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
METHYL BROMIDE
This pesticide was evaluated at the Joint Meeting in 1965 (FAO/WHO
1965c) and was reviewed extensively in 1966 and 1967 (FAO/WHO 1967b,
1968b) and briefly in 1968 (FAO/WHO 1969b).
Reference should be made to Appendix IV, where Section 3 of the report
on the 1971 meeting (FAO/WHO 1972a) is reproduced. It contains a
discussion of general principles relating to the occurrence of
residues of fumigants.
RESIDUES IN FOOD AND THEIR EVALUATION
Use Pattern
Methyl bromide, which has been in widespread use for some 30 years, is
used for all types of stored dry foodstuffs, particularly for produce
in bags, cases or other packages. Its use for large loose bulks of
foodstuff, when it may be used alone or in admixture with ethylene
dibromide or with carbon tetrachloride, is more limited. It is also
used for the treatment of soil before sowing or planting, especially
in glasshouses, against nematodes, weeds and other organisms; also in
plant quarantine operations, including disinfestation of fresh fruit
and vegetables.
Residues resulting from supervised trials (Post-harvest uses)
From the extensive information previously reported it can be concluded
that after treatment of produce the physically sorbed residue
diminishes either by loss to the atmosphere or by reaction with the
foodstuff. The main reaction products with cereals and cereal products
have been characterized. In these materials methylation of nitrogen
and sulfur containing groups in the protein fraction accounts for most
of the reaction. In other foods the reaction appears to be similarly
associated with the protein fraction. These reactions also leave a
"fixed" residue of bromide which is in the water-soluble, ionic form.
Considerable attention has been given to the determination of this
inorganic bromide occurring in food after fumigation and many data are
available for amounts in foodstuffs moving in commerce as well as from
supervised trials.
The significance of these residues of inorganic bromide is discussed
in Appendix IV.
Less attention has been given to study of the residue of unreacted
methyl bromide. The earlier work supported the assumption that normal
airing, processing and cooking of foodstuffs would ensure that no
residue of the unreacted compound could reach the consumer. However,
the development of analytical methods, based on gas-chromatography,
which enable much lower levels to be detected and measured allows a
reassessment of the persistence of these unreacted residues. Scudamore
and Heuser (1970) present data for a wide range of dry foods showing
the rate of disappearance of methyl bromide under controlled
conditions from both sealed samples and well ventilated samples. The
rate of disappearance under commercial conditions of storage and
handling must be between these extremes.
For most products held at 25°C the amount of residual methyl bromide
fell to below 1 ppm within a few days. In some foods such as
groundnuts and cocoa beans the rate of loss was lower. The rate was
also lower at lower temperatures, small amounts of methyl bromide
being extracted from several commodities one month after treatment.
These experiments were carried out at a constant concentration of
approximately 14 mg per litre, which is a little above the level
normally found in the latter part of a commercial treatment. It was
found that the amounts of the initial residues of unchanged methyl
bromide (one hour after fumigation) were more closely proportional to
the concentration than to the concentration-time product. From a close
study of these data, and considering for each commodity the normal
extent of storage, processing and cooking to which it is subjected, it
is reasonable to conclude that there is insignificant risk that
residues of unchanged methyl bromide will reach the consumer.
Fate of residues
Little, if any, new information on the nature of the reactions of
methyl bromide with foods has been obtained since the studies on
cereals and cereal products by Winteringham and his colleagues
(Bridges, 1955, Winteringham et al., 1955 and Winteringham, 1955)
reviewed at earlier Joint Meetings. In cereals, methylation, mainly of
nitrogen and sulfur-containing groups in the proteins accounts for a
large part of the decomposition of sorbed fumigant.
At the same time a residue of inorganic bromide is formed and there
have been many studies of the amounts produced in treated foods. Where
the amount of inorganic bromide directly resulting from treatment with
methyl bromide is known, this provides an indication of the amount of
methylation or of other reactions that has taken place.
