WHO Pesticide Residues Series, No. 1
1971 EVALUATIONS OF SOME PESTICIDE RESIDUES IN FOOD
THE MONOGRAPHS
The evaluations contained in these monographs were prepared by the
Joint Meeting of the FAO Working Party of Experts on Pesticide
Residues and the WHO Expert Committee on Pesticide Residues that met
in Geneva from 22 to 29 November 1971.1
World Health Organization
Geneva
1972
1 Pesticide Residues in Food: Report of the 1971 Joint Meeting of
the FAO Working Party of Experts on Pesticide Residues and the WHO
Expert Committee on Pesticide Residues, Wld Hlth Org. techn. Rep.
Ser., No. 502; FAO Agricultural Studies, 1972, No. 88.
These monographs are also issued by the Food and Agriculture
Organization of the United Nations, Rome, as document AGP-1971/M/9/1.
FAO and WHO 1972
1,2-DIBROMOETHANE
This pesticide was evaluated at the Joint Meeting in 1965 (FAO/WHO
1965c) and it was reviewed in 1966, 1967 and 1968 (FAO/WHO 1967b,
1968b, 1969b). Previously it has been referred to as ethylene
dibromide.
Reference should be made to Appendix IV which includes Section 3 of
the report on the 1971 Meeting (FAO/WHO 1972a), where general
principles concerning the occurrence of residues of fumigants are
discussed.
RESIDUES IN FOOD AND THEIR EVALUATION
Use pattern
Post-harvest use on dry foodstuffs
1,2-dibromoethane is rarely used alone except in certain small-scale
treatments. Thus, in West Africa, gelatine capsules have been used to
deliver small doses of 1,2-dibromoethane (5 ml, 10.8 g) in the
treatment of individual bags of cereal or cereal products, each
enclosed in a polythene bag.
More commonly it is used in mixtures with other "liquid fumigants",
such as 1,2-dichloroethane and carbon tetrachloride, for treating
cereals in bins or in bulks on floors. Mixtures with methyl bromide
are also used for this purpose, particularly in parts of India, where
these mixtures may also be applied on products other than cereals.
They are also used as "spot" fumigants for the treatment of individual
items of machinery in milling plants.
Other uses
As indicated in previous monographs, the compound is also used for the
treatment of soil before planting; also of certain fruits for
quarantine purposes.
Residues resulting from supervised trials
The fumigant is sorbed strongly by cereal grains, cereal products or
other produce during the exposure period and, even when normal airing
procedures are followed, the residue of fumigant is dispersed very
slowly. Nearly all the fumigant taken up is physically sorbed and at
normal temperatures there appears to be only a very small reaction
leading to the formation of inorganic bromide. On occasion, however,
in produce at higher temperatures and moisture contents a rapid
breakdown to inorganic bromide has been noted (Heuser et al., 1969).
Interpretation of some of the early work is uncertain because the
analytical procedures adopted did not effectively differentiate
between inorganic and organic bromide. Heuser (1961) examined the
conditions for recovery of unchanged 1,2-dibromethane from wheat and
milled products and followed the slow diminution of residual fumigant
during laboratory and field trials. When wheat containing 80 ppm of
total bromide was removed from the upper layers of a bin 49 days after
fumigation a large proportion of the residual bromide was in the form
of unchanged fumigant and most of this was retained in the fractions
on milling, particularly in the bran. Other workers confirm the
concentration of the sorbed fumigant in the seed coat.
The effect of processing of wheat treated with a fumigant mixture
containing 3.5% by volume of 1,2-dibromoethane was studied in the
Netherlands by Wit et al., (1969). Discrepancies between the results
obtained by two participating laboratories using the same analytical
procedure, involving steam distillation followed by
gas-chromatography, make it difficult to summarize the results. After
storage with some ventilation for six or more weeks the two
laboratories reported amounts of ethylene dibromide in the wheat and
processed products as follows.
