Factors Influencing DPA Residues on Apples
Most apple growers in New England use diphenylamine (DPA) as a postharvest dip or drench to control scald on apples. Although these treatments have been generally effective in controlling scald, results of a given treatment are not always the same. One reason for this is that fruit vary considerably in their susceptibility to scald, depending on variety, fruit maturity, fruit nutrition, and growing conditions...especially the temperature that occurred during the days shortly before harvest.
Another reason for variable results is that a number of factors affect the amount of residue left on fruit by a treatment. (It is this residue that protects the fruit from scald during and following storage.) A recent study by Drs. Shih-Lo Lee, Asela Carag, and Hesh Kaplan of Decco Tiltbelt Division, Pennwalt Corporation, Monrovia, California demonstrated some important factors influencing this residue.
Most of their studies were with Granny Smith apples, which are very sensitive to scald, and most employed No Scald DPA EC-283, 31% a.i., as the test material. Their results are summarized in the following sections.
Effect of DPA Temperature
Cold apples were dipped in solutions at 41°, 55°, 70° and 95° F for 30 seconds, 1 minute, or 2 minutes. When dipped for 30 seconds, solution temperature had no effect on DPA residue, but when dipping time was 1 minute a solution temperature of 95° F. doubled the residue left by solutions at the lower temperatures. It should be noted that solutions of 41°, 55°, and 70° F. all left approximately the same residue regardless of solution temperature or treatment time.
Effect of Fruit Temperature
Cold apples were kept at 40°, 55°, or 72° F. for 10 to 12 hours to warm before dipping in a 70° F. solution for 1 minute. The coldest apples (40° F) retained the least residue, but there was no difference between those at 55° and 72° F.
Effect of Dipping Time
Apples were dipped for periods varying from 15 seconds to 4 minutes. A dip of only 15 seconds produced less residue than the other dipping periods, but all periods of 30 seconds or more produced the same residue.
Effect of DPA Concentration
When apples were dipped in concentrations of DPA varying from 500 to 2500 ppm, the amount of residue increased with concentration. There was twice as much residue from 2500 ppm as from 500 ppm, but perhaps of greater interest is the finding that there was about one-third more residue from 2000 ppm than from 1000 ppm, since our recommendation usually calls for use of "1000 to 2000 ppm".
Effect of Additives
Addition of calcium chloride at 24 lbs. per 100 gallons did not affect DPA residue, and neither did the addition of a surfactant.
The authors compared Granny Smith apples with Golden Delicious, Delicious, Jonathan, and Rome Beauty. When the fruit were all dipped under identical conditions, the residues were similar on all varieties except Delicious, which retained almost twice as much residue as did the other varieties.
Effect of DPA Formulation
Three different commercial formulations of DPA were tested under identical conditions, and one of the formulations left twice as much residue as did the other 2 formulations.
These results indicate that apples should be treated for at least 30 seconds, but that prolonged periods provide no additional benefit unless the dipping solution is hot. They also show that cold apples will retain less residue than warm ones. However the temperature of the dip solution is of little consequence unless a quite warm solution is used, which will greatly increase the amount of residue. They also illustrate that the same treatment can protect different varieties to different extents, and that different DPA formulations can produce different results. Within the solution itself, DPA concentration was highly important, but addition of CaCl or surfactant had no effect on DPA residue. Addition of fungicides to the solution was not tested.
The results of this study should be of considerable interest to apple growers, who are frequently concerned about how treatment conditions affect scald control.
Lee, S-L, A. Carag, and H. J. Kaplan. 1984. Factors influencing the uptake of diphenylamine by apples. HortScience 19(1):94-95
William J. Bramlage
Department of Plant and Soil Sciences, University of Massachusetts
Post Harvest Pomology Newsletter, 2(3):18-19