Evidence of residues in food in commerce or at consumption
Nearly all the available data relate to inorganic bromide. In most
cases it is not possible to attribute the amount found to any
particular treatment with methyl bromide.
Methods of residue analysis
Most of the earlier studies on bromide residues after fumigation with
methyl bromide used methods which measured total bromide in all forms.
A widely used procedure involved a preliminary alkali digestion
process followed by ashing under controlled conditions, extraction
with water and determination of bromide by an oxidation and titration
method (Shrader et al., 1942, Lewis and Eccleston, 1946, Kolthoff and
Yutzy, 1937, van der Meulen, 1931). Some workers have preferred oxygen
combustion to the alkaline fusion procedure (Dow, 1970, Kretzschmann
and Engst, 1968a, 1968b, 1970). Other studies of total bromide content
have been made by the use of neutron activation analysis (Guinn and
Potter, 1962; Lindgren, et al., 1962) or X-ray fluorescence
(Getzendaner, 1961; Getzendaner et al., 1968; Shuez et al., 1971).
When it was suspected that unreacted methyl bromide was present some
workers removed this by prolonged aeration or by solvent extraction
before determining bromide and estimated the organic bromide by
difference from the total bromide found in an unreacted sample. This
procedure was clearly incapable of determining small amounts of
unreacted methyl bromide.
Gas-chromatography now provides a ready means of direct determination
of unreacted methyl bromide. A number of methods of extracting the
residual fumigant from food samples have been examined. Methyl bromide
was among six fumigants used by Malone (1969) in a comparison of three
recovery procedures; a sweep co-distillation method based on the
procedures of Ragelis et al., (1968) and Storherr et al., (1967); a
steam distillation procedure adapted by Bielorai and Alumot (1966)
from the method of Kennet and Huelin (1957); and the acid reflux
procedure described by Ramsey (1957) for the determination of carbon
tetrachloride. Malone (1970) further developed her preferred method,
the acid reflux procedure, for the multi-residue determination of
volatile fumigant residues in cereal grains. At low levels of residual
methyl bromide (3 ppm) recoveries were low (60 to 70%).
Heuser and Scudamore (1968) developed a simple extraction procedure
applicable to methyl bromide in wheat and flour. The sample was soaked
in a 5:1 acetone water mixture in a flask at room temperature and an
aliquot of the supernatant liquid injected into the gas-chromatograph.
The procedure was developed into a multiresidue system (Heuser and
Scudamore, 1969) and was further evaluated in a study of the residues
of unreacted methyl bromide in a wide range of foods (Scudamore and
Heuser, 1970). An extraction period of 24 hours was sufficient for all
the foods tested but for a finely divided product such as flour the
maximum recovery was obtained after six hours. A polypropylene on
Chromosorb W column was used with a flame ionization detector.
Ultimate sensitivity for 1% of full-scale deflection was equivalent to
1.5 × 10-10 g, or 0.1 ppm when 15 g of commodity was extracted with
30 ml of solvent mixture and a 3 µl injection was used. Satisfactory
recoveries were obtained with all the foods tested with the exception
of cocoa beans (about 60% recovery).
Heuser and Scudamore (1970) have developed a method for the selective
determination of inorganic bromide by gas-chromatography with
simultaneous measurement of residual methyl bromide if desired. The
method is based upon the quantitative reaction of inorganic bromide
present in the commodity with excess of ethylene oxide in an acidified
solvent to form ethylene bromohydrin. Methyl bromide (and also
ethylene dibromide) remains intact under these conditions. The
bromohydrin is determined by gas-chromatography with electron-capture
detection, with a sensitivity of 0.5 ppm of ionic bromide.
National tolerances (as reported to the meeting)
Residues of unreacted methyl bromide
Australian and Canadian regulations assume that after post-harvest use
of methyl bromide, residues of the unreacted fumigant will disappear
before food reaches the consumer.
