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The amount of residue that remains in fumigated materials is determined by the conditions existing during the fumigation and the treatment of the material afterwards. In some cases residue levels may be held to a minimum if the various factors that lead to residue accumulation are taken into account before the treatment is done. A few general statements on residue accumulation can be made. However, it must be emphasized that no one condition is likely to apply equally for all fumigants or for different commodities.
TYPE OF FUMIGANT
Fumigants with high boiling points tend to be sorbed to a greater extent and remain as residues longer than more volatile compounds. For example, acrylonitrile was found to remain in wheat for many days, whereas methyl bromide dissipated in a few hours (Dumas and Bond, 1977). Fumigants that react with plant or animal constituents may also leave appreciable residue. This may be fixed residue such as inorganic bromide, chloride, phosphate or other compound, depending on the fumigant, or it may be a volatile material such as ethylene chlorohydrin from ethylene oxide or dimethyl sulphide from methyl bromide. Solubility in water can also influence residue accumulation. HCN is not used on some moist materials because of the burning effect of the acid formed when it combines with water.
TYPE OF COMMODITY
Some materials will sorb and retain more fumigant than others. Foods with high oil and fat content may retain more residue than cereals. Rhodes et al (1975) indicated that methyl bromide is readily absorbed by lipid materials and they suggest that care should be taken to avoid contamination of high fat content foods such as butter, cheese, margarine, meat etc.
Different fractions of seeds contain different amounts of residue. The shells of walnuts have been found to contain 70 percent of the total residual bromide remaining after fumigation with methyl bromide (Adomako, 1974).
A substantial portion of residual carbon tetrachloride in treated wheat appeared in milled fractions, especially the bran (Jagielski et al, 1978). The gluten fraction of wheat flour contained 80 percent of the total decomposed methyl bromide (Winteringham et al, 1955).
Finely divided materials can often absorb more fumigant and retain more residue than whole seeds. Some materials are not treated with certain fumigants because of the reaction products that remain as residue. Thus, sulphur-containing goods are not treated with methyl bromide, and materials containing copper or copper salts may react with phosphine, depleting concentrations of the fumigant from the atmosphere and forming undesirable residues.
CONCENTRATION AND EXPOSURE TIME
The amount of residue accumulating during a fumigation can be influenced by the dosage applied and the length of time the material is exposed. The amount of bromide retained by citrus fruit after fumigation with methyl bromide and ethylene chlorobromide WAS found to be greater with higher dosages and longer exposure times (Lindgren and Sinclair, 1951; Lindgren et al, 1968). Similar observations have been made on other commodities treated with methyl bromide, carbon tetrachloride, ethylene dichloride and ethylene dibromide (Whitney, 1963; Rowe et al, 1954).
MOISTURE CONTENT AND HUMIDITY
The retention of sorbed gases and the reaction of fumigants with components of treated goods are influenced by the moisture content of the goods and by the relative humidity of the sir around them. Usually, sorption is higher in materials with higher moisture content. In dried fruit fumigated with HCN, moisture content was found to be the main factor governing retention of cyanide; fruit of 19 percent moisture content retained four times as much free cyanide and had eight times as much laevulose cyanohydrin as fruit of 8 percent moisture content (Page and Lubatti, 1948). Maize at 15 percent moisture content retained twice as much ethylene dibromide as at 9 percent (Sinclair et al, 1964). Humidity of the atmosphere also appears to be an important factor in the dissipation of fumigant. Greater desorption of ethylene dibromide from layers of wheat was found to occur at high rather than low humidities (Dumas and Bond, 1979).
TEMPERATURE
The rate of desorption of fumigant is usually related to temperature, with less abreacted fumigant residue remaining at high temperatures. However, residue from chemical reaction is likely to be greater at higher temperatures. The inorganic bromide residues in flour increase with increases in temperature, even when the dosage is decreased (Lindgren et al, 1962; Vardell, 1975).
MULTIPLE TREATMENTS
If commodities are refumigated with some fumigants, the level of residue may be expected to increase with each treatment. Cereal grains given repeated treatments with methyl bromide were found to contain increasingly higher levels of inorganic bromide both in the whole grains and in the flour milled from them (Kawamoto et al, 1973; Vardell, 1975; Banks et al, 1976). Similarly, flour fumigated several times with methyl bromide has more residue after each treatment (Brown et al, 1961). On the other hand, flour refumigated with phosphine contained no more measurable residue than when only treated once (Vardell et al, 1973).
Since there is considerable possibility of re-infestation and subsequent retreatment of goods in international trade, and the history of such treatment may not be known, some precautions may be needed to ensure that the residue levels do not exceed permitted tolerances.
PROCESSING AND COOKING
Milling of fumigated grain will usually remove or reduce residues. All of the residual dust from formulations of aluminium phosphide is entirely removed from grain during the milling process (Liscombe, 1963). Grinding will promote desorption of unreacted fumigant and liberation of volatile reaction products. Chang and Kyle (1979) found that the seed coat offered considerable resistance to desorbing carbon tetrachloride from wheat grains and removal of the seed coat greatly increased the rate of Resorption. However, considerable residue may remain after milling; Heuser (1961) found a large proportion of the residual ethylene dibromide remaining in flour produced from fumigated wheat.
Cooking can further reduce residue levels in most fumigated foods. In the preparation of baked and steamed products from flour treated with ethylene oxide, 20 to 100 percent of the original residue was lost (Scudamore and Heuser, 1971). Residual carbon tetrachloride and ethylene dichloride virtually disappear end most of the free ethylene dibromide dissipates on baking (Jagielski et al, 1978).
However, small amounts of some fumigants can remain after the cooking process. Unchanged ethylene dibromide can be detected in bread, using sensitive analytical methods, and sufficient residues from methyl bromide can remain to cause objectionable odours in bread made from treated flour. For food materials that are intended for consumption without cooking, the witholding period may need to be extended after treatment with fumigants such as ethylene dibromide or carbon tetrachloride, otherwise the food may have to be selected from lots that have not been so treated (FAD/WHO, 1980).
For residual fumigants that may desorb from treated material into the atmosphere of the work place, sensitive, easy to use methods of detection are available. These have been outlined in Chapter 3.
The analysis of residues in food and other materials is a highly specialized procedure that is normally done in a laboratory. The techniques used for such determinations are beyond the scope of this manual; however, some information is given under individual fumigants in Chapter 6 and the following publications are given as a guide to sources of more detailed information: FAD/WHO (1980); Alumot and Bielorai (1969); Bielorai and Alumot (1975); Dumas (1973, 1978, 1980); Dumas and Bond (1975, 1977, 1979); Fairall and Scudamore (1980); Heuser and Scudamore (1968, 1969b, 1970); Jagielski et al (1978); Msjumder et al (1965); Scudamore and Heuser (1971); Stijve (1977).
In this chapter the more important fumigants are discussed, and condensed information on their pertinent physical and chemical properties is presented. Some fumigants that were used extensively in the past, but have since been replaced by others with more favourable characteristics, are still included to indicate both adverse and beneficial properties. Fumigants having a limited field of use are considered more briefly.
Carbon disulphide and hydrogen cyanide (HCN) were the first chemicals to be used for this type of treatment and HCN remained the primary fumigant for some years. However, with the discovery of methyl bromide and, more recently, phosphine its use has declined significantly. Currently, methyl bromide and phosphine are the most commonly used fumigants for the treatment of stored grain and similar commodities.