Using Chlorine in the Packinghouse
Chlorination is a water purification treatment, and chlorine is a surface sterilant. It does not disinfect the internal tissues of infected fruits but does prevent the buildup of decay producing organisms in the water of the dump tank, washer, hydrocooler etc. Fruit coming into the packing-house from CA storage, from regular storage or directly from the field contains some decayed fruits as well as dirt and debris in bins and boxes. When decayed fruit or dirt and debris are dumped into a tank of water, the amount of fungal spores (decay organisms) increases dramatically.
Spore count often determines how much decay will occur due to infection in punctures and stem ends of healthy fruits in packinghouses. Chlorination is used to reduce the number of spores in the water, thus reducing or preventing inoculation of healthy fruit. Much storage loss is due to fungus diseases caused by Penicillium expansum (blue mold), Botrytis cinerea (gray mold), and Mucor piriformis. Chlorine is toxic to the spores of these decay organisms.
A contact fungicide, chlorine quickly kills fungal spores but has poor penetrating ability and no residual activity.
Chlorine gas, sodium hypochlorite and calcium hypochlorite are the most common chlorine disinfectants. Chlorine gas, Cl, is supplied as a compressed gas in a cylinder. Sodium hypochlorite, NaOCl, is available as household bleach (Chlorox 5.25%), or in commercial formulations (such as Agchlor, Sunny Sol, or Purechlor which are 12½%) and are supplied as liquids. Calcium hypochlorite, (CaClO) is supplied as swimming pool chlorine and is granular. Various organic complexes of chlorine are available as well.
All of these chlorine containing chemicals form the same spore-killing active ingredient, hypochlorous acid when added to water.
The disinfectant activity of a chlorine solution is influenced by its pH. The lower the pH (more acid), the greater the amount of hypochlorous acid formed from the chlorine compound. However, at very low pH levels, odor problems and corrosion of equipment will occur.At levels above pH 7.5 (slightly alkaline) hypochlorous acid dissociates to form the hypochlorite ion, which is less toxic to microorganisms. Therefore, the pH should be kept between 6.0 and 7.5 for greatest effectiveness without damage to equipment.
The pH of chlorinated water in the packinghouse can be checked periodically with litmus paper (which changes color different pHs) test kits, or pH meters. The pH can be adjusted and maintained at the desired level by buffering the solution, except if pear flotation materials are used. See Buffering Chlorine Solutions for further cautions.
In summary, to effectively control the spore-killing process two things must be tested frequently: the amount of chlorine in the solution and the pH of the water in the dump tank after the chlorine is added.
Organic matter reacts with chlorine to lower its effectiveness. The more organic matter in solution, the more chlorine is tied up. Thus organic matter in the water can be a critical problem. Consequently, flumes and dump tanks should be cleaned at regular intervals. Timing will depend upon the volume of fruit run over the line as well as the amount of dirt and debris in the bins. The tank of a packingline into which presized apples are dumped probably needs less chlorine than the dump tank in the presize line.
Chlorine exhibits a complex reaction to temperature. Generally, as temperature increases the chlorine becomes more active as a disinfectant. However, raising the temperature increases the ability of organic matter to tie up chlorine. Raising the temperature also speeds up the loss of chlorine into the atmosphere.
As exposure time to chlorine is reduced, less disinfection occurs. Higher concentrations of chlorine are necessary when fruits and vegetables are exposed to chlorine for only a few seconds, as in spray wash, in contrast to a dump tank where the fruit may be exposed for several minutes.
Maintaining Chlorine Levels
Equipment to use chlorine in gas, liquid, or granular form ranges from very complicated to very simple. However, efficient disinfection depends upon accurately monitoring concentration of chlorine in the tank.
Monitoring may be performed continuously with automatic equipment or periodically with kits. Two types of chlorine test kits are available. One measures "total chlorine" in the solution; the other type measures "free available chlorine."
Total chlorine test kits measure all compounds containing chlorine in the solution. Some of these compounds may not have the spore-killing power of other compounds. Although the study of these compounds is not yet completed, it would be a good idea to measure pH along with total chlorine.
Free available chlorine test kits use color comparisons to determine the amount of spore killing power in the solution. According to some sources it is not as important to measure pH since the kit measures spore killing power.
Treatment is most effective when chlorine is metered continuously. The most ineffective method is to dump chlorine into the tank occasionally. Although 70 ppm of chlorine is probably more than adequate if that concentration is maintained, the recommendation is for 100 ppm when using the batch method.
