CASE STUDY -- Methyl Bromide Alternative
Temperature Control for Stored Product Insects at Pillsbury
The Pillsbury Company uses a method known as superheating or heat sterilization (HS) to control stored-product insects (SPI) in flour handling equipment and areas at its production facilities, which produce dough, dry mixes for baking, and pizza (Heaps 1995). This method has been in existence since the turn of the century and uses extremely high temperatures (130 -140 F, 54-60 C) for extended periods of 18 to 24 hours to kill the egg, larvae, pupae, and adult stages of stored product insects. Heat sterilization has proven to be extremely successful. In addition, because heat sterilization does not require toxic chemicals or chemical pesticides, there are no such related threats to worker health (Heaps 1988) or regulatory or commercial barriers to its use.
Several other national food companies (Nabisco, Quaker Oats, Con Agra, General Mills) are also using heat sterilization to eliminate insects from their facilities. From its experiences, Pillsbury believes that heat sterilization can be used to replace 100 percent of the methyl bromide that it has had to use to control stored product insects. About 3 percent of the total methyl bromide consumed in the United States is used to treat flour mills (Heaps 1995).
In 1992, the Pillsbury Company established a program to use heat sterilization in its flour-based operations. The impetus to switch to heat sterilization stemmed from the company's desire to eliminate its reliance on chemical pesticides. In November 1993, the company had wanted to curtail its use of DDVP (Vapona), because EPA was revoking the use of it on food due to concerns regarding its carcinogenicity. Because Pillsbury feared that the use of other pesticides, including methyl bromide, might be restricted or banned, it established a goal of replacing all conventional pesticides by the year 2000 (Heaps 1995).
Pillsbury began its heat sterilization program by first experimenting and learning with heat treatment in one of its facilities, gradually expanding the technique to three other plants. Pillsbury plans to start treating another plant with heat sterilization in the near future. When testing the heat sterilization method, the company performed all research in-house and received no outside funding for its work (Heaps 1995).
Overview of Methyl Bromide Usage
Methyl bromide has been widely used in the United States for controlling insects in flour mills because it is able to penetrate cracks and crevices where flour dust penetrates and insects may breed. Pillsbury did use methyl bromide, but has been successful in virtually eliminating the use of methyl bromide over the past several years. In 1994, Pillsbury reported using no methyl bromide (Heaps 1995).
Discussion of Alternative Strategy
The heat sterilization technique uses high target temperatures ranging from 130 F to 140 F (54 C to 60 C) to kill stored product insects by inducing dehydration and/or protein coagulation or enzyme destruction. The length of time required to destroy stored product insects varies from a few hours for a piece of machinery up to 24 hours for a large facility. The key to successfully eradicating the insects is achieving the target temperatures for a long enough period of time to reach lethal temperatures in areas such as machinery interiors, voids, dust collectors, and piping where stored product insects may be found (Heaps 1988).
Heat sterilization can be conducted using heaters with fans fueled by either high or low pressure steam, water, gas, or oil. The heaters may be installed permanently in the facility or can be portable. In general, heat sterilization is conducted on weekends or holidays when production has ceased for the week. The frequency with which heat sterilization needs to be performed varies depending on the climate where the facility is located. In general, heat sterilizations can be performed every 6 to 7 weeks during the warmer months and every 10 to 12 weeks during the colder months. However, in colder climates, heat sterilizations may only need to be conducted 2 to 3 times per year. Pillsbury has gained knowledge from each heat sterilization that have improved the technology and made it more efficient. Heat sterilization should be done according to need based on stored product insect activity as determined by plant inspection/sanitation programs plus monitoring stored product insects using specie specific pheromone traps.
Several factors must be taken into consideration when conducting heat sterilizations. First, the area to be heated must be sealed to maintain target temperatures. Second, anything that could damaged by high temperatures, such as fire extinguishers, aerosol cans, flammable liquids, computers, and other electrical equipment, should be moved to cooler areas. Finally, the effectiveness of heat sterilization requires that general cleaning and sanitation procedures be followed in the facility to ensure pest control (Heaps 1988).
