Blowing hot and cold

by Teresa Acklin
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With fumigation restrictions on the increase, flour millers and grain handlers look at temperature-based systems to control insects.

   Spurred by environmental and health concerns, international authorities are placing more and more constraints on the use of pesticides, fumigants and other chemical means of knocking out insects in the food chain. While the restrictions may help to assure food safety and reduce environmental damage, they also are forcing the grain handling and processing industries to find other weapons in their perpetual battle against bugs.

   Fumigant restrictions and potential temperature-based pest management techniques were major topics at the recent Association of Operative Millers Technical Conference and Trade Show in Salt Lake City, Utah, U.S. Indeed, some have called these issues the most significant operating issues currently facing the grain storage and milling industries.

   The pressure primarily stems from planned restrictions on methyl bromide, a fumigant widely used for decades in the grain storage and flour milling industries. It is inexpensive, fast-acting and highly effective in killing a wide variety of insects in most stages of development.

   Methyl bromide had been somewhat controversial for many years because of its toxicity, which requires respiratory protection and other safety measures during application. But its death knell sounded when scientists identified the chemical compound as an ozone-depleting substance, making it a threat to the global environment.


   The first step in restricting methyl bromide for fumigation took effect in January 1994, when annual production was capped at the 1991 level. This action was taken under terms of the Montreal Protocol, an international agreement to control ozone-depleting substances (see article on page 12).

   The United States and some other countries are pushing for an eventual global ban on methyl bromide; indeed, the United States already has acted to prohibit the substance in the U.S. by 2001.

   But many U.S. flour millers have tended to overlook the approaching ban, assuming “something will be done” to permit its continued use in mills. A panelist at the A.O.M. food safety forum, Jerry Heaps of The Pillsbury Co., Minneapolis, Minnesota, U.S., commiserated with this view.

   “We can't believe as an industry that they're saying we can't use methyl bromide anymore,” Mr. Heaps said.

   Mill fumigation makes up a relatively small percentage of total methyl bromide use; soil fumigation is the most prevalent and most environmentally threatening use. In addition, mills have few or no options available that are as economical and reliable as methyl bromide, Mr. Heaps said.

   He noted that the recent shift in the U.S. political climate favors increased cost-benefit analysis of regulations on business, which could provide an opportunity for the milling industry to receive a legal exemption from the methyl bromide ban. But unless the U.S. Congress passes such an exemption, millers eventually will have to do without methyl bromide, Mr. Heaps said. And at an international meeting earlier this year, the U.S. continued to press other Montreal Protocol countries to accept a ban.

   “You may wake up one morning and methyl bromide use as we know it won't be allowed,” he cautioned the millers at the A.O.M. meeting.

   Options to methyl bromide have been under study within the milling industry for several years. Scientists around the world have been working to develop new fumigant gases or to improve the efficacy of existing gases; experiments also have been conducted using “combination fumigation methods” which mix elements such as heat and phosphine.

   But the use of any fumigant gas carries the risk of toxicity, requires lengthy testing and registration procedures and faces potential future restrictions if new dangers are identified. So, many in the industry are looking at “more natural” solutions.

   Presentations at the A.O.M. meeting focused on two non-chemical methods of insect control in flour milling and grain handling — the use of heat and cold. Neither method is new, but neither typically has been as effective and as inexpensive as fumigation.


   At the food safety forum, panelists described what is called “facility heat-up,” the literal increase in a flour mill's internal air temperature to a level high enough to kill insects. Research in this area in the past several years has ranged from controlled experiments on the effects of heat on specific insects to demonstration heat-ups in working mills.

   Ole Dosland of ConAgra, Inc., Omaha, Nebraska, U.S., described the results of various experiments to assess the effective “heat parameter”; that is the lowest temperature that can kill an insect while, importantly, minimizing structural, electrical and mechanical damage to the mill plant and equipment.

   Heat as a pest management tool poses a series of extreme complexities because its effectiveness depends on so many variables, Mr. Dosland said. Various research has indicated that heat treatment is effective at temperatures of 44°C to 72°C, held at anywhere from 12 to 36 hours; but those ranges are far too imprecise to make heat-ups practical for the typical operating mill, he noted.

   Among the factors that influence the effective killing temperature are the type of insect and its stage of development; the temperature to which the insect already is acclimated; ambient air conditions, including the humidity level; and the building's structure, Mr. Dosland said.