Laboratory A Laboratory B
Wheat 10-20 5-13
Flour 4 2
Coarse and fine offals 18-23 2
White bread 0.002-0.003 0.018-0.040
Wholemeal bread 0.006 0.12-0.16
In a subsequent collaborative study by these two laboratories flour
was treated directly with 1,2-dibromoethane, aired thoroughly and
baked into loaves. Good agreement was obtained in results showing
residues of 20 to 24 ppm in the flour which were reduced to 0.33 to
0.47 in the bread. In tests of a method of treating 200 lb lots of
white maize with 5 ml of 1,2-dibromoethane applied from a gelatine
capsule Heuser et al., (1969) found up to 50 ppm of unreacted fumigant
in samples taken shortly after a seven-day exposure to the fumigant.
This was reduced to approximately 15 ppm after free exposure to air
for one week, but a sample in a sealed tin retained the original
amount of 1,2-dibromoethane. The amount retained in the original bag,
kept closed between samplings, fell progressively over a period of
three months. Similar samples of maize containing around 30 ppm of
unreacted 1,2-dibromoethane were prepared and cooked according to
Ghanaian custom. In dough the amount of unreacted fumigant was reduced
to 3 ppm and the amount in the cooked foods was less than 0.5 ppm.
Fate of residues
Using 1,2-dibromoethane labelled with bromine-82, Bridges (1956)
showed that in spite of the high physical sorption of the fumigant by
wheat and its slow rate of airing, the amount of chemical reaction
between it and the wheat is very small at room temperature. The
precise nature of the reaction was not determined but it appeared to
occur mainly with the wheat protein. On heating wheat containing
sorbed 1,2-dibromoethane for 30 minutes at 180°C about one third or
one half of this residue broke down to ethylene glycol and inorganic
bromide and the remainder volatilized.
Sorption of 1,2-dibromoethane by and reaction with various cereals and
cereal constituents has been extensively investigated in Israel
(Olomuchi and Bondi 1955, Bondi and Alumot 1966). It was shown that
the presence of fat increased the sorption of 1,2-dibromoethane but
not its reaction with the grains. The reaction was mainly with the
protein fraction. From tests with different proteins it was concluded
that the amount of reaction was little affected by their chemical
composition but was in accordance with the extent of the initial
sorption which depended upon the physical structure and condition of
the protein.
The biochemical effects of ingestion of 1,2-dibromoethane in chicks
and rats has also been extensively investigated in Israel (Nachtomi,
Alumot and Bondi, 1965, 1966, 1968). Nachtomi (1970) has recently
studied the reaction with glutathione in vitro and in vivo.
Methods for residue analysis
Since unchanged 1,2-dibromoethane residues are of much greater
toxicological significance than is ionic (inorganic) bromide, it is
important that the two forms are distinguished by the analytical
methods employed. Many methods hitherto employed for determining
bromide residues arising from treatments with 1,2-dibromoethane have
included both unchanged fumigant and the ionized bromide fraction in a
total bromide figure. Such methods include those which employ a
preliminary alkaline hydrolysis step followed by an ashing procedure,
e.g. Sinclair and Crandall (1952) and also methods based on
neutron-activation (Guinn and Polter, 1962) and X-ray fluorescence
(Getzendaner, 1961).
If dilute (2-5%) alcoholic potash is employed for hydrolysis, the
1,2-dibromoethane molecule is split with volatilisation and loss of
50% of the bromine as vinyl bromide, enabling some differentiation to
be made if an alternative total hydrolysis step is also employed
(Olomuchi and Bondi, 1955).
Heuser (1961) found that the ionic bromide could be separated from the
unchanged 1,2-dibromoethane in cereals by extraction with water after
preliminary aeration. However, methods which rely on subtraction of
one component from a total bromine figure to obtain the value for the
second component are necessarily limited in sensitivity and accuracy.
Bridges (1956) and Heuser (1961) found that adsorbed 1,2-dibromoethane
could not be successfully removed from cereals, even when finely
divided, with organic solvents such as ether, chloroform or methylene
chloride, to allow differentiation from the insoluble ionic bromide,
though Shrader et al., (1942) and Heuser and Scudamore (1970)
successfully separated adsorbed methyl bromide from ionic bromide in
this manner. This means that the total bromide methods mentioned above
cannot be used with any confidence to determine the ionic bromide
after attempting removal of the 1,2-dibromoethane in this way.
