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Postharvest Information Network

Sunday, March 24, 2019

WSU-TFREC/Postharvest Information Network/Proper Management of Your DPA Program Can Save You Money and Headaches!

Proper Management of Your DPA Program Can Save You Money and Headaches!


This was an interesting and different fall for the Washington State apple harvest. The color came late because of the high temperatures and it appeared many strains of Red Delicious were going to be left on the tree. When color did develop, blocks previously color picked were being re-picked in an attempt to salvage remaining fruit. This translated into a hectic and unusual harvest pattern, which put a lot of pressure on the packers who are responsible for maximizing the marketing of the product. It is known that pre-harvest air temperatures are an important parameter in scald potential. Hot weather shortly before harvest increases scald susceptibility and one of the concerns for this year was the warin fall. It is generally accepted that there is a high potential for scald during this storage season.

Storage Scald

Storage scald is a physiological disorder characterized by skin browning that becomes a concern when fruit is stored for several months. It occurs in both air stored and CA stored fruit. The symptoms arise from the death of cells in the peel and it is generally agreed that this is the result of the oxidation of alpha farnesene to conjugated trienes. These trienes kill epidermal cells, hence the browning. DPA (diphenylamine) is an antioxidant that has been used for years to prevent storage scald.

The DPA is applied, via a drench system, immediately following fruit harvest. Typically the drencher is located at the packinghouse in the delivery area. Both drive-through and chain driven systems are used and in each case the DPA solution is pumped from a holding tank and cascaded down over the fruit in bins. By far the majority of packinghouses use the drive-through even though there are relatively fewer problems associated with the chain driven systems. However, the draw back with the chain driven system is that it is slower and more labor intensive. In the drive through system the truck is pulled into the drencher and the first bins positioned under the delivery nozzles. When the system is turned on the DPA solution begins to drench the fruit and the driver of the truck pulls forward, continually exposing the untreated fruit to the cascading sheet of DPA solution. Recormnended exposure time will vary with the drencher system. This process typically takes a few minutes for a 64 bin load.

Once all the fruit has been exposed, the truck is pulled forward and the excess product allowed to drain from the fruit and truck, to the pavement, then back into the holding tank, The fruit is then unloaded and placed into storage.

This seemingly simple operation has many potential pitfalls and requires very stringent management in order to apply the product safely and obtain the correct residue levels on the fruit to ensure protection during storage.

Key Areas

There are key areas to watch when running a DPA drenching program. For example:

  • The concentration of the drench solution.
  • The exposure time within the drench system.
  • Delivery volume of the system.
  • The number of bins treated per tank charge with fresh DPA.
  • Cleanliness of the system.
  • The safety of the operation.


    Drenching product with DPA is only one of many jobs to be done during harvest so being well prepared from day one and starting out with all systems properly primed will save a lot of aggravation and guarantee proper residue levels on the fruit. The first step is to make sure that the DPA levels in the system are those recommended for the variety being treated. For Red Delicious apples the recommended label rate is 2000 ppm. The majority of tanks are rated at 2000 gallons and the recommended dilution, using 15% product, is 1 gallon of DPA per 74 gallons of water. Therefore adding 26.7 gallons of DPA should charge a 2000-gallon tank.

    At each location there are constant checks to assure that the tank concentrations are correct. When these checks show discrepancies from the expected level there are a number of possible explanations. For example, to correctly determine the volume of solution within your system it is essential to have an accurate measure of the tank volume. It is also important to include the volume of water contained in the delivery piping. A tank filled to the 2000-gallon level that has a delivery system of pipes holding water should actually be rated to include that extra volume. If this extra volume is not taken into consideration then the final concentration will fall below the recommended DPA level. This is a common problem experienced in the field.

    Another way in which concentration errors can occur is by improper transfer of DPA to the tank. It is common to buy the DPA in bulk and dispense it into the tank using buckets. Applicators are warned that it is possible, during the charging of the tank, for mis-counts to occur due to distractions occurring at the drencher. If a mis-count does happen, the final strength will be low or high depending on the direction of the error. Pressures of the harvest period create this scenario.

    Exposure Time

    The EPA tolerance for DPA residue on the fruit is 10 ppm; the recommended range by the manufacturers is 2 to 7 ppm. Manufacturers, as a service to the user, continuously check that residues are within the proper range for adequate control. The amount of time that the apples are in contact with the DPA solution will have an impact on the residue levels. For example, if you dip an apple in and out of a DPA solution in less than a second you would get a small amount of residue on that apple. If you put the apple in the same solution but left it there for one minute you would get a higher residue value. Field research has shown that the drencher system requires careful monitoring of the exposure time in order to get residue levels on the fruit to fall within the acceptable range.

    The key to achieving the correct exposure time for a given drenching system is in timing the movement of the trucks. Experienced operators know, from their experience, the correct time for each truck to spend under the cascading DPA solution. This can be judged accurately by watching the solution that gushes from the bottom opening of the bottom bins. In effect, the delivery of solution is such that the solution begins to back up in the bin slightly.