   “A building with 12-inch (30-cm) floors will heat up differently than one with 16-inch (40-cm) concrete floors,” he said. “You need engineers to make some of these determinations.”

   To illustrate the effects of the many variables, Mr. Dosland reported on the results of specific tests conducted with red flour beetles, warehouse beetles and cockroaches obtained from the University of Nebraska. In the series of tests, the insects were monitored while being subjected to different temperatures and humidity levels.

   In one test, temperatures were increased slowly to 50°C with a relative humidity at 26% to 28%; insect mortality was 100%, Mr. Dosland said. In another test, a 46°C temperature with humidity at 74% to 76% produced inactive insects within 10 minutes; but all the insects recovered when temperatures were lowered. Finally, in a slow heat-up to 46°C from 30°C, held for three hours, some 27% to 36% of insects survived.

   In general, the tests showed that higher humidities generally reduce the rate of insect mortality, he said. The results also indicated that with humidity at 26% to 28%, minimum lethal temperatures were 44°C to 46°C for cockroaches, 46°C for warehouse beetles and 47°C to 49°C for red flour beetles.

   Mr. Dosland said millers considering facility heat-ups needed to make sure they had accounted for all the variables.

   “Know your pest, your plant, your process, the degree of your problem and your heat parameter,” he said. “Then the question is how to get that parameter to the insect; it could be a moving target.”

   Mark Mire, with Aggreko, Inc., a New Iberia, Louisiana, U.S., company that specializes in facility heat-ups, noted that the industry was advancing along the learning curve with heat-up methodology. Every demonstration of the process generally offers new insights that eventually will improve the technique, Mr. Mire noted; for example, a 1994 experimental heat-up of the pilot flour mill at Kansas State University, Manhattan, Kansas, U.S., revealed the need to tape doors and other small cracks to improve efficiency, he said.

   One of the most negative aspects of the heat-up method currently is the costs associated with it. The investment needed for heating systems, as well as energy costs for heating up and then cooling down the plant, can be steep, especially compared with fumigation.

   “The capital cost of installing a heat (source) is equal to 125 years of methyl bromide use,” Mr. Mire said.

   Possible energy sources and their costs are among the areas of the heat-up method that remain under study. Options discussed at the forum included use of existing plant heat systems, portable steam and portable electric heaters.


   Another option to control pests approaches the problem from the other extreme — cold. Although cooling down flour mill facilities to eradicate insects also has met with some success in experiments, the method is not particularly practical for flour mills in warmer climates.

   But for controlling insects in grain in storage, chilling has proven itself as a cost effective method, according to an A.O.M. presentation by Dirk Maier, assistant professor and extension agricultural engineer at Purdue University, West Lafayette, Indiana, U.S.

   Dr. Maier said chilling effectively halted insect infestations at all developmental stages. The technique also offers the benefits of preventing mold and reducing shrinkage, he said.

   Grain chilling systems have been widely used in Europe for the past several decades, Dr. Maier said, and rapidly are becoming popular in Southeast Asian markets, where grain is particularly susceptible to insect and mold problems inherent with the climate. But because of the availability of cheap fumigants, the systems never gained a foothold in the United States, he said; research and demonstration projects have been under way here since 1988, but the first commercial U.S. chiller was installed just last year in California for rice handling.

   Because of the increasing regulatory limits on fumigants, as well as the fact that many insects are becoming resistant to the most widely used chemicals, chilling is likely to attract more interest, Dr. Maier said. It also can be more economical than aeration, he said.

   Specific tests on wheat in storage illustrate these points, he said. Wheat with a temperature of 30°C and 14% moisture was stored in bins in the U.S. state of Michigan during the first week of July, when the area's ambient temperatures are among the year's warmest. One week later, wheat that had been chilled had dropped to a temperature of below 15°C, low enough to avoid insect infestations and prevent mold, and shrink loss was 0.7% to 0.8%. Aerated wheat reached the same temperature more than three months later, and shrink loss was 2% to 3%.

   Overall, preliminary studies of the comparative economics of aeration/fumigation systems versus chilling systems show chilling can reduce per-bushel costs by about 23%, Dr. Maier said. The analyses compared the capital investment, energy, labor, maintenance and depreciation costs of chilling systems with the fumigant, labor, monitoring, safety, energy, licensing and liability costs of aeration/fumigation, he said.