Acetic acid fumigation can prevent pear rots
By Geraldine Warner
Canadian researcher has found that fumigation with acetic acid is a promising treatment for killing the fungal spores that cause decay in pears during storage.
Dr. Peter Sholberg, a plant pathologist at the Agriculture and Agri-Food Canada Research Centre at Summerland, British Columbia, said the acetic acid acts as a sterilant, somewhat like bleach, and kills most of the pathogenic organisms on the fruit.
Sholberg said he began the project about four years ago after seeing reports that fumigation with acid aldehyde could control rhizopus rot on strawberries. A chemist he was working with suggested trying acetic acid, which vaporizes quite easily. This was important if the product was to be used as a fumigant.
Acetic acid can be used as a drench, in which case it is similar to vinegar, which is a five percent acetic acid solution. Sholberg said he and Dr. Rodney Roberts with the U.S. Department of Agriculture in Wenatchee, Washington, found that an acetic acid dip could control fireblight bacteria on fruit, but it was not pursued because other treatments were more effective.
He said acetic acid appears to work better when used as a vapor than when mixed with water.
In tests comparing fumigation with acetic acid and treatment with the fungicide thiabendazole (TBZ), Sholberg found that both killed spores on the fruit, but only TBZ was effective against infections that had already begun. He believes the two could be used together. Some pathogens are showing resistance to TBZ, and the acetic acid might kill spores that are left after the TBZ treatment. He said acetic acid could also be used with biological postharvest disease controls to reduce the disease pressure.
Acetic acid is cheap and is manufactured in large volumes. It is used as a solvent and in the manufacture of rubber, plastics, and acetate fiber.
It is classified by the U.S. Food and Drug Administration as Generally Regarded As Safe, and can be used in food in concentrations as high as five percent. Sholberg said a dill pickle might have a five-percent concentration, and the residues on a fumigated pear would be much lower. Taste tests have shown that the acetic acid could barely be tasted on pears right after fumigation.
In his tests, Sholberg has used the same fumigation chambers as for methyl bromide. Basically, all that is needed is a tightly closed room. The acid comes as a liquid and is evaporated into the room by a heater and circulated by a fan. He said it seems to penetrate well, and the effect on fruit at the bottom of boxes has been as good as on fruit at the top.
The longer the fruit is exposed to the acetic acid, the better it works. Sholberg said it probably gives about 98% control after four hours, but overnight is better. It appears to work well at low temperatures and has not left burn marks on the fruit.
There is some concern about the explosiveness of acetic acid, but Sholberg said it would have to be used at much higher concentrations and in higher temperatures than in a pear storage room before there would be any chance of explosion.
One concern that would have to be addressed before the treatment could be used on a commercial scale is worker safety, although the product is much less hazardous to humans than methyl bromide, he said. The Occupational Health and Safety Act (OSHA) has standards concerning the levels to which people may be exposed.
The fruit and the bins absorb much of the acetic acid, and the little that remains in the air is exhausted out of the room. Sholberg said the fact that the bins absorb the acetic acid may be an advantage because it means the bins are sterilized also. It does not clean them, he stressed, but just kills the spores.
Sholberg has received $15,000 from the Washington Tree Fruit Research Commission to continue the project. This year, he will try to pinpoint the optimum concentration of acetic acid to use, the best temperature, and the optimum exposure time.
By next year, he may be ready to do tests in commercial packing houses. He said the treatment is probably two to three years away from being an option for commercial packers, depending on the industry's interest in it.
"If they are really interested in it, they might pick it up and push it through," he said. "First of all, we have to demonstrate that it does have effectiveness on a commercial level.
"It's got good promise," he added. "From our knowledge so far, it looks exciting."
Copyright, November 1997, Good Fruit Grower,105 South 18th Street, Suite 217, Yakima, Washington
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