    Moving the larger trucks through the drencher at a slow speed is not popular with the truck drivers. If they are being paid by the load, time sitting is money lost. In addition, it is hard on the clutch of these heavily loaded trucks to creep through the drencher with a stop, start, stop procession. It takes patient drivers and a firm and experienced drencher operator to control traffic through these systems, but that firmness and experience may save a lot of fruit from scalding due to under-exposure.

    On the opposite end, fruit over-exposed runs the risk of chemical burn. In our company's experience this is very rare but can happen. Usually, over exposure results from drenching fruit in bins with liners that prevent drainage. This important information is stated on the product label.


    By virtue of the system design the cascade of DPA solution is generally concentrated on one section of the load and the slow movement of the truck allows the cascading solution to drench all bins in turn. However, distribution can vary from system to system. For example, the delivery of solution may come from a series of nozzles in a single row or from several rows of nozzles. In the former case the cascade of solution will hit a single row of bins across the truck whereas, in the latter case, two rows of bins may be treated at once. This may imply that, in the latter case, the total exposure time will be less since more fruit is being drenched at a given time. However, the true exposure depends on the capacity of the delivery system, i.e., how much solution is cascading down in gallons per minute, as well as the time spent under the cascading solution.

    The volume of DPA solution being cascaded over the fruit is a key factor in achieving the proper residue levels and there are differences in the rate at which the solution is being delivered at the different drencher sites. Also the distribution pattern of the cascading solution varies. For example, most of the drenchers deliver both over the top of the bins and have jets coming in from the side. Positioning of the side jets will have a major impact on their effectiveness. In some instances there are lower nozzles shooting DPA solution below the truck bed thus having no input at all. In other cases they are aligned to shoot right into the lower bins. The variability in the systems is not a problem, but management of the system to compensate for that variability is critical.

    Bins per Tank

    The label states that the number of bins that can be treated per 100 gallons of DPA solution is 30. The majority of DPA tanks are listed at 2000 gallons and thus are charged with 26.7 gallons of DPA 15%. A 2000 gallon solution should treat 600 bins of apples and then should be discarded and the tank recharged with fresh solution.

    Recharging is done in order to maintain the full efficacy of the product and to keep clean solution in the tank. Continuous drenching of bins from the field may cause the accumulation of fungal spores and debris in the drencher solution and put additional pressure on the fungicides used to prevent post harvest decay, as well as decrease the active DPA levels.


    Diphenylamine (DPA) is an organic product that readily binds to organic debris in the tank. Experienced operators know the importance of keeping the level of debris to a minimum. The debris that collects on the screen, covering the return, should be removed regularly. Also, the dirt that is washed down from the truck during the drenching process should be removed routinely. When there is a busy time, with a line-up of trucks waiting to use the drencher, it is difficult to stop and do house-cleaning. However, an experienced operator will be aware of the cleanliness of his operation and will make sure that build-up is minimized. Dirty DPA tanks rob you of active product. This decreases product effectiveness and increases your costs.


    Safe use of the product in the field is the concern of everyone involved from the drencher operator to the manufacturer of the product. Safety factors have been well researched and the product is classified as low toxicity; however, there are no winners if any product is not used with appropriate caution. The manufacturers are bound by EPA regulation in how the product is manufactured and handled up to the point of delivery. A safety meeting is arranged each year by the manufacturer and is designed to educate the drench operator as to how the product should be handled and how to deal with any mishaps that may occur. These seminars, provided by the manufacturer, should be attended by all those involved in the drenching of fruit.

    The first line of defense, from a safety point of view, is for the handler to understand the toxicity of the product; the second is to have the correct protective clothing when working with the product. This means wearing coveralls, a facemask and suitable gloves when handling the concentrated product. There are common sense practices like not eating; drinking or smoking while handling the product heeded by experienced operators. Also, handlers should immediately wash material from their skin, if exposed, and should properly clean clothing that is worn during the day. Since wind can carry fine mists from the drench, proper positioning of barriers to protect the operator from drift are necessary to minimize exposure. All these safety concerns are clearly stated on the product label and taught at the safety meetings.


    DPA is a product used world wide to prevent storage scald and it is used extensively in the Washington State apple industry. Although there are subtle variations in the drenching programs from house to house, DPA application is carried out smoothly and efficiently, by the industry, using standard drive-through or chain driven drenchers. The use of DPA in a year such as 1998 has likely resulted in major savings for the industry.

    Successful implementation of DPA programs depend on the experience of the drencher operators who know how to get the job done correctly and safely. In spite of the volume of fruit drenched with DPA in Washington State in 1998, monitoring programs show that the vast majority of that fruit is treated without difficulty. Understanding the systems that are used to apply the product, pre-planning your program, and utilizing a trained operator will result in a smooth operation and significant savings.

W.J. McPhee

Pace International LLC

October 1999

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