Advances in Organic Hermetic Storage and Vacuum-Hermetic
Fumigation (V-HF) or Gas-Hermetic Fumigation (G-HF)

Philippe Villers
President
GrainPro, Inc.
200 Baker Avenue, Suite 309
Concord, MA 01742
e-mail: pvillers@igc.org

Introduction

In response to the international agreement to phase out the use of methyl bromide, many alternatives for protecting stored commodities have been proposed in recent years, but none are as simple and as environmentally benign as hermetic storage. Modern hermetic storage uses large, flexible, polyvinyl chloride (PVC) envelopes called cocoons to create a low-oxygen environment in which insects quickly die for lack of oxygen. Attendant benefits of this flexible low-oxygen environment include the maintenance of constant moisture levels, the prevention of fungal growth and its associated aflatoxins, and the prevention of rodent access. Hermetic systems make safe, long-term storage feasible without the use of pesticides and are now in use in some 20 countries. In addition, hermetic storage is now widely used to preserve seed germination potential for many months even in hot, humid environments without requiring refrigeration.

The storage takes three forms, depending on the mechanism used to reduce oxygen levels to about 2 percent. Organic hermetic storage relies on the respiration of the infesting insects themselves to reduce the oxygen level to this unsurvivable level. Vacuum-hermetic fumigation (V-HF) uses an attached vacuum pump. Gas-hermetic fumigation (G-HF) introduces carbon dioxide to drive out air and replaces it with carbon dioxide, maintaining a normal atmospheric pressure. Each of these three processes has its natural niche, depending on whether the interest is in use as long-term storage or rapid fumigation, and whether the commodity is resistant to vacuum or is fragile and might be crushed, as in the case of figs and dates. In all cases, these gas-tight envelopes (cocoons) are made from approximately 1/32-inch-thick PVC, which has low gas permeability and high resistance to UV radiation.

Scientific Basis of Hermetic Storage and Current Usage

When cocoons are used for organic hermetic storage, insect respiration typically causes the oxygen level to reach below 2 percent within a few days and the carbon dioxide level to climb. At this point, adult insect life cannot be sustained. Over time oxygen infiltrates and eggs, which are more resistant to low oxygen levels, may hatch and a second cycle of reduced intensity can take place. However, in all cases, the cumulative losses at room temperature and above are generally below 1 percent before all insect activity ceases. Safe, long-term storage of paddy and milled rice, which is even more demanding, is possible with no quality deterioration over extended periods of time, up to 6 months or more. Wheat has been stored with no significant degradation for 5 years in cocoons. In Central America, cocoons are now used to retain flavor in coffee beans over multimonth storage.

GrainPro, Inc., research shows that fungal population is a very strong function of relative humidity. Hermetic storage, by preserving constant relative humidity, prevents the growth of fungi and therefore aflatoxins. In addition, fungi do not thrive in low-oxygen, high-carbon dioxide atmospheres.

Recent Results

V-HF field tests in the last several years have shown that, with high-value commodities such as coffee, cocoa, and Narcissus bulbs, the use of vacuum allows quick, complete fumigation by using a commonly available vacuum pump. Under this patented process, adult insects die even more rapidly, but even the most resistant life forms, such as eggs, typically are killed in less than 3 days at room temperature. Faster kill times are observed at higher temperatures. In the summer and fall of 2003, additional field tests were performed on apples, matzo flour, corn chips, cocoa, semolina, wheat flour, Narcissus bulbs, rice, almonds, chick peas, sunflower seeds, and peanuts.

Where the commodity is fragile and crushable, such as figs, dates, and cut flowers, G-HF is used, where the vacuum pump is replaced by a carbon dioxide source. This is now being used on a significant basis in Turkey with figs. Recently a high-throughput fumigation facility using a special type of cocoon has been developed, called the PIT system (Pesticide-Free Integrated Transportation and Storage). This system allows for rapid in and out processing. In this system, the use of an insect-resistant membrane, called PalletWrap, protects a skidload once it has been "fumigated" with V-HF or G-HF. This approach makes the skid-mounted commodities easy to handle in all stages of shipment—from fumigation to transportation in 20-foot containers to storage at the country of destination—and all without danger of reinfestation.

Practical Aspects

PVC cocoons are lightweight and easy to transport. For instance, a 10-metric-ton capacity cocoon weighs only 49 kg. Cocoons are available in sizes that range from a capacity of 5 to 300 metric tons. GrainSafes, which allow dispensing of bulk grains at will, are designed for small users and have capacities of 0.5 to 1.0 metric tons. Both of these storage types are easy to maintain and repair. Cocoons are sealed with a zipper originally designed for space suits and any tear that may accidentally be introduced can easily be repaired with a patch kit, much as is done with a bicycle inner tube. When cocoon sides are made taut, which is done with tensioning straps, rodents cannot get a tooth-hold on its tough, slippery surface. Cocoons are currently used to store a large variety of commodities ranging from rice and corn seed to paddy, wheat, corn, beans, sorghum, dates, figs, and nuts.

Conclusion

Use of hermetic storage (cocoons) avoids the need for pesticides in medium- and long-term storage. Storage can take place either indoors or outdoors, in warehouses or on unprepared ground. Cocoons effectively control storage insects and fungi, either alone or with the addition of vacuum or carbon dioxide for rapid fumigation. Rapid fumigation, in 3 days at room temperature with V-HF or G-HF, has now been proven effective for a number of commodities, and experiments are ongoing.

Much of the recent experimental work was done under a grant from the US-Israel Science and Technology Foundation under the project title "Development of Alternative Fumigation Systems and Methods to Reduce Methyl Bromide Emissions in Post-Harvest Treatment."

Last Updated: April 9, 2004