Before the development of gas-chromatography techniques, methods for
removal of residual 1,2-dibromoethane from citrus fruits and cereals
were published (e.g. Kennet and Huelin, 1957; Mapes and Shrader, 1957)
which included a steam distillation stop, after which the volatile
organic bromide was decomposed by alkaline hydrolysis or by a
catalytic method, and then determined as bromine. These and similar
methods, whilst giving good recoveries of 1,2-dibromoethane added to
substrates, failed to characterize the fumigant and would have
included other volatile bromine compounds such as
1-bromo-2-chloroethane or 1,2-dibromo-3-chloropropane in samples of
unknown history.
One early attempt at separation and characterization of the two forms
of bromine was the method of Tanada, Matsumoto and Scheuer (1953) who
distilled 1,2-dibromoethane from fresh fruits in a benzene extract,
reacting the 1,2-dibromoethane in the distillate with potassium iodide
and acetic acid to give potassium tri-iodide in quantitative amount,
and also determined ionized bromide in the distillation residue.
Gas-chromatographic methods
In recent years gas-chromatographic methods have been developed which
are capable of identifying fairly conclusively and determining with
accuracy and high sensitivity amounts of 1,2-dibromoethane in solution
(e.g. Bielorai and Alumot, 1966; Heuser and Scudamore, 1967, 1969).
The choice of a method of extraction which will allow the removal of
1-2-dibromoethane from the substrate efficiently and without chemical
breakdown is therefore the major consideration in the application of
this technique to the analysis of the residual fumigant.
Heuser (1961) showed that even moderate heating of a dry cereal
substrate containing adsorbed 1-2-dibromoethane rapidly caused its
decomposition to water-soluble ionic-bromide; which suggested that
methods for recovery of the unchanged fumigant involving heating
should be avoided. Nevertheless, Bielorai and Alumot (1965)
successfully adapted Kennet and Huelin's (1957) steam-distillation
apparatus for the recovery of 1,2-dibromoethane to allow determination
in the distillate by gas-chromatography. Malone (1969) compared this
method and two other methods involving removal of 1,2-dibromoethane by
heating, namely acid-reflux distillation (after Mapes and Shrader,
1957) and sweep-codistillation using gas-chromatography with
electron-capture detection as the determinative step. With spiked
substrates, Malone found that sweep-codistillation was only partially
effective in recovery of 1,2-dibromoethane and that the acid-reflux
method gave slightly lower results than the Kennet-Ruelin technique.
Percentage recovery figures were not obtained for any of these methods
using previously fumigated samples.
Heuser and Scudamore (1967, 1969) with a solvent extraction method at
room temperature employing a 5:1 by volume mixtures of acetone and
water or acetonitrile and water, obtained 98-100% recovery of known
amounts of 1,2-dibromoethane from fumigated cereals, determined by
gas-chromatography using flame-ionization and electron-capture
detectors. These authors used a total recovery technique involving
partial aeration to establish the amount of 1,2-dibromoethane
remaining to be determined in the substrate, and also established the
rate at which the fumigant residue was removed from it.
Heuser and Scudamore (1970) later extended their method for the
determination of residual 1,2-dibromoethane to the selective
determination of ionic bromide by gas-chromatography. The ionic
bromide was reacted with ethylene oxide in a specified solvent mixture
to give ethylene bromohydrin in quantitative yield. 1,2-dibromoethane
and methyl bromide were shown to remain intact under these conditions.
The bromohydrin was determined by gas-chromatography with
electron-capture detection, enabling the two forms of bromide residue
to be determined in one operation, the ionic bromide with a limit of
detection of 0.5 ppm and 1,2-dibromoethane with a limit of detection
of 0.02 ppm. The procedures of Heuser and Scudamore therefore are
preferred for residue analysis at this time.
National tolerances (As reported to meeting)
Residues of unreacted ethylene dibromide
In the United States of America the following grains: barley, corn,
oats, popcorn, rice, rye, sorghum (milo), wheat, are exempted from
requirements. Canada and Australia similarly consider that there is no
necessity for a tolerance on the grounds that the residue of the
unchanged compound will disappear before the food reaches the
consumer. Australia requires that residues of the unchanged compound
must not be present in or upon foods as consumed.
