Research in California has shown that the liquid bleach sodium hypochlorite, which is commonly used in apple packing systems, can cause skin damage on apples.
Dr. Beth Mitcham at the University of California, Davis, said when dump tank water is used to treat large quantities of fruit, the chlorine reacts with the organic matter, but the sodium stays behind and can build up to such high levels that it damages the fruit. The damage is superficial and doesn’t penetrate into the fruit flesh. It can be found on different parts of the apple, depending on how high the sodium concentration is. Sometimes, the damage is in the stem cavity, and other times in the calyx end, but browning might also be seen around lenticels or other openings in the skin. Mitcham said she’s seen symptoms on Granny Smith, Gala, and Fuji apples.
Generally, it is not an issue where the water is changed daily.
In one case, where there was significant damage, the packer was hydrocooling the apples and not changing the water frequently enough. The packer was using an automated chlorine system that added chlorine as needed to maintain a certain level of disinfection, with the result that the sodium would build up to fairly high levels.
Dr. Gene Kupferman, postharvest extension specialist with Washington State University, said similar damage has been seen in Washington. Some packers avoid adding large amounts of bleach, and instead dribble it in with a meter, based on the oxygen reduction potential, which indicates how much chlorine is in the water and how active it is.
However, Steve Agnew at U.S. Syntec in Yakima, Washington, said another option is to use calcium hypochlorite as a sanitizing agent, rather than the liquid bleach sodium hypochlorite. Calcium hypochlorite, which comes in a powder form, is used in swimming pools and water-treatment systems and does not form damaging salts.
He said the product was not used by fruit packers in the past because it didn’t come in a user-friendly form. It did not dissolve easily in water and couldn’t be metered. However, the manufacturer, Arch Chemicals, now makes calcium hypochlorite in briquettes, which are placed into a hopper and slowly dissolve into water to maintain a constant 2 percent solution of calcium chloride.
U.S. Syntec has an exclusive license to sell the product in the tree fruit market and installs the necessary metering devices.
Agnew said he believes some packers have been avoiding using sanitizers because of the fruit damage caused by sodium hypochlorite, but he feels sanitation is necessary to avoid food-safety issues.
Though calcium hypochlorite is more expensive than liquid beach, Agnew noted that the pH level is lower, so less buffer is needed, and that helps offset the cost. Another advantage of calcium hypochlorite is that it is more stable. It can be stored in briquette form for years, whereas sodium chloride degrades from the day it’s manufactured.
Mitcham said she was not familiar with the new system that Arch had developed, but said it sounded promising. "If the system is providing an easy way to get that into solution, it could be very interesting, particularly because we know calcium does some particularly good things in fruit."
Kupferman also thought calcium hypochlorite could be a good option for avoiding the build-up of salts, but said there was no evidence that the calcium in the dump tank would have a lasting effect in terms of increasing calcium in the fruit.