This is not a case where "if a little is good, more is better." In some situations, the level of chlorine has been doubled in order to "effect a better kill." If the concentration of chlorine is too high, or if the pH of the chlorine solution is too low, chlorine gas is released into the air. This creates an odor problem, wastes chlorine, and corrodes equipment. It is better to meter in the chlorine and control the pH of the solution than to double the amount of chlorine in the solution.
In pear lines, flotation materials make the situation more complex. Sodium sulfate used for flotation does not appear to reduce chlorine activity since the pH is fairly close to neutral. Sodium silicate and sodium carbonate (soda ash), however, raise the pH of the dump tank water significantly.
Since a great deal of the pear industry uses sodium silicate, research by Dr. Bob Spotts at OSU's Mid-Columbia Research Station is seeking methods to improve chlorine effectiveness at high pH levels.
Both chlorine gas and liquid will control spore germination and significantly reduce decay in laboratory tests at lower concentrations than used in the packinghouse. However, when the lower concentrations were tried in a packinghouse, decay control was poor due to inadequate penetration of wounds. Therefore, the recommended level of 100 ppm for dump tanks should be retained under present conditions.
Dr. Spotts is currently working to improve the penetrating ability of chlorine by adding surfactants to the dump tank. With certain surfactants he has been able to reduce decay by an additional 10-20% over that obtained with chlorine alone.
Addition of certain surfactants (0.5% active ingredient) to packinghouse dump tank water containing 5% sodium silicate resulted in reductions in decay. (See Tables 1 and 2). These tests were run on artificially wounded fruit. Fungal spores were added to the dump tank water.
Dr. Spotts found that the surfactants did not alter the pH of the dump tank water (pH 10.9 when sodium silicate was used) but appeared to help chlorine penetrate wounds.
When d'Anjou pear fruits were immersed in 0.5% ortho X77 or VWR Spread Number 1 and stored for 2 months without rinsing, no adverse effects on fruit finish were observed. These surfactants remained effective in the dump tank over a three week period.
At this time only Triton X100 and AG-98 are registered for use in packinghouses. Surfactants change the solubility of flotation salts and may increase foaming. If chlorine concentration is not carefully maintained when using a surfactant, decay may actually increase. Additional research is in progress.
Dr. Spotts warns that the addition of a surfactant with fungicides other than chlorine (such as Captan, SOPP, etc.) actually reduces fungicide residue on the fruit and allows increased decay.
Buffering Chlorine Solution
The use of a pH buffer in dump tanks in which chlorine is used will keep the pH at the desired level. However, using a buffer or reducing the pH with acid, when sodium silicate is used as a flotation for pears can turn the solution into a gel. Therefore, do not attempt to adjust (lower) the pH of dump tanks containing pear flotation chemicals.
Hydrocooling of Pears
Recently, a packinghouse hydrocooled d'Anjou pears before putting them in storage. When the pears were removed to be packed, serious injury was observed on the skin of the fruit. Chlorine in the hydrocooler may have been responsible for the damage to this very sensitive variety. Use of chlorine in hydrocooling Bartletts or cherries has not presented a problem.
Use of chlorination is a fast, inexpensive, and effective method of killing fungal spores in water in packinghouses. However, if used incorrectly it can cause unpleasant working conditions and corrosion of equipment.
The pH (acidity/alkalinity) alkalinity) of the solution in the tank and the concentration of the chlorine should be measured regularly. Dr. Charlie Pierson's recommended levels of chlorine are 100 ppm in the dump tank of a presizer and 30 ppm in water flumes. Dr. Spotts recommends 100 ppm in all packinghouse water. Lower levels, especially in the dump tank, may be used if the fruit is clean, if the tank is cleaned regularly, and if free available chlorine in the system is monitored regularly.
For further information request a copy of the following from E. Kupferman (509-663-8181 x239):
"Postharvest Decay Control of Apples & Pears After Immersion Dumping" (SP545).
This OSU publication outlines the differences between SOPP and chlorine for packinghouse use.
"Effect of a Surfactant on Control of Decay of Anjou Pear with Several Fungicides" by Dr. Spotts.
"Use of Surfactants with Chlorine to Improve Pear Decay Control" by Dr. Spotts and Ms. Peters.
Dr. Eugene Kupferman, Postharvest Specialist
WSU Tree Fruit Research and Extension Center
1100 N. Western Ave., Wenatchee, WA 98801
Post Harvest Pomology Newsletter, 2(4): 5-9