Irradiation-Quality Issues

Blueberries are plagued by quarantine pests including the blueberry maggot, apple maggot, and plum curculio. Currently, methyl bromide is the only approved treatment that will allow blueberries to be shipped to places that don't have these pests, like California, Ontario, and British Columbia. ARS scientists at Miami and Orlando, Florida, have shown that irradiation should control these pests without affecting the quality of the blueberries.

"Most fruits and vegetables will tolerate ionizing radiation at low doses with minimal detrimental effects on quality, making irradiation a potential quarantine treatment," ARS horticulturist Roy E. McDonald stated at the San Diego Conference on Methyl Bromide, Nov. 5-8, 1995. "It is essential that a quarantine treatment be effective and efficient without harming the commodity's quality."

McDonald and colleague William R. Miller, with the U.S.Horticultural Research Laboratory in Orlando, Florida, have studied the issue of how gamma radiation affects fruits and vegetables--their quality, condition, and susceptibility to decay.

"Some commodities can be damaged at radiation doses between 0.25 and 1.0 kilogray," McDonald said. "Generally, nonfruit vegetables like lettuce are much more sensitive to irradiation stress than fruits like apples or fruit-vegetables like tomatoes."

When gamma radiation was proposed as a potential quarantine treatment for fruits in 1956, ARS scientists in Hawaii investigated its effects on fruit flies in papayas and other tropical fruits. McDonald said that though most of the research since then has dealt with insect mortality, some studies have evaluated the effect of irradiation on fruit quality.

"If the quarantine treatment reduces the value of the commodity, then that treatment is not fully effective," he said. "Any adverse change in shelf life, appearance, flavor, texture, aroma, or susceptibility to decay organisms constitutes damage, because these factors determine marketability."

Preharvest factors like climate and cultural practices may influence a commodity's response to irradiation stress, he reported. The way radiation is administered can also affect the response.

In some crops--like blueberries--irradiation is the only apparent alternative to methyl bromide because other fumigants, and physical treatments like heat or cold, are not viable treatments.

Blueberries tolerate irradiation without adverse effects. However, the effects of irradiation have not been studied for insect disinfestation for many other crops. McDonald suggested several areas of future research on how irradiation affects the quality of horticultural commodities:

1. Identify preharvest or postharvest treatments that reduce possible damage.
   
   Improved horticultural practices--irrigation, hormonal sprays, fertilization, increasing calcium uptake--may increase resistance to irradiation stress. Maturity of the commodity at harvest, time between harvest and treatment, and post-treatment storage conditions also need to be considered.
   
   
2. Determine the physiological basis for conditioning.
   
   The conditioning phenomenon, a pretreatment process that has been found to reduce damage from heat and cold treatments, could alleviate some irradiation damage. The physiological and biochemical basis of irradiation damage needs to be determined.
   
   
3. Develop objective methods to measure irradiation damage.
   
   Objective methods are needed to provide uniformity in assessing quality and damage. These methods would provide quantifiable information that could be related to biochemical indices. These indices could indicate thresholds of damage or predict potential damage, allowing greater flexibility in developing irradiation quarantine treatments.