Chlorinated Water Sanitation of Leafy Green Vegetables For Fresh Produce Processors

AEX-262
Agriculture and Natural Resources
Date: 
05/05/2017
Ryan A. Gehringer, Graduate Student (Present address: ConAgra Foods), and Gönül Kaletunç, Professor, Food, Agricultural, and Biological Engineering

Following harvest, leafy green vegetables are treated with sanitizers to reduce bacteria that have come in contact with the produce prior to or during harvesting, and to prevent cross-contamination. Although several liquid sanitizers that could be used to decrease bacterial load—including chlorine dioxide, ozone and peroxyacetic acid—chlorinated water is the frequently used sanitizer in the fresh produce industry. Chlorinated water treatment is cost-effective and easiest to manage among all of the Environmental Protection Agency (EPA) approved sanitizers for fresh produce. Since organic material interferes with the disinfection capacity of chlorinated water, produce initially covered in dirt must be rinsed with cool water prior to the application of the sanitizer. Chlorinated water treatment alone reduces the bacterial load 1.3 to 1.7 log units, and usually is part of a sanitation program (2)

This fact sheet is intended to provide information to small food processors about sanitation of fresh produce. While chlorine bleach is used in commercial produce processing facilities, it is not recommended for home or foodservice use. 

What chlorine sources are commercially available to prepare chlorinated water?

Chlorinated water is produced by adding either chlorine gas, calcium hypochlorite (Ca(ClO)2), or sodium hypochlorite (NaOCl) to water (Table 1)(5).

Table 1. Comparison of three commercially available chlorine sources.
Chlorine source Purchasable Form Purchasable Concentrations Cost Safety
Chlorine Gas compressed liquid 100% free active chlorine low capital, low operation threat of gas leak or explosion
Calcium Hypochlorite tablet or granulated powder 65 or 68% free active chlorine high capital, high operation hazardous due to higher percent chlorine
Sodium Hypochlorite water-based formulation 5 to 15% free active chlorine high capital, high operation low level safety concerns

The cost of each sanitizer varies due to the active chlorine concentration and shipping cost. Because sodium hypochlorite readily absorbs water from air, it is sold in liquid form, increasing the cost (7). Although more costly, sodium hypochlorite is often favored because it is safer and is relatively easy to use.

How is chlorinated water prepared?

 
  Figure 1. smart-fertilizer.com/articles/chlorine (8)

Chlorinated water with a concentration range of 75 to 200 parts per million (ppm) active chlorine is commonly used (6). Active chlorine concentration of 200 ppm is the maximum amount allowed to be used by law (Code of Federal Regulations Title 21 Part 178) (1). Depending on the chlorine source used, the procedures below describe the necessary amounts of chlorine sources to produce 200 ppm chlorinated water.

Chlorine gas, stored in high-pressure vessels, can be injected into water by an automated system that requires close monitoring. The pH at the inlet must be carefully monitored at all times and pH must be adjusted as the use of chlorine gas results in a pH below 6.5. In large operations, where high levels of organic material and debris are present on the produce, chlorine gas is preferred (Figure 1). 

Calcium hypochlorite (Ca(ClO)2) is available in granulated powder or compressed tablets and contains 65 to 68 percent active ingredient. Powder or tablets are dissolved in water prior to treatment of fresh produce. An additional water tank containing warm water is often required to assure that the Ca(ClO)2 is completely dissolved prior to mixing with the cooler water for sanitation (6). If calcium hypochlorite is not completely dissolved, bleaching of the produce can be observed. The pH needs to be checked and lowered to assure the chlorinated water pH is below 7.5.

 
  Figure 2. smart-fertilizer.com/articles/chlorine (8)

Sodium hypochlorite (NaOCl), also known as bleach, is available as liquid sanitizer that can be mixed with water and used immediately. Since minimal monitoring is required, NaOCl is a common chlorine source for chlorinated water in smaller scale operations. Sodium hypochlorite will cause pH to rise above 7.5 so the pH must be reduced prior to treatment of produce (Figure 2).

For leafy green vegetables with a high surface area to volume due to crevices and uneven surfaces, liquid sanitizers may not have a direct contact with all surfaces contaminated by bacteria. Adding EPA-approved surfactants reduces the surface tension of the water to allow a uniform distribution of chlorinated water on leaf surface and deeper penetration into pores (6).

Is it safe to use any type of bleach when making chlorinated water?

Food grade bleach is the only bleach that is approved by the EPA for use in food processing. Food grade bleach is defined as sodium hypochlorite solution without any additives such as fragrances, stabilizers or thickeners. Always be sure to read the warning label and use the product only as directed.

How often does a batch of chlorinated water need to be changed?

Chlorinated water should be prepared fresh daily to assure the intended concentration is achieved. Once prepared, the disinfecting properties of the solution begin to degrade. The chlorine concentration and pH of chlorinated water should be monitored by using chlorine test strips or kits. It is important to choose the test strips or kits that measures free chlorine levels available to interact rather than total chlorine.

How should chlorine sources be stored?

Chlorine sources used to prepare chlorinated water should be stored in a cool, dark, dry place for no more than the times shown in Table 2.

Table 2. The ideal storage temperatures and times for chlorine sources.
Chlorine Source Maximum Storage Time Ideal Storage Temperature
Chlorine Gas infinite below 125°F
Calcium Hypochlorite several years below 70°F
Sodium Hypochlorite 3 months 50 to 70°F

Note that stabilizers are often added to sodium hypochlorite solutions to slow the rate of decomposition. Since food-grade bleach does not contain stabilizers, it has a shorter shelf life than household bleach with additives.

How is chlorinated water applied?

Chlorinated water is applied by either submerging or spraying the fresh produce. Submerging involves loading produce into a long channel filled with cool, chlorinated water, allowing for surface contact. Submersion for one minute, in a concentration of 75 to 200 ppm total chlorine, commonly achieves a microbial reduction of approximately 1.3 to 1.7 log units (2). Spray application of chlorinated water can occur during hydrocooling, in which case the produce is placed on a conveyor belt that simultaneously cools and sanitizes in a longer period of time.

Why is pH of chlorinated water important for effective sanitation?

The effectiveness of chlorinated water is highly dependent upon the pH of the solution. Depending on the pH of the solution, three products (Cl2, HOCl, or OCl-) can be present alone or in combination (Table 3) (4). At low pH values, toxic chlorine gas is produced, so the pH of the solution should never drop below 5. Values below pH 6.5 are corrosive to processing equipment and will reduce equipment life. Above pH 7.5, the formation of hypochlorite ion (OCl-) occurs and reduces the effectiveness of sanitation. The solution should be held between pH values of 6.5 to 7.5 to produce hypochlorous acid (HOCl). Recirculated chlorinated water should be monitored for pH and changed as needed (4).

Table 3. Percentage of three compounds obserced in chlorinated water at various pH levels.
pH % Cl2 % HOCl % OCl¯ Chlorinated Water Description
2 30 70 0   Dangerous Cl2 levels, too corrosive for equipment and food products  
3 5 95 0
4 0 100 0   Too corrosive for equipment and food products 
5 0 100 0
6 0 97 3
6.5 0 90 10   Ideal range
7 0 75 25
7.5 0 50 50
8 0 35 65   HOCl level too low for proper sanitation of food products
9 0 2 98
10 0 0 100

How is the pH of chlorinated water checked and adjusted?

Since all three methods of creating chlorinated water should result in pH values between 6.5 to 7.5, checking and adjusting the pH of the solution is important. Once the appropriate pH is achieved, it must be checked frequently so that adjustments can be made to offset the addition of the organic matter from the fresh produce. For an accurate pH reading, a pH meter should be used, but pH test kits and even litmus paper can provide general pH ranges. Since pH is crucial for proper sanitation of fresh produce, making accurate measurements is essential. If the pH is found to be outside of the necessary range, sodium carbonate is added to raise the pH, and hydrochloric acid, vinegar or sodium bisulfate is added to lower the pH (3). A test kit will allow the user to determine how much acid or base must be added to be within the desired range.

Is the produce ready for packaging after chlorine treatment?

After sanitation, fresh produce should be rinsed with a lower concentration (2 to 7 ppm) of chlorinated water by spray application or submersion in a separate tank (4). The low concentration of chlorinated water is used to prevent recontamination by recirculated water.

References

  1. 21 CFR Part 178 www.accessdata.fda.gov/scripts/cdrh/cfdocs/cfCFR/CFRSearch.cfm?fr=178.1010
  2. Beuchat, R. L., B. V. Nail., B. B. Adler., & M. R. S. Clavero. 1998. Efficacy of Spray Application of Chlorinated Water in Killing Pathogenic Bacteria on Raw Apples, Tomatoes and Lettuce. Journal of Food Protection, 61: 1305-1311.
  3. Grubinger, Vern. 2008. Post-Harvest Washing of Fresh Produce to Reduce Food Safety Risks. University of Vermont Extension.
  4. McGlynn, W. 2004. Food Technology Fact Sheet: Guidelines for the Use of Chlorine Bleach as a Sanitizer in Food Processing Operations. Oklahoma State University.
  5. Shah, J., & N. Qureshi. 2008. Chlorine Gas vs. Sodium Hypochlorite: What’s the best option? Disinfection. American Water Works Association. 24-27.
  6. Suslow, T. 1997. Chlorination in the Production and Postharvest Handling of Fresh Fruits and Vegetables. Fruit and Vegetable Processing. University of California – Davis, 6: 1-15.
  7. Suslow, T. 1997. Postharvest Chlorination Basic Properties and Key Points for Effective Disinfection. University of California-Davis. (ucfoodsafety.ucdavis.edu/files/26414.pdf)
  8. Chlorine chemicals used for water disinfection. Accessed at: smart-fertilizer.com/articles/chlorine. Not intended for endorsement.

Reviewers: Denny Heldman, Food Science and Technology; Sanja Ilic, Human Sciences, Human Nutrition.

Topics: 
Ohioline http://ohioline.osu.edu