To minimize food safety risks associated with microbial pathogens, fruits and vegetables that have visible soil or organic matter residues on their surface should be thoroughly washed before sale. Washing should be done with potable water only. Washing with potable water is a simple way to reduce microbial populations on the surface of fresh fruits and vegetables.

If potable water is not available, wash water should be chlorinated. Removal of soil and organic debris prior to washing in chlorinated water (by brushing or pre-washing) is essential because organic matter reacts with chlorine in solution to lower its effective concentration. The more organic matter in solution, the more chlorine is tied up.

The purpose of chlorination is to drastically reduce the number of microorganisms in the water and thus reduce or prevent inoculation with post-harvest diseases or contamination with human pathogens. To do this, chlorine must be in all water that contacts the produce prior to it being packed or sold. Chlorine should also be used in cleanup water to sanitize packing equipment, picking containers, and other sites that come in direct contact with the produce.

Chlorine gas, sodium hypochlorite (household bleach), and calcium hypochlorite (chlorinated lime) are the most common disinfectants containing chlorine in general use. All these chlorine containing chemicals form the same active ingredient, hypochlorous acid, when added to water. The disinfectant activity of a chlorine solution is determined by its pH. The pH level should be maintained between 6.0 and 7.0 to provide for greatest effectiveness without damage to produce and equipment. At a higher pH, chlorine is not a very effective germicide. At a lower pH, the solution becomes corrosive to metals. If the pH is decreased below 4.0, deadly chlorine gas is formed. Thus, the pH of the wash water should be checked periodically. This may be done with litmus paper (which changes color at different pH's), available test kits, or pH meters. The pH can be adjusted to a desired level by adding a weak acid or base, as is done in swimming pools.

It is essential that the concentration of active chlorine in the fruit or vegetable wash water be monitored so that an adequate level can be maintained. This may be done continuously with automatic equipment, or it may be done intermittently with commercial chlorine test kits. Avoid kits used to check chlorine in swimming pools since these are designed for low levels, between 2 to 6 ppm. The most effective treatment results when chlorine is metered into the wash water continuously, whereas the most ineffective method is to dump some in the tank occasionally. In the later case, the chlorine concentration can vary tremendously, and during rush periods when the need for chlorine is most critical, the tendency is to forget it. Although 50 ppm of chlorine is probably more than adequate if that concentration is maintained, the general recommendation is for 100 to 150 ppm. Higher concentrations provide some protection against unusual conditions resulting from overloading the systems or from loads containing excessive amount of organic matter.

To make an effective sanitizing agent containing approximately 70 ppm chlorine for use on fresh produce, food handling equipment and machinery add one pint of household bleach (5.25% sodium hypochlorite) to 50 gallons of water. Adding 2 pints of 40 grain distilled white vinegar should adjust the solution's pH to between 6 and 7. Adding 1/4 pint wetting agent, such as liquid soap, will help the chlorine get into the nooks and crannies on the surface of produce.

Note that the national organic standards board has proposed that certified organic vegetables be washed with chlorine solutions not exceeding 4 ppm chlorine, which is the maximum allowed in drinking water under the Safe Water Drinking Act. It is questionable whether this low level of chlorine will provide effective protection against pathogens on produce.

Chlorine is highly poisonous and must be handled with care according to instructions. For the safety and comfort of the workers, provisions must be made for adequate ventilation to remove chlorine fumes from enclosed packinghouses. All workers handling chlorine and chlorine equipment should use protective equipment recommended for the particular form that is being used. Read the label before opening any container.

References:

Coon, S. "Chlorination Cuts Postharvest Losses." American Vegetable Grower, Sept. 1990.

Hicks, J.R. and R.H. Segall. "Water Chlorination for Vegetable Packinghouses". Fact Sheet
VC-1, University of Florida Cooperative Extension, Gainsville, FL

Schlimme, D. "Cleaning and Sanitizing Fresh Produce and Fresh Produce Handling Equipment, Utensils and Sales Areas". Fact Sheet 715. University of Maryland Cooperative Extension, College Park, MD

Suslow, T. "Microbial Food Safety is Your Responsibility!". Dept. of Vegetable Crops, University of California, Davis (web site info)