IRRADIATION AS AN ALTERNATIVE TREATMENT TO METHYL BROMIDE FOR INSECT CONTROL
B. AKINBINGÖL
Ankara Nuclear Agriculture and Animal Science Research Center
Nucleer Agriculture Department, Saray, Istanbul Road 30. Km, 06983 Ankara TURKEY
Many of the practical applications of food irradiation have to the with preservation.Radiation inactivates food spoilage organisims, including bacteria, moulds and yeasts. It is effective in lengthening the shelf -life fresh fruits and vegetables by controlling the normal biological changes associatiated with ripening, maturation, sprouting, and finally aging. For example no radiation delays the ripening of green bananas, inhibits the sprouting of patatoes and onions,and prevents the greening of endive and white potatoes. Radiation also destroy disease-causing organisms, including parasitic worms and insect pests, that damage food in storage. As with other forms of food processing, radiation produces some useful chemical changes in food. For example, it softens legumes (beans), and thus shortens the cooking time. It also increases the yield of juice from grapes, and speeds the drying rate of plums (Table l.).[l]
The radiation dose - the quantity of radiation energy absorbed by the food- is the most critical factor in food irradiation. The special name for the unit of absorbed dose is the gray (Gy). It is defined as the mean energy imported by ionizing radiation to matter per unit mass. One Gy is equal to one joule per kilogram. ( An older unit of radiation measurement, the rad, equals 0.01 Gy ). At present, the dose of radiation recommended by the FAO/WHO Codex Alimentarius Commission for use in food irradiation does not exceed 10 kGy. This is actually a very small amount of energy, equal to the amount of heat required to raise the temparature of water by 2.4 °C.
There are four sources of ionizing radiation for use on food: Gamma irradiation using the isotope of 1) Co 60 or 2) Cs 137 3) Electron beam, or 4) x-ray (bremsstrahlung) irradiation produced when an electron beam strikes a high density converter. X-ray radiation is very similar to gamma ray from isotopes both can penetrate pallet-loads of produce. A major difference is that X-ray radiation is consantrated in the same direction as the electron beam; gamma rays from isotopes are emitted in all directions uniformly. Electron beam radiation penetrates only a few centimeters and, thus, is limited to small products, such as shallow boxes of berries, passing the irradiation source on a conveyor line. All other thingsbeing equal, such as dose rate, close uniformity, and product density, there is no difference in effect amaong the fourionizing radiation sources used on food items. Most radiation research has been done with the radioactive isotopes cobalt60 and cesium137. Stored products such as cereals and legume, spices, dried fish and meat and even fresh fruits and vegetables are routinely fumigated by various chemicals to destroy insects and microorganisms. As there are widespread concerns among the consumer of health risks associated with the use of certain fumigants such as ethylene
dibromide (EDB), methyl bromide (MB), ethylene oxide (ETO) and phosphine. In the past, EDB was widely used for fumigation of such produce prior to importation. The prohibition of EDB by the U.S. Environmental Protection Agency (EPA) in 1984. Alternative treatments to EDB fumigation such as vapour and dry heat treatment, hot water dips, refrigeration at near 0 oC for specific duration, and other chemicals such as methyl bromide, phosphine and cyanide are commodity specific and have been used with varying degrees of success. [3]
Table 1 . Dose requirement in various applications of food irradiation
Purpose Dose (kGY) Products Low dose ( up to 1 kGy )
1. Inhibition of sprouting 0.05 - 0.15 Potatoes, onions, garlic,ginger root, etc. 2. Insect disinfestation and
parasit disinfection
0.15 - 0.5 Cereals and pulses, fresh and dried fruits, dried fish and meat, fresh pork, etc.
3. Delay of physiological process (e.g.ripening)
0.5 - 1.0 Fresh fruits and vegetables
Medium dose ( 0 - 10 kGy )
1. Extension of shelf-life 1.0 - 3.0 Fresh fish, strawberries,etc. 2. Elimination of spoilage
and pathogenic organisms
1.0 - 7.0 Fresh and frozen seafood, raw or frozen poultry and meat, etc.
3. Improving technological properties of food
2.0 - 7.0 Grapes ( increasing juice yield ), dehydrated vegetables ( reduced cooking time ), etc.
High dose ( 10 - 50 kGy )
1. Industrial sterilization ( in combination with mild heat )
30 - 50 Meat, Poultry, seafood, prepared foods, sterilized hospital diets
2. Decontamination of certain food additives and ingredients
10 - 50 Species, enzyme preparations, natural gum, etc.
