An Italian researcher is experimenting with a nonchemical method to prevent scald developing in stored apples, and has tested it successfully in some commercial packing houses.
Scald is a serious disorder of apples around the world. It typically appears after several months of storage in the form of irregular brown areas on the skin of the fruit. The flesh below the affected area may become soft and discolored in severe cases. Symptoms may not show up until the fruit has warmed up to room temperature, after it’s been shipped.
Treatment with diphenylamine (DPA) has generally been used to control the disorder. However, the antioxidant treatment is now banned in northern Europe because of increasing concerns of environmentalists and consumers about postharvest treatments on fruit, Dr. Angelo Zanella, a researcher at the Laimburg Experiment Station in the South Tyrol, Italy, reported during a recent Washington State University Tree Fruit Quality School.
Three large Italian retail chains will not accept any postharvest treatment on apples, whether fungicides, DPA, or SmartFresh (1-methylcyclopropenl, MCP). The two largest retailers in the European Union have adopted a zero-residue policy, "and they’re not kidding," Zanella added.
They have set up expensive monitoring programs, at their own expense, to test the residues on the fruits and vegetables they sell. Recently, they sold no grapes at all for a month because the grapes from Spain had residues.
"This is the market reality we’re confronted with," Zanella said. "We have to get rid of any treatment that’s causing a residue."
Apple growers in the South Tyrol are in a good position to supply residue-free fruit because of their long history of using integrated pest management and are looking for alternatives to DPA for scald control, he said. Apart from leaving residues, drenching with DPA can cause fungal spores to spread in the water, potentially leading to more rot on the fruit. In addition, disposing of the spent drench water can be a problem. Producers have tried thermofogging with DPA, but are seeking another alternative because the DPA was not uniformly distributed in the storage room or on the fruit, and it resulted in staining on the fruit where the residues where too high.
An extremely low oxygen level in storage can reduce scald, but if the fruit has insufficient oxygen, it can turn anaerobic and develop off flavors. One nonchemical treatment that’s been tested is called Initial Low Oxygen Stress. This involves holding the fruit at an oxygen level of 0.4 percent for the first 14 to 20 days in storage to stress the fruit. The level is then raised to 1.0 percent, which is termed ultra-low oxygen but is above the danger level for anaerobic fermentation. Ultra-low oxygen levels also inhibit ethylene production, providing similar benefits to SmartFresh in terms of fruit firmness, he said. However, Initial Low Oxygen Stress has not always given complete control of scald, and packing houses want full scald control, not 50 percent, Zanella said. "We cannot have any fruit with scald."
Now, he is testing a technique called Dynamic CA, which involves changing the oxygen level during the storage period according to the needs of the fruit. Several years ago, Dr. Robert Prange, postharvest physiologist with Agriculture and AgriFood Canada in Nova Scotia, developed a sensor called HarvestWatch that signals when the apples need oxygen.
It actually measures the fluorescence of the chlorophyll in the fruit, using four pulsating light sources, and a monitor detects anaerobic stress. This allows the oxygen levels to always be set just above the anaerobic threshold and enables the atmosphere to be tailored to suit the actual fruit in storage. The fruit’s oxygen requirement might vary depending on the cultivar, maturity, and –seasonal variation. More mature fruit needs more oxygen.
The carbon dioxide level, on the other hand, is relatively high—possibly around 1 percent, depending on the variety. High carbon dioxide levels result in better storage quality, and less energy is needed to scrub the carbon dioxide from the room, Zanella explained.
In 2003, he tested Dynamic CA on 600 bins of Granny Smiths in a commercial storage and was able to store the fruit at 0.4 percent oxygen and 1.3 percent carbon dioxide with a temperature of 33°F. The fruit had no scald, –compared with 70 percent scald in an ultra-low oxygen treatment.
By 2005, four packing houses in the South Tyrol area were ready to adopt it. In 2006, 15 packing houses tested it in 81 storage rooms containing 76,000 boxes of various varieties, which were stored for between five and eight months.
A side effect, apart from the scald control, was better fruit quality, Zanella reported. Red Delicious in the treatment had less core browning and better firmness than the ultra-low oxygen treatment. The firmness was comparable with a SmartFresh treatment when the fruit was brought out of storage, though after seven days at room temperature, the Dynamic CA fruit was slightly less firm. Unlike SmartFresh, the treatment has no continuing effect after storage.
The Dynamic CA treatment was also used successfully for Braeburn apples to control internal browning.
Zanella said it is a great advantage to be able to supply the consumer and retailers with fruit of this quality –without any residues.
Dr. Gene Kupferman, postharvest extension specialist with Washington State University, said the Washington tree fruit industry has been relying on DPA and MCP to enhance postharvest quality, but neither of those are options for the organic fruit industry, which is increasing in importance. "What are we going to do in postharvest to help them extend the storage life of their fruit so they have a good marketing condition?" he asked.
The HarvestWatch sensor that Prange developed is manufactured by a company called Satlantic in Nova Scotia, but the company does not want to be the marketer of the instrument, Kupferman said. Agriculture Canada holds the patent and is seeking a partner in North –America to sell it.
Kupferman said it might need some refinement for use in Washington, but he would like to work with a group of packers to try to learn how to use the Dynamic CA system without putting the fruit at risk.
"We need to develop this," he said.