. 3d ed. New York: Van
Nostrand Reinhold.
http://anrcatalog.ucdavis.edu/
http://vric.ucdavis.edu/
http://vric.ucdavis.edu/
You’ll also find detailed information on many aspects
of postharvest technology in these titles and in other
publications, slide sets, and videos from UC ANR:
Postharvest Biology of Horticultural Crops: An Overview,
slide set 84/117
Postharvest Chlorination: Basic Properties and Key Points
for Effective Distribution, publication 8003
Postharvest Technology of Horticultural Crops, 2d edition,
publication 3311
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Publication 7256
© 2001 by the Regents of the University of California,
Division of Agriculture and Natural Resources.
All rights reserved.
4 • Water Disinfection: A Practical Approach to Calculating Dose
Values for Preharvest and Postharvest Applications
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WATER DISINFECTION
Why Water Disinfection Is Needed
Minimum Effective Doses
Chlorine and Hypochlorite (Bleach) Treatment
Calculating Required HOCl Additions
Table 1. Guidelines for meeting the disinfection hurdle in postharvest water treatment
Table 2. Current projected value of Dh in postharvest water at pH 7.0
Glossary of Technical Terms
Additional Information
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