The major fumigant for post-harvest disinfestation is now methyl bromide (MB) for food and agricultural products against pests such as insects and nematodes, will likely suffer the same fate as EDB. MB has been listed under the Montreal Protocol (an international treaty for the regulation of ozone depleting substances world wide and under the auspices of the United Nations Environmental Programme) as one of the substances which causes depletion of ozone layer. The original phase-out schedule of MB was seet for the year 2000. However, the last meeting of the Parties to the Montreal Protocol held in Montreal, Canada in September 1997 had revised the phase-out schedule of MB according to the following :
Advanced countries: 25 % reduction by the year 1995
50 % reduction by the year 2001
70 % reduction by the year 2003
Phase-out by the year 2005
Developing countries: 20 % reduction by the year 2005
Phase-out by the year 2015
The USA which under its Clean Air Act of 1990 had earlier prohibited the production and consumption of MB by the year 2001. As the USA is the major producer of this chemical, it remains to be seen how other countries could obtain MB when there is no production in the USA after 2005. Already the cost of this chemical, has increased significantly in recent years. The limited availability of MB after 2005 could lead to non-competitive of the use of this chemical for insect control especially in developing countries. [4]
Another fumigant which is widely used for control of insect pests in grain is phosphine. Phosphine is slow acting, but even with this limitation it is internationally the predominant grain fumigant. There are several reports to indicate that many insect pests of stored products have developed resistance to phosphine, which necessitates increased levels of this chemical.[5]
The major advantage of MB is the short time needed to disinfest grain, 5-24 hours with MB compared to 5-12 days for phosphine. This makes it ideal for disinfestation of grain found to be infested at export or as a phytosanitary requirement of a contact of sale. [6]
Hydrojen cyanide is one of the most toxic insect fumigants. The fact that it is very soluble in water makes it unsafe to use on moist materials such as fruit and vegetables. At certain dosages and under spesific conditions, sodium cyanide is used to kill mites and surface insects such as scales, mealybugs and white fly nymphs on citrus. [7]
Therefore, satisfactory long term storage of staple crops maybe in jeopardy as these traditional fumigants are to be phased-out or because of increased insect resistance.
One of the alternatives to fumigation is temperature maniplation includes the use of heat and cold. The factors that must be considered in temperature maniplation are the effective temperature needed to kill the pest (efficacy), the tolerance of the commodity that is being treated (phytotoxicity) and the time of exposure. [7]
Grain of cereals, maize, sorghum, birdseeds, and legume seeds for human and animal consumption is protected against insect and mite pests by a range of pest management procedures. Options include insectisides additives, temperature reduction by natural or refrigerated aeration, reduction of moisture content, hermetic sealing of storages after fumigation or atmosphere modification with carbon dioxide or nitrogen, insect proof packaging, heat disinfestation and manipulation of handling operations such as cleaning or grading procedures, vertical impact when grain is relocated in vertical or horizantal storages involving high velocities and rapid decelerations. [6]
Insects are capable of developing resistance to most chemicals. Irradiation offers a good potential to replace MB fumigation whenever the use of this chemical is prohibited and an efficient alternative to phosphine. Low-dose irradiation between 0.2 and 0.7 kGy can control effectively insect infestation of grain and stored products. [8]
The most radiation tolerant stage will be the most developed one found in the commodity. Table 2 recommends objectives for irradiation treatments based on the most advanced stage present at the time of harvest.
Doses against several groups of pests are presented in Table 3. Many groups of pests from the important orders DIP, COL and HOM are controlled with relatively low doses which are tolerated by many plant hosts. Other important groups, such as tetranychid mites and LEP, are controlled by moderate doses ( 0.2 -0.3 kGy). [2]
Table 2 . Recommended objectives of irradiation treatments based on most advance insect stage found at time of treatment
Most Advenced Stage Objective of treatment
Egg Prevent development beyond first instar
Early instars ( simple metamorphosis ) Prevent late instar or adult development Early instars ( complete metamorphosis ) Prevent pupariation or pupation
Late instars Prevent pupation or adult emergency
Pupa Adult sterility
Table 3 . Absorbed dose ranges of several pest groups
Pest group Objective Dose ( Kgy )
Bruchid weevils Sterilize adult 0.07 - 0.1
Aphids, whiteflies Sterilize adult 0.05 - 0.1 Tephritid flies Prevent adult emergence from third instar 0.05 - 0.25
Scarab beetles Sterilize adult 0.05 - 0.15
Curculionid Weevils Sterilize adult 0.1 - 0.2 Noctuidae and Tortricidae Prevent adult emergence from late instar 0.1 - 0.3 Pyralidae and Tortricidae Sterilize late pupa 0.2 - 0.3
Tetranychid mites Sterilize adult ~0.3
Stored product beetles Sterilize adult 0.05 - 0.4
Acarid mites Sterilize adult ~0.5
Stores product moths Sterilize adult 0.1 - 1
Root-knot nematodes Sterilize adult ~4
Radiosensitivity is a basic requirement for radiation disinfestation of agricultural products; it is gauged by determining the lethal or sterilizing effects on the stage of the insect involved. Typically, lethality is gauged by determining the emergence of adults from the irradiated pre adult stages or by determining survival of metamorfic stages from previous stages that were irradiated. In situations where the lethal effects are not required, sterilizing effects would be the important criterion in determining both the effectiveness of disinfestation and the level of irradiation that will fulfil the requirements for disinfestation. Sterilizing doses are lower and would therefore be more economical than lethal doses. However, the presence of any live insects, even sterile ones, maybe unacceptable. Thus, selection of sterilizing versus lethal doses must be based on an array of factors. [9]
With all other treatments efficacy is measured by acute mortality. Radiation doses required to provide near 100 % acute mortality of insects would be well over 1 kGy, and would damage almost all agricultural commodities. Therefore, very little product damage occur even if larvae and adults survive for some periods following treatment. The advantages of radiation processing include short treatment time in comparison to several days of exposure required in chemical fumigation (e.g. phosphine), no undesirable chemical residues in the food, no resistance developed by the insects as in the case of phosphine, and no significant changes in the physicochemical and functional properties or the nutritive value of the treated products.[10]
Prepackaged cereals, legumes, flour etc. in consumer packs can be treated which is not feasible with fumigants. Since irradiation has no residual effect, it is necessary to prevent reinfestation by proper storage management and the use of insect resistant packaging materials. To prevent re-infestation, grain and stored products must be properly packaged in insect proof containers.
