For a more complete discussion of water disinfection,
see Postharvest Chlorination (ANR Publication 8003).
Briefly, the proper use of a disinfectant in postharvest
wash and cooling water can help prevent both
postharvest diseases and foodborne illnesses. Because
most municipal water supplies are chlorinated and the
vital role of water disinfection is well recognized,
organic growers, shippers, and processors may use
chlorine within specified limits. All forms of chlorine
(e.g., liquid sodium hypochlorite, granular calcium
hypochlorite, and chlorine dioxide) are restricted materials
as defined by existing organic standards. The
application must conform with Maximum Residual
Disinfectant Limit under the Safe Drinking Water Act,
currently 4 mg/L (4 ppm) expressed as Cl2. The
California Certified Organic Farmers (CCOF) regulations
have permitted this threshold of 4 ppm residual
chlorine, measured downstream of the product wash
(due to food safety concerns, CCOF has recently modified
this threshold to permit 10 ppm residual chlorine
measured downstream of the wash step). Growers certified
by other agencies should check with their certifying
agent.
As a general practice, field soil on product, bins,
totes, and pallets should be kept to a minimum by prewashing
the produce before loading it. This will significantly
reduce the demand for disinfectant in the water
and lower the total required volume of antimicrobial
agents. Prewashing also removes plant exudates
released from harvest cuts or wounds, which can react
rapidly with oxidizers such as hypochlorite and ozone,
and so requires higher rates of the chemical to maintain
the target 4 to 10 ppm downstream activity.
For both organic and conventional operations, liquid
sodium hypochlorite is the most common form used.
For optimum antimicrobial activity with a minimal concentration
of applied hypochlorite, the pH of the water
must be adjusted to between 6.5 and 7.5. At this pH
range, most of the chlorine is in the form of hypochlorous
acid (HOCl), which delivers the highest rate of
microbial kill and minimizes the release of irritating
and potentially hazardous chlorine gas (Cl2). Chlorine
gas will exceed safe levels if the water is too acidic.
Products used for pH adjustment also must be from a
natural source such as citric acid, sodium bicarbonate,
or vinegar. Calcium hypochlorite, properly dissolved,
may provide benefits of reduced sodium injury to sensitive
crops (e.g., some apples varieties), and limited
evidence points toward extended shelf life for tomatoes
and bell peppers due to calcium uptake. Amounts of
sodium hypochlorite to add to clear, clean water for
disinfection are given in the table on the next page.
Ozone is an attractive option for water disinfection
and other postharvest applications. Ozonation is a