Residues of inorganic bromide
Many countries have established tolerances of inorganic bromide, or
total bromide, in foods fumigated with methyl bromide. The United
States and Canadian regulations provide extensive lists of tolerances
related to different treatments. The tolerance of 50 ppm in raw cereal
grains is widely adopted.
Appraisal
For many years methyl bromide has been widely used for dry stored
foods especially those in bags, cases or other packages. It is also
used for quarantine purposes, on a wide range of fresh fruits and
vegetables, and in the treatment of soil against nematodes and
soil-borne insects and fungi.
After fumigation the physically sorbed fumigant is rapidly given off
into the atmosphere: some may react with the food and it has been
generally assumed that none of the unreacted compound will reach the
consumer. Recent work employing more sensitive and specific analytical
methods based upon gas-chromatography has demonstrated that this
residue can, in some circumstances, be more persistent than has been
hitherto supposed. For any given food the amount depends upon the
concentration during treatment, the moisture content of the food, the
duration and temperature of storage and the amount of ventilation
allowed.
A multi-residue method is available using cold solvent extraction and
gas-chromatography by which unreacted methyl bromide can be detected
to a limit of about 0.1 ppm. The method is suitable for regulatory
purposes when a limit of Ca. 0.5 ppm should be determined with
reliability.
The reactions of methyl bromide with cereals and cereal products have
been characterized as mainly methylation of nitrogen- and
sulfur-containing groups in the proteins with simultaneous formation
of a "fixed" residue of inorganic (ionic) bromide. Reaction in other
foods appears to be similarly associated with the proteins. A
considerable amount of attention has been given to the amounts of
inorganic bromide formed in this way and to the setting of tolerances
for inorganic bromide. Unfortunately, in regulatory work it is not
usually possible to assign a particular fraction of the total
inorganic bromide as being derived from methyl bromide. Most
analytical methods determine the total bromide in the sample, but a
method using gas-chromatography is now available which specifically
determines ionic bromide in the presence of methyl bromide (or
1,2-dibromoethane) with a limit of determination of 0.5 ppm. The
general problem of residues of inorganic bromide in food is discussed
in the Report (see Appendix IV).
Food moving in commerce shortly after fumigation with methyl bromide
may contain high levels of unreacted fumigant. This treatment
sometimes takes place immediately before shipment or during transit in
ship holds or in containers. The amount of unreacted methyl bromide
usually diminishes rapidly during handling, storage and processing.
From the available information on the occurrence of unreacted methyl
bromide in or on the foods listed below, after fumigation in
accordance with good practice the following amounts need not be
exceeded (provided that the commodity is freely exposed to air for a
period of at least 24 hours after fumigation before sampling). It is
recommended that these residue levels be used as guidelines:
In the following raw
agricultural products at
point of entry into a country: nuts, peanuts, 100 ppm
raw cereals, cocoa beans 50 ppm
dried fruits 20 ppm
In milled cereal products
which will be subject to
baking or cooking: 10 ppm
In the following foods at the
point of retail distribution
or when offered for
consumption: nuts, peanuts, 0.5 ppm
bread, other cooked (i.e. at or
cereal products, cocoa about the
products, dried fruits present limit
of
determination)
Even though the content of bromide ion per se may be considered to
be of minor importance, nevertheless, to ensure that the content of
methylated reaction products is not excessive it is recommended that
the previously recommended tolerance of 50 ppm of bromide ion in raw
cereals and wholemeal flour should stand.
It is recommended that temporary tolerances previously recommended for
residues of bromide ion in other foods be suspended (see Report).
Further work desirable
1. Additional data on residues of unchanged methyl bromide occurring
in food in commercial practice including data for fresh fruits
and vegetables.
2. Information on the nature of the reaction products of methyl
bromide with foods other than cereals and cereal products.
3. Additional data on amounts of "background" bromide ion occurring
in foods before post-harvest fumigation.
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