Most of the research reported on the requirements for insect resistance packages for agricultural products in order to prevent post-irradiation infestation. Bhuiya et al. (Paper IAEA-RC-273.3/2) compared bags made of jute, polyvinyl chloride (PVC), polyethylene (PE) lined jute or polypropylene with traditional jute bags for storing irradiated pulses and oilseeds. The pulses and oilseeds were infested before irradiation and stored for 8 months. The three experimantal bags contained fewer insects at each periodic examination than did the standard jute bags; PVC bags and PE lined jute bags were the most effective. [8]
After irradiation to eliminate internal infestations, packaged products must be protected from external insects that could subsequently be encountered in shipping, storage and transportation systems. Thus, to develop the technology for irradiation disinfestation and post irradiation protection from reinfestation, developmental packages must be tested appropriately. The tests must include the storage and exposure to insect species that are likely to be encountered until the time the package is opened by the consumer.
The cost of irradiating food has been estimated at between US$ 0.02 and US$ 0.40 per kilogram. This wide range results from many variables involved in any one irradiation operation. Among them are:
-the dose of radiation employed (which can vary widely depending on the purpose of the treatment)
-the volume and type of product being irradiated -the type and efficiency of the radiation source
-whether the facility handles one or a variety of food products -the cost of transporting food to and from the irradiator -special packaging of the food
-cost of supplementary processing such as freezing or heating.
Construction of an irradiation plant large enough to permit economic operation has been estimated the cost in the order of several million US dollars.
The existing limited industrial experience with food irradiation makes it diffucult to assess how the cost of this process might compare with those of other food processing technologies. It seems reasonably certain, however from knowledge gained through research and development as well as practical application, that the benefits of food irradiation make its costs competitive. [1]
CONCLUSION
Turkey is the leading country in the world, in production and exports of dried fig, apricot, raisin and hazelnut. One of main problem in the export trade is infestation by stored product insects.
Using MB is very effective for controlling stored product insects in Turkey. MB has also listed as an ozone depleting substance and worldwide production will be phased out in the near future, than Turkey will be faced very serious problem for export dry fruits and hazelnut.
Use of irradiation to disinfest agricultural products has obvius advantages, most of which are influenced by environmental, cultural, economic, commercial and govermental factors. The first two factors, effectiveness and economy, are adressed principally. Research conducted world wide in the past four decades have shown that radiation processing is an effective and safe method for controlling insect pests of stored products. Irradiation offers an effective alternative quarantine treatment which is more environmentally friendly and sustainable as compared to fumigants. In view of the phasing out of the currently used post harvest chemical fumigants, irradiation either alone or in conjuction with other post-harvest procedures can contribute towards the goals of achieving food security in developing and less developed countries by effectively reducing post-harvest losses.
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3. Loaharanu, P., 1998. Irradiation as an Alternative to Chemical Treatments of Food. IAEA.
4. Loaharanu, P., 1999. International Developments of Food Irradiation. IAEA Documents. 5. Muthoo, M. K., 1999. Opening Statement. FAO/IAEA/WHO International Conference on
Irradiation to Ensure to Safety and Quality of Food. Antalya, 19-22 Oct. 1999.
6. ANONYMOUS, 1990. Use of Irradiation as a Quarantine Treatment of Food and Agricultural Commodities. IAEA, Venna 220 p.
7. ANONYMOUS, 1999. Irradiation as a Quarantine Treatment of Arthropod Pests. IAEA- TEC DOC-1082, Vienna, 19-27.
8. IAEA. 1991. Report of a Task Force Meeting on Irradiation as a Quarantine Treatment of Fresh Fruits and Vegetables, Convered by the International Consultative Group on Food Irradiation (ICGFI). IAEA, Vienna.
9. ANONYMOUS, 1991. Insect Disinfestation of Food and Agricultural Products by Irradiation. IAEA Panel Proceedings Series. Vienna. 174 p.
10. WHO. 1994. Safety and Nutritional Adequacy of Irradiated Food, World Health Organisation, Geneva (1994).
