Quarantine treatments for sweet cherries

By Lisa G. Neven and Stephen R. Drake, U.S. Department of Agriculture-Agricultural Research Service, Yakima Agricultural Research Laboratory, Wapato, Washington

USDA-ARS laboratories have been involved in identifying and developing alternative quarantine treatments for sweet cherries to meet export quarantine restrictions. During the past five years, we have been researching the use of irradiation, heat treatments, cold treatments, controlled atmospheres, and even microwaves. We will discuss the status of development of treatments, the acceptability both nationally and internationally, and the potential for implementation on a commercial level.

IRRADIATION

Twenty years ago, the thought of irradiating food was a very scary proposition to most people. Although irradiation technology was not any more advanced than now, the notion that "nuclear" energy was used on food going into human mouths was not widely accepted. Today, the idea is being revisited, especially in the wake of E. coli, and other food-borne disease outbreaks.

Let us say this right now: irradiated food does not itself become radioactive. There is no evidence that radiation used at the levels (<1,000 Gray or Gy) approved by the U.S Food and Drug Administration (FDA) causes any substantial reduction in vitamin or nutrient levels or produces excessive levels of free radicals any greater than conventional heat cooking does. Research on the use of radiation for disinfesting fresh commodities has been ongoing since the 1940s. This initial work looked at levels required to kill the insects, levels not always tolerated by the commodities.

The FDA has set the acceptable upper limit dose of radiation on fresh foods at <1,000 Gy. Today, researchers realize that instantaneous death of insects is not always possible at the levels approved by the FDA. Therefore, proper tests must be conducted to ensure that insects treated with radiation are not able to reproduce. The problem is with the presence of nonviable, but moving, insects.

Typically, an inspector will examine a commodity to look for prohibited insect pests. If a live one is found, the shipment is either treated or destroyed. If the commodity has been treated with radiation, it is possible that a live, though nonviable (unable to develop or reproduce) insect may be found. How can the inspector determine whether or not the live insect is nonviable?

Holding the fruit while checking for continued development of the insect would result in the loss of the perishable commodity. For example, an irradiated third instar codling moth can linger, without continuing its development, for over two months before it dies. No cherry could withstand being held for that length of time and still be marketable.

Both federal and international plant protection bodies have begun to establish guidelines for relying upon documentation which certifies a shipment has been treated with the appropriate dose using dosimetry (tests used to ensure the appropriate radiation dose was indeed given to the commodity). The United States Department of Agriculture, Animal and Plant Health Inspection Service (USDA-APHIS) is the only regulatory body to establish, approve, and operate under guidelines to handle irradiated fresh foods.

To date, only fresh commodities from the state of Hawaii to the U.S. mainland have been approved for irradiation to control insect pests, though this option for treatment has not been used. No other country has approved irradiation as a quarantine treatment, and thus irradiation is illegal in most countries, even in the United States, except for the above special case of Hawaiian fruit to the U.S. mainland.

Irradiation is quick, efficient, effective, leaves no residues, and causes, in most cases, almost no damage to the commodity. In general, at the dose required to prevent development of both codling moth and western cherry fruit fly (300 Gy), there is no significant change in cherry fruit quality. At the 300 Gy level, codling moth larvae cannot pupate and western cherry fruit fly cannot emerge as adults. We have not been able to identify a "marker" system which an inspector could use to determine whether an insect has received the appropriate dose to render it nonviable. Although other researchers have found a connection between phenoloxidase and irradiation in Caribbean fruit fly, the test is too inconsistent to be used accurately by inspectors in the field.

However, perhaps a marker on the insect is not the answer. There is a company which has produced a rather inexpensive and reliable dosimeter which can be put on every box that would indicate the dose the box (and its contents) received. This would provide an additional level of certification that a commodity has been appropriately treated.

Our research shows that irradiation is a viable treatment for sweet cherries. There are a few gaps in the data on the effects of radiation on codling moth and western cherry fruit fly (WCFF). Previous research on codling moth was performed on either naked insects or insects in apples. Nothing was performed on insects in cherry fruit--a potential requirement of any importing country. We are now in the process of generating that data. Also, the published information of the effects of radiation on western cherry fruit fly is very limited. Only the third instar was treated, and of those treated, over 90% were parasitized, thereby confounding the results. We have now treated more than 2,000 third instar WCFF in a dose response experiment.

The problem with this project is that the pupal cases (not necessarily pupae) have to be held for four months or more at low temperature to overcome the diapause requirement of this insect, before adult emergence data can be collected. We will be proceeding with examining the effects of radiation on the other larval instars and egg stages this season. This type of testing would most likely be required by any importing country to ensure that the treatment is efficacious.