Residues of inorganic bromide
Many countries have adopted tolerances for bromide in specified foods
arising from a particular source, or from all sources. Some are
defined in terms of inorganic bromide only, others as total bromide.
The lists are too extensive to reproduce here. (But see Appendix IV
for a discussion of the general significance of such residues).
Appraisal
Is used for the treatment of cereals in bins or in bulks on floors
commonly in admixture with other "liquid fumigants" such as
1,2-dichloroethane and carbon tetrachloride. The total post-harvest
use outside the United States of America and India is probably small.
Used in the treatment of fresh fruits and vegetables for quarantine or
other purposes; also as a soil fumigant against nematodes and
soil-borne insects.
There is ample evidence, from supervised trials, that
1,2-dibromoethane is strongly sorbed on foods. The residues of
unchanged fumigant are only very slowly lost by aeration and under
normal storage conditions the rate of loss by breakdown or reaction
with food constituents is low so that the residues of unchanged
fumigant persist for long periods and are not readily eliminated by
processing, although they are normally destroyed by cooking or baking.
At normal temperatures there is a small amount of reaction on wheat
which appears to be with the proteins, with formation of inorganic
bromide, but this reaction is much smaller than that of methyl bromide
with wheat and, for this reason, the nature of the reaction appears
not to have been studied. On heating, any residual unreacted fumigant
breaks down to ethylene glycol and bromide ion.
The residue of unchanged 1,2-dibromoethane is of more importance and
concern than the residue of inorganic bromide. There is little
information on the residues of unreacted fumigant occurring in
commerce and most of the data on bromides refer to determinations of
total bromine. Sometimes these determinations were made after
extraction of the sample with non-aqueous solvent but it has been
shown that this removes only part of the organic bromide.
In the collection of residue data or in obtaining evidence for any
regulatory action it is desirable to determine amounts of unreacted
1,2-dibromoethane and of bromide ion separately and specifically.
Analytical procedures are now available for both these determinations
using gas-chromatography, enabling bromide ion to be determined with a
limit of detection of 0.5 ppm and 1,2-dibromoethane with a limit of
detection of 0.02 ppm and with 0.1 ppm as a reliable limit of
determination for regulatory purposes.
From the available information on the occurrence of unreacted
1,2-dibromoethane in or on raw cereals or cereal products after
fumigation in accordance with good practice it appears that the
following amounts need not be exceeded and it is recommended that
these residue levels be used as guidelines.
In raw cereals at point of entry into a country
or when supplied for milling, provided that the
commodity is freely exposed to air for a period
of at least 24 hours after fumigation before
sampling 20 ppm
In milled cereal products which will be
subjected to baking or cooking 5 ppm
In bread and other cooked cereal products
(i.e. at or about the present limit of
determination) 0.1 ppm
Insufficient information is available on the residues of unchanged
1,2-dibromoethane occurring in fresh fruits and vegetables fumigated
with this compound for quarantine or other purposes to permit similar
practical limits for these foods to be proposed.
Even though the content of bromide ion per se resulting from
fumigation of food with 1,2-dibromoethane may be small and may be
considered of minor importance, nevertheless, in order to guard
against the excessive use of this or other brominated fumigants it is
recommended that the previously recommended tolerance of 50 ppm of
bromide ion in raw cereals should stand.
It is recommended that temporary tolerances previously recommended for
residues of bromide ion in other foods should be suspended (see
Report).
Further work desirable
1. Further data on residues of unchanged 1,2-dibromoethane occurring
in foods in commercial practice including data for fresh fruits
and vegetables.
2. Further information on the nature and amount of the reaction
products of 1,2-dibromoethane in cereals and in a selection of
other foods.
REFERENCES
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cereals fumigated and aired at two different temperatures. J. Agric.
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feed on animals. Final report of research conducted under grant
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See Also:
Toxicological Abbreviations
Dibromoethane, 1,2- (EHC 177, 1996)