242
J. R. Plimmer
1,3-dibromochloropropane, chloroform, trichloroethylene, and chloroethylene)
are carcinogenic and/or heptatotoxic.
Conventional Fumigants. One of the most important fumigants in the U.S.
is methyl bromide, which is widely used in the stored food and beverage industry.
In 1974, 13.8 million kg were produced in the U.S. (Fowler and Mahan, 1976).
Of 11 million kg produced in 1972, 1 million kg were exported, 4 million kg
were used for soil fumigation, and 6 million kg were used commercially, mainly
for treatment of stored grains and feeds (von Rumker et al., 1974).
Methyl bromide is low boiling (b.p. 3.6°C) and is rapidly dissipated by
desorption or reaction. The most common residue that results from its use is
inorganic bromide. However, methyl bromide is a reactive methylating agent,
and in a classic series of studies with 14(:-labeled methyl bromide, Winteringham,
Bridges, and their co-workers at Slough (Winteringham et al., 1955) were able
to show that it reacted with certain components of flour; if too much methyl
bromide was used, the reaction products gave off-flavors to baked goods. Levels
of bromide ion have been determined in many commodities after fumigation with methyl bromide, but the presence of unknown background levels of
bromide ion of natural origin has cast some doubt on the significance of these
figures.
The effect of 14C-labeled methyl bromide on wheat flour provided an indication of the fate of the compound in commodities. Free mercapto groups such
as those present in wheat germ as reduced glutathione were methylated rapidly.
The gluten or protein fraction of wheat accounted for 80% of the decomposition
of methyl bromide. Methylation occurred, and N-methyl derivatives (50%),
dimethyl sulfonium derivatives (30%), and methoxy and methylthio derivatives
(20%) were formed.
Hydrolysis of the protein fraction from methyl-bromide-treated flour showed
that most basic sites were methylated. The histidine residue was the principal
site of reaction (Bridges, 1955), and I-methylhistidine, 3-methylhistidine, and
1,3-dimethylhistidine were present in the hydrolysate. The nutritional and toxicological implications of these findings are probably not significant. Winteringham (1955) considered the possibility of human exposure to very small quantities of methanol, methyl glucose, S-methyl cysteine, methyl methionine
sulfonium salts, and N-methyl derivatives of histidine. He concluded from feeding studies that there was little potential for risk to human health from these
residues. There was little evidence of nutritional loss, and the major problem
was that of "sulfide taint," the unpleasant odor that arises when dimethyl sulfonium derivatives, which are formed in flour treated with unduly high levels
of methyl bromide, decompose and release sulfide compounds when the flour
is baked.
J. R. Plimmer
1,3-dibromochloropropane, chloroform, trichloroethylene, and chloroethylene)
are carcinogenic and/or heptatotoxic.
Conventional Fumigants. One of the most important fumigants in the U.S.
is methyl bromide, which is widely used in the stored food and beverage industry.
In 1974, 13.8 million kg were produced in the U.S. (Fowler and Mahan, 1976).
Of 11 million kg produced in 1972, 1 million kg were exported, 4 million kg
were used for soil fumigation, and 6 million kg were used commercially, mainly
for treatment of stored grains and feeds (von Rumker et al., 1974).
Methyl bromide is low boiling (b.p. 3.6°C) and is rapidly dissipated by
desorption or reaction. The most common residue that results from its use is
inorganic bromide. However, methyl bromide is a reactive methylating agent,
and in a classic series of studies with 14(:-labeled methyl bromide, Winteringham,
Bridges, and their co-workers at Slough (Winteringham et al., 1955) were able
to show that it reacted with certain components of flour; if too much methyl
bromide was used, the reaction products gave off-flavors to baked goods. Levels
of bromide ion have been determined in many commodities after fumigation with methyl bromide, but the presence of unknown background levels of
bromide ion of natural origin has cast some doubt on the significance of these
figures.
The effect of 14C-labeled methyl bromide on wheat flour provided an indication of the fate of the compound in commodities. Free mercapto groups such
as those present in wheat germ as reduced glutathione were methylated rapidly.
The gluten or protein fraction of wheat accounted for 80% of the decomposition
of methyl bromide. Methylation occurred, and N-methyl derivatives (50%),
dimethyl sulfonium derivatives (30%), and methoxy and methylthio derivatives
(20%) were formed.
Hydrolysis of the protein fraction from methyl-bromide-treated flour showed
that most basic sites were methylated. The histidine residue was the principal
site of reaction (Bridges, 1955), and I-methylhistidine, 3-methylhistidine, and
1,3-dimethylhistidine were present in the hydrolysate. The nutritional and toxicological implications of these findings are probably not significant. Winteringham (1955) considered the possibility of human exposure to very small quantities of methanol, methyl glucose, S-methyl cysteine, methyl methionine
sulfonium salts, and N-methyl derivatives of histidine. He concluded from feeding studies that there was little potential for risk to human health from these
residues. There was little evidence of nutritional loss, and the major problem
was that of "sulfide taint," the unpleasant odor that arises when dimethyl sulfonium derivatives, which are formed in flour treated with unduly high levels
of methyl bromide, decompose and release sulfide compounds when the flour
is baked.