The Pillsbury company has conducted extensive research to ensure the effectiveness of their heat sterilization technique. In one experiment, Pillsbury tested the use of rented electric diesel-fueled heaters to heat sterilize a 350,000 cubic foot food plant dry mix area. The area was heat sterilized at temperatures approaching 130 F (54.4 C) for at least 24 hours. The company used confused flour beetle (Tribolium confusum) bioassays (eggs, larvae, and adults) to test the effectiveness of this technique. In addition, leavening ingredients were exposed to the heat and evaluated for damage. Equipment used in the heat sterilization was also observed for damage (Heaps and Black 1994).
Based on the results of this experiment, Pillsbury determined that insect mortality for adults/larvae combined was 95 percent. The 5 percent of test insects that were not killed were placed in areas where it was believed heat would not penetrate. The company found no equipment damage aside from two computer input cards which needed replacement. In addition, the leavening ingredients did not appear to be damaged (Heaps and Black 1994).
Costs Associated With Alternative Technologies
The costs of heat sterilization vary depending upon the scope of the area to be treated, the amount and source of heat to be used, and the equipment required to generate that heat. Costs also depend on whether a facility must rent or buy equipment to generate the required temperatures or can utilize its own heat system. Other costs that must be considered include labor and plant downtime. In general, if a facility must rent or purchase heating equipment, the initial costs of heat sterilization are higher than those associated with methyl bromide use because of this capital investment. However, in the long term, heat sterilization may be cheaper than methyl bromide because much less sealing, and therefore less labor, is required for heat sterilization than for treatment with methyl bromide. In addition, the costs of insurance to use methyl bromide are quite high. Pillsbury has estimated that for a million cubic foot production plant, it would cost $0.5 million to 1 million to install heaters, while it would cost about $20,000 per application to rent the heaters to perform the same work. Pillsbury also believes that the cost associated with methyl bromide use, and its restrictions, are likely to increase each year at a greater pace than those associated with heat.
Although rental or purchase of heating equipment may be initially more expensive than using methyl bromide, utilizing existing heating equipment in a facility tends to be less expensive than methyl bromide since fuel is relatively inexpensive, and the cost of labor when using heat sterilization are considerably less than associated with the use of methyl bromide. Table 1 presents a cost comparison of the treatment methods (Heaps 1995).
Comparison of Treatment Costs
Treatment Method Costs (per 1,000,000 cubic feet):
Heat Sterilization: $747-830 (1)
Methyl Bromide: $2,000-4,500 (2)
1 - This estimate includes the cost of the heating fuel and assumes that the facility is utilizing its own heating equipment. Calculation is based on the following estimates; 7 million BTUs/hour, 135,000 BTUs/gallon of kerosene fuel, $0.80/gallon of kerosene fuel, and a heating period of 17-20 hours.
2 - This estimate includes cost of purchase of methyl bromide.
Source: EPA, 1985; Heaps 1995.
EPA. 1985. Compilation of Air Pollutant Emission Factors. U.S. Environmental Protection Agency, Office of Air and Radiation, Office of Mobile Sources. Ann Arbor, Michigan.
Heaps. 1995 (January 3 and February 1). Personal Communication. Jerry Heaps, The Pillsbury Company. Minneapolis, Minnesota.
Heaps. 1988. Turn on the Heat to Control Insects. J. W. Heaps. Dairy and Food Sanitation, Volume 8, pp. 416-418.
Heaps and Black. 1994 (July). Using Portable Rented Electric Heaters to Generate Heat and Control Stored Product Insects. J.W. Heaps and T. Black. Association of Operative Millers - Bulletin, pp. 8408-8411.
Please note that this publication discusses specific proprietary products and pest control methods. Some of these alternatives are now commercially available, while others are in an advanced stage of development. In all cases, the information presented does not constitute a recommendation or an endorsement of these products or methods by the Environmental Protection Agency (EPA) or other involved parties. Neither should the absence of an item or pest control method necessarily be interpreted as EPA disapproval.
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