PESTICIDE RESIDUES IN FOOD - 1979
Sponsored jointly by FAO and WHO
EVALUATIONS 1979
Joint meeting of the
FAO Panel of Experts on Pesticide Residues
in Food and the Environment
and the
WHO Expert Group on Pesticide Residues
Geneva, 3-12 December 1979
BROMOMETHANE (Methyl Bromide)
Explanation
This fumigant pesticide was evaluated at the Meeting in 1965, reviewed
in 1966, 1967 and 1968 and re-evaluated in 1971. These evaluations
were related almost exclusively to post-harvest uses. Since 1971,
both pre- and post-harvest uses have increased considerably.
The report of the 1971 Meeting contained a statement of general
principles relