Horticulture Fact Sheet
H-00-050

Cooperative Extension Service/The University of Georgia College of Agricultural and Environmental Sciences

Packinghouse Management of Hydrocooler pH and Chlorine Content

Kathryn C. Taylor, Stone Fruit Horticulturist, Horticulture Department, CAES
University of Georgia, 21 Dunbar Rd., Byron, GA 31008

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In the post harvest handling of stone fruit the hydrocooling operation serves two purposes: removal of field heat from fruit, with cooling to ~ 40 degrees F, and reduction of fruit rotting organisms through the use of chlorine. Both effects improve the shelf-life of fruit. While hydrocooling can have positive effects on fruit quality, it can also have negative effects, if the chemistry of the hydrocooler water is mishandled. The two most important components of hydrocooler water chemistry are the pH and the chlorine content. When the pH is outside of the 5.5-6.5 range, risk of peel discolorations caused by preharvest sprays or high mineral content in the hydrocooler water will be markedly enhanced. A commercial buffer, such as Decco 311 Buffer Concentrate is effective in maintaining this range. At pH 5.5-6.5, a free chlorine level of 25-50 ppm is ideal for reducing microbial populations without damaging the peel. Although as little as 1 ppm is sufficient to kill microbes, more is advisable to ensure that the level stays above 1 ppm over the course of the day during packing.

Table 1 contains a list of several commonly used chlorine products and the percent chlorine/chloride in the product. Since there are a number of chlorine-containing products available, from household bleach at 5 percent hypochlorite to granulated materials at 68 percent free chlorine, tables have been created that give specific values of chlorine addition per 100 gallons of hydrocooler water. These tables will enable you to determine the amount of product for your initial chlorine application (Table 2), as well as the appropriate amount for recharge (Table 3) of the hydrocooler to a concentration of 50 ppm free chlorine.

When adding the chlorine product, it is important to add it to the 30-35 degrees F hydrocooler water in a liquid form. It is an easy extra step to predissolve granulated forms in a 5-gallon bucket before adding to the hydrocooler water.

Clearly, the dry formulation is less costly. However, liquid formulations give very quick, reliable recharge to the final chlorine concentration. The dry form can be used in the same way by dissolving the compound in warm or tepid water prior to adding it to the hydrocooler water. Even during the initial charge of the hydrocooler, predissolving the granular form is wise.

Estimates of the amount of product to add/100 gallons to obtain a free chlorine level of 50 ppm in hydrocooler water are made on the basis of percent active ingredient (free chlorine) in each product. If the water has had no chlorine added previously, use the guidelines in Table 2.

Calcium hypochlorite is a 68 percent granular material. To obtain a 50 ppm solution of chlorine using calcium hypochlorite, add approximately 1 ounce/100 gallons water (27.82 grams/100 gallons of water).

Commonly available liquid sodium hypochlorites are 12.5 percent chlorine. To get a 50 ppm chlorine solution in the hydrocooler using these products, add 5.86 fluid ounces/100 gallons of water.

When adding chlorine to hydrocooler water during the day, test the chlorine level every 1-2 hours and if, for example, you need to add enough chlorine to bring your level to 50 ppm, use the following table. To test for chlorine using pool test strips, dilute 1 part cooler water with 3 parts of tap water. Multiply the chlorine concentration result from your test strips by 4.

It is ideal to change the hydrocooler water daily. However, many operations base the timing of such cleanings on the amount of fruit run through the hydrocooler. In either case, additions of chlorine must be made either manually or by injection throughout each day or between hydrocooler cleanings. Check the chlorine status throughout the day (at the beginning of the day and every 1-2 hours during operation) to maintain the chlorine level between 25-50 ppm.

In Table 3, the amount of product addition required to recharge the hydrocooler water to 50 ppm has been calculated. For a 68-percent compound like calcium hypochlorite, if testing indicated that the residual chlorine content of the hydrocooler water was 20 ppm, approximately 6 ounces of product would be added per 1,000 gallons of hydrocooler water (~0.6oz/100 gal, from Table 3) to recharge the chlorine to a final concentration of 50 ppm.

The hydrocooler operation is a necessary part of the peach packing process. When managed properly, hydrocooling serves to inhibit rather than enhance other sources of damage to fruit that occur during fruit culture, harvest and handling. Appropriate maintenance of water pH and chlorine content is essential to maintaining high post harvest fruit quality.

Departmental Fact Sheet H-00-050/July, 2000