Sanitizers for Vegetables in Harvest and Post-Harvest Water for Small Farmers

Harvest/Post-Harvest Water

The U.S. Food and Drug Administration’s (FDA) Produce Safety Rule (PSR) defines agriculture water as “water intended to, or likely to, contact the harvestable portion of covered produce or food- contact surfaces” (FDA, 2023). Furthermore, the PSR divides agricultural water into pre-harvest water and water used in production activities (harvest/post- harvest). For example, cleaning water for grading lines, wash tanks, spray lines, or any other water used for cleaning purposes is harvest/post-harvest. Harvest/post-harvest water must meet two conditions before it can be used.

The water must be “safe and of adequate sanitary quality for its intended use” and meet the performance standard of zero detectable generic Escherichia coli (E. coli) per 100 mL of water. It is important to maintain your post-harvest water to comply with the PSR. Even if a farm is exempt from the PSR, this is still a best practice worth maintaining. There are several ways to maintain post-harvest water, including adding clean water to the dump tank (overflow) or emptying and refilling the dump tank when the water gets too cloudy or dirty (high turbidity). Additionally, chemical sanitizers can be added to the post-harvest water to help reduce the number of microorganisms and produce a cleaner product. This publication will focus on commercially available sanitizers that have chlorine, peracetic acid, or hydrogen peroxide formulations for use in produce production.

Sanitizer Use Factors

Adding sanitizers to post-harvest water is an important risk management strategy since sanitizers can reduce microorganisms’ levels in production water and decrease the risk of any cross- contamination between produce. It is important to know specific attributes of post-harvest water because it can dictate your decision for which sanitizer to use, such as:

• Water characteristics (pH level and mineral content).
• Water temperature.
• Amount of total dissolved solids in the water (soil, dirt, other debris).
• Contact time of the sanitizer with the produce.
• Surface texture of the produce being washed.

Chlorine

Chlorine is commercially available as an over-the- counter product ready to purchase. Make sure to check the label to confirm it is approved for use with food. It is inexpensive, and effective when properly used. However, it can corrode stainless steel and aluminum and is pH dependent. It degrades in sunlight and heat, making proper storage paramount.

Two forms of chlorine can be purchased for mixing with post-harvest washing water. The first is calcium hypochlorite (CaCl₂O₂) which comes as a tablet or powder. The second form is liquid sodium hypochlorite (NaOCl) commonly called bleach.

When either of these compounds are dissolved in water they form hypochlorous acid, which can kill the target microorganisms on the surface of the produce. Both products are commonly used by small to medium growers.

An example of chlorine formulation is to prepare a chlorine mixture concentration of 100-150 ppm of free chlorine with clean post-harvest water at a pH between 6.0 and 7.5. The pH matters because if it is outside the recommended range, it will be less effective due to a change in the charge of the chlorine molecules. Make sure to remove all physical and organic materials (organic load) with the produce beforehand since this will diminish the concentration of the free chlorine in the solution. 1-2 minutes of contact with the produce is recommended, followed by a rinse with clean water. The washing techniques will be determined by what vegetables are being washed. Some vegetables may be cleaned best by submersion and others may need a wash and rinse.

Peracetic Acid

Peracetic acid (PAA) is a mixture of hydrogen peroxide and acetic acid. The advantages of PAA are that it is noncorrosive to postharvest equipment (stainless steel and aluminum), effective at varying pH levels, and not as sensitive to organic matter as chlorine. The disadvantages are that PAA is more expensive than chlorine and loses its sanitizing effectiveness when the pH is not within the range of 3 to 7.5 and when the water temperature is outside the range of 32 F to 104 F. It is important to note that PAA for fresh vegetables should never exceed 80ppm when used for washing.

Farmers can mix a solution for a submersion clean in the water at a concentration of 60 ppm. The submersion technique is used by having a wash bin and two rinse tanks. Make sure to remove as much excess soil or organic matter with a hose before adding the produce to the wash bin that has the sanitizer solution. Once added, the produce must be left in the solution for at least 1 minute. Afterward, move the produce to the next two rinse buckets to remove any residual sanitizer. Allow the produce to drip dry. Before reusing the sanitizer and wash bins, test your water’s pH, turbidity, and temperature before adding new clean rinse water and/or new PAA solution as needed. Be sure to consistently check the PAA levels in the water since PAA can break down quickly.

Hydrogen Peroxide

Hydrogen Peroxide, also called hydrogen dioxide, can be used as a sanitizer for fruits and vegetables in either a liquid or gaseous form. An advantage of hydrogen peroxide is that it is “generally recognized as safe” (GRAS) by the FDA and Environmental Protection Agency (EPA) and is environmentally friendly due to its ability to break down water and oxygen molecules. Hydrogen peroxide is affected by the organic load in the post-harvest water, but not by pH level.

In recent years operations have adopted a method of combining hydrogen peroxide and PAA when sanitizing post-harvest water. It is recommended that no more than 59 ppm of a sanitizer mixture of hydrogen peroxide and PAA is applied to fresh produce. This can be used on other produce as well, but not for all produce. Be sure to check the label before using this sanitizer for certain produce, such as cucumbers.

Monitoring Post-Harvest Water

When it comes to monitoring your post-harvest water for effectiveness, it is paramount to remember that the quality of the water changes over time. Consistent testing is key to preserving the quality of the washing water. Consistently keeping records of additions, changes, or other modifications to the post-harvest water will help to keep producers aware of its state.

To start, make sure your bulk/batch water tank as well as the water in it are clean by conducting an E. coli test to make sure it meets the PSR standard for clean water when using surface or well-water. Surface water should be tested at least three times per season and well water tested at least one time per season. Municipal water does not need to be tested. Check pH levels with pH strips, turbidity levels with a turbidity meter, and remove any organic materials (leaves, rocks, and other field debris) you may find in the batch/bulk tank. Check the water’s temperature to make sure it is appropriate for the produce and sanitizer’s temperature recommendations to avoid infiltration.

Infiltration is when the outer layer of the produce becomes compromised and lets potential contamination enter the inside of the produce. Additionally, be sure to routinely clean and sanitize any bins, tanks, sprayers, and other washing line equipment routinely to stop cross-contamination. Lastly, be sure to keep records of cleanings to make sure that proper monitoring and cleaning are continually completed.

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