Propylene Oxide: The Registered Fumigant,
the Proven Insecticide as a MeBr Alternative

Tom Griffith & Morris Warren, ABERCO, INC.

This report is an attempt to demonstrate that the use of the New Old Fumigant, propylene oxide, can replace methyl bromide in many applications. With 40 years of food use history, we expect propylene oxide will satisfy all the tests of availability, effectiveness, safety and practicality. ABERCO, INC. has held the EPA Registration since 1984 and is now the sole registrant for the use of propylene oxide for fumigation purposes. The EPA label reads, "to aid in the control of microbiological spoilage and as an insecticidal fumigant for the control of stored product insects, to reduce bacterial and mold contamination in processed spices, cocoa and processed nutmeats (except peanuts)."

Worldwide, over ten billion pounds of propylene oxide are produced annually. Since propylene oxide is a basic chemical intermediate, and it is used to produce a wide variety of products such as urethane foams, cosmetics, polymers, starch modifiers, food emulsifiers and, of course, propylene glycol, it is designated as a GRAS (Generally Regarded as Safe) food additive. Propylene glycol is also the basis for the non-toxic Sierra antifreeze. The hydrolysis of propylene oxide to propylene glycol will be explored later.

The total effectiveness of sterilization doses of propylene oxide against the pathogenic bacteria Salmonella and E. coli 157 in nuts, cocoa and spices has been well documented, while growers routinely treat their products with propylene oxide to reduce other bacteria, molds and yeast as well.

Industries use methyl bromide to control stored products insects. With the accelerated timetable for banning all production and importation of methyl bromide beginning January 1, 2001, our top priority was to demonstrate the effectiveness of propylene oxide in killing the insects found in cocoa beans, in-shell nuts and spices. This study is underway at the Dried Fruit Association of California. The early results are: propylene oxide, at doses similar to typical methyl bromide doses, kills all stages of the confused flour beetle, warehouse beetle, red flour beetle, cigarette beetle and the Indian meal moth. Propylene oxide is particularly effective on eggs. A study on the rate of kill on the Codling Moth larvae in walnuts, as well as, off gassing studies on raisins, prunes and figs are scheduled.

The allowable residue tolerance for propylene oxide is 300 ppm. However, these residues are not persistent as they will evaporate rapidly from the substrate due to the relatively high vapor pressure of propylene oxide. Furthermore, propylene oxide will react in the human stomach to form (GRAS) propylene glycol. A research program was established utilizing radio labeled propylene oxide for 100% accuracy and irrefutability.

The test was conducted in two parts: 1) Using simulated stomach juice at pH 1.0, the PPO half life was 63 seconds. 2) Using human stomach juice at pH 1.5, the PPO half life was 111 seconds.

Notes: All PPO converted to (GRAS) propylene glycol. No propylene chlorohydrin, a suspected by-product, was found. These facts indicate that propylene oxide is not likely to be carcinogenic or harmful.

The conversion of propylene oxide to propylene glycol is catalyzed by both acid and base and the rate of reaction is determined by the concentration of the catalyst. This is also why propylene oxide will hydrolyze to propylene glycol in the soil. Both propylene oxide and propylene glycol are biodegradable.

The initial fumigation work shown above was done with flammable 100% PPO in vacuum chambers at ~80°F for 4 hours @ 100mm Hg according to our label. Note, it is very important not to confuse propylene oxide with ethylene oxide because their structure is similar. Ethylene oxide may not be used on food products since it is a known carcinogen, whose by-products are also toxic. Furthermore, it is explosive at concentrations of 1% to 100% in air. Propylene oxide, on the other hand is flammable from 3% to 37% in air, much like propane. Since many of the new uses contemplated will be better served by a nonflammable insecticide, a collaborative study with the USDA at Fresno will determine the efficacy of a nonflammable mix of 8% PPO/92% CO2 under atmospheric conditions. This mixture is currently on the EPA label. There is a history of using this mix at ambient atmospheric conditions in stack and structural fumigation, but the insecticidal efficacy data on the commodities in question needs to be confirmed. Delivery systems for this gas mixture can be tailored for the application. The inhalation limits for 100% propylene oxide are: OSHA–100 ppm and EPA–20 ppm. Worker safety issues are easily handled by the most basic precautions. Researchers at the University of Tennessee have determined that methyl bromide is 92.8% effective against Aspergillus flavus and A. parciticus, the molds required for the production of aflatoxin. Phosphine, however is reported to be only 7.1% effective at the same dose. Since propylene oxide is widely used to reduce mold and yeast in nutmeats and spices, the registrant has commissioned the Dried Fruit Association of California to determine the efficacy of propylene oxide against these two molds. The control of the target insects and these molds in the post-harvest environment will reduce the formation of aflatoxin in the commodity. This dual function will offer another portion of the advantages obtained by using methyl bromide.

There are references indicating that propylene oxide is a possible nematicide effective against both egg masses and larvae of Meloidogyne hapla in soil. Propylene oxide has been reported to be highly mobile in soil and has physical characteristics that appear to be ideal for soil fumigation. Certainly, its biodegradability and its conversion to non-toxic biodegradable propylene glycol in ground water answer some of the key environmental questions. ABERCO, INC. is planning some preliminary studies in this area.

Propylene oxide is already approved for use on food products and it passes the key tests of availability, effectiveness, safety and practicality. Although more information is needed to confirm its efficacy, it is readily apparent that propylene oxide is an important viable contender for replacing methyl bromide in some critical applications.

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Last Updated: May 4, 2000