COLD TREATMENTS

The pursuit of a cold treatment to control codling moth in sweet cherries is pretty much dead in the water. Research on codling moth in apples has shown that the larvae can survive more than 120 days at 32°F, not a treatment you want to use for sweet cherries. Some packing houses have been using modified atmosphere packaging (MAP) on their sweet cherries to extend the storage life for up to ten weeks, but this is not useful to kill codling moth. Although the MAP alters the levels of atmospheric gases (lower oxygen and elevated carbon dioxide), the levels are not sufficient to control codling moth larvae. Tests with larvae in apples show that they can survive more than 90 days at 32°F under a 1% O₂ and 3% CO₂ atmosphere. Again, this is not recommended as a treatment for sweet cherries.

HIGH TEMPERATURE

The axiom "pick it, get it cold, and keep it cold" is driven into the minds of most cherry growers. The first time we suggested heating cherries, we almost got laughed out of the room. The reason we tried heating cherries is based on fruit physiology. If you were to go out to a cherry tree in the middle of the afternoon and check the internal temperature of a cherry exposed to the sun, you'd be surprised to find the thermometer reads somewhere between 112 to 120°F. We have monitored fruit temperatures here, as have scientists in New Zealand, and have found cherry fruit temperatures reach as high as 120°F. Amazing!

How does the fruit handle that high temperature without turning into mush? The answer is, in part, due to heat-shock proteins and other stress-responsive mechanisms commonly used by higher plants. Heat-shock proteins help prevent protein denaturation, maintain transport of important proteins within the cell, and may also help maintain cell wall integrity.

Another class of stress proteins, sometimes called dehydrins, helps prevent water loss under stressful conditions. These proteins, along with other physiological mechanisms, help the fruit to withstand environmental stresses.

When a heat treatment is applied to a freshly harvested cherry, the ability of these stress mechanisms to operate are still fairly intact. A short period of heat exposure is tolerated fairly well by the fruit. The key to the development of a heat treatment to disinfest fruit is finding a time/temperature combination that will maintain fruit quality while killing the target pest.

When we first started out in this project, with the help of Dr. Elizabeth Mitcham at the University of California, Davis, we found that insect tolerances to direct heat were well above commodity tolerances. It took more than 90 minutes at 45°C (112°F) to kill codling moth in sweet cherries. A very long treatment for such a perishable commodity. Then we hit on an idea combining controlled atmosphere with the heat treatment. Not a novel idea, since the initial work on combination heat with controlled atmospheres (CA) was performed way back in the 1930s, but one we thought we had the technology to perform on a commercial scale.

HEAT PLUS CA

Why try a combination heat with CA treatment? Insects, like plants, have heat shock proteins and physiological mechanisms to withstand environmental stress. They also need oxygen to fuel the energy used to withstand these stresses. When the level of carbon dioxide is too high, or the level of oxygen is too low, insects can simply close their spiracles (or breathing holes) and wait until conditions improve before breathing again--like a temporary suspended animation. This control is also influenced by temperature and humidity. We searched for a proper combination of time/temperature/oxygen/carbon dioxide treatment that would kill the insect and maintain fruit quality. During our search for the proper combination, we also observed a critical relationship between the rate of heating and insect mortality. The rate of heating is so critical that it can mean the difference of up to 30 minutes in the duration of the treatment. We finally came up with two treatments, using a combination of heat with low oxygen and high carbon dioxide, which would kill the insect and preserve fruit quality.

What happens to the cherry with this combination heat/CA treatment? Doesn't the fruit also need oxygen to live? Yes, but cherries also have a capacity for short-term anaerobic metabolism. They will produce ethanol during the treatment, which is later converted back to sugar once the fruit is cooled down. The ethanol that is produced by the cherry is no longer detectable within a couple of hours following treatment.

In July 1997, we supplied the participants of the annual Washington Tree Fruit Research Commission (WTFRC) apple postharvest review with a comparison of various alternative treatments of Bing and Rainier sweet cherries. There were little differences detected in visual or taste quality between irradiated, CATTS (Controlled Atmosphere/Temperature Treatment System) and control fruit, except for the high rate of pitting in the CATTS fruit. We believe this problem of pitting can be eliminated. Off flavors were detected in methyl bromide-treated fruit.

Overall quality parameters measured over the course of the study showed that the only two major problems with CATTS-treated fruit were shortened storage life (14 days as opposed to 28 days) and increased pitting and bruising. The shortened shelf life is most likely the result of sub-optimal post-treatment packaging. We hope to correct this by applying MAP after treatment. Bruising in both cultivars was most likely due to increased handling of the fruit during the CATTS treatments. Pitting was most likely the result of the formation of carbonic acid during CATTS treatment or prolonged exposure of hot cherries to 100 parts per million (ppm) chlorine water in the hydrocooler.

1998 Season Data: This past season, we addressed the problems of pitting and storage duration. We compared treatments of no heat, heat (47°C for 25 minutes), heat plus 1% O₂, 10% CO₂; and heat plus 1% O₂, 15% CO₂. We reduced the level of chlorine in the hydrocooler water to 50 ppm. We also tried regulating the dew point in the chamber, keeping it at two degrees Celsius below the surface temperature of the fruit to reduce water condensation on the fruit, thereby reducing the formation of carbonic acid. Following hydrocooling, we packed fruit in either regular packaging materials or in modified atmosphere packaging (MAP). After packing, the cherries were stored for 14 and 21 days at 2°C. Typical evaluations of quality parameters were made.

We found that although the number of pits was the same as last year, the size of the pits was greatly reduced. Control fruit showed about 20 pits per fruit for Bing and 25 pits per fruit for Rainier. Heat-treated fruit, regardless of packaging or atmospheres during treatment, showed an average of 35 pits per fruit. It appears that perhaps the chlorine may still be the culprit, since even control fruit exhibited elevated pitting. Perhaps hydrocooling in warmer water and/or water without chlorine might alleviate this problem. More research in this area is required.

The good news is the extension of storage life. By using MAP, we were able to get 21 days of fruit storage. Firmness, soluble solids, and titratable acidity were comparable to control fruit. The fruit were visually comparable to methyl bromide-treated fruit, with stems being slightly greener and gloss much improved.

Boxes of treated cherries were provided to the participants of WTFRC's annual apple postharvest review in July. Kyle Mathison, of Stemilt Growers, also provided comparisons of methyl bromide-treated Bings and fruit kept at room temperature for three days. The general consensus was that the CATTS-treated fruit compared quite favorably with and perhaps was even better than methyl bromide-treated fruit.

Although we have shown that the combination heat/CA treatment is not that much worse than irradiation or traditional methyl bromide treatments, there is still a huge opposition to the procedure: it goes against traditional practices of the industry.

OTHER ALTERNATIVES

We have looked at the possibility of using microwaves as a heat treatment. Heating something faster would mean a shorter treatment time and more effective treatment all around. There are a number of parameters that need to be optimized before microwave technology can be used for quarantine purposes. We are encouraged that with the gifted and bright scientist Dr. Jumming Tang of Washington State University working on the project, something will come up soon.

We haven't looked into other fumigants as potential treatments because so many are either carcinogenic, hazardous to humans, ozone depleters, or they have not been registered for use on fresh food.

We will continue to keep the industry updated on the progress we make in the area of alternative quarantine treatments for sweet cherries.

ACKNOWLEDGMENTS

The authors wish to express appreciation to the Washington Tree Fruit Research Commission for grant funds which partially supported this research; Fred Scarlett of Northwest Fruit Exporters for supplying fruit; and, Jim Doornink and Kyle Mathison for providing cherries for the 1998 season. We also are very grateful to Bill Bernard of CVP Systems for the loan of the MAP unit and to Tom Bailey of Orchard View Farms for educating us in the procedures of MAP.

ADDITIONAL READING

Neven, L.G. and E. Mitcham, "CATTS: Controlled Atmosphere/Temperature Treatment System. A Novel Approach to the Development of Quarantine Treatments," American Entomologist 42(1): 56-59, 1995.

Drake, S.R. and L.G. Neven, "Quality responses of Bing and Rainier sweet cherries to low dose electron beam irradiation," Journal of Food Preservation and Processing 21: 345-351, 1997.

Neven, L.G., "Respiratory response of fifth instar codling moth to rapidly changing temperatures," Journal of Economic Entomology 91: 302-308, 1998.

Neven, L.G., "Effects of heating rate on the mortality of fifth instar codling moth," Journal of Economic Entomology 91: 297-301, 1998.

Neven, L.G. and Morford, M.W., "Effects of irradiation on phenoloxidase levels in codling moth larvae,"Journal of Economic Entomology 91: 534-538.

Drake, S.R. and L.G. Neven, "Irradiation as an alternative to methyl bromide for quarantine treatment of stone fruits," Journal of Food Processing and Preservation 21: 345-351, 1997.

Drake, S.R. and L.G. Neven, "Irradiation as a quarantine treatment for stone fruits,"Good Fruit Grower Feb. 1998. Pp. 46-49. *

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Quarantine treatments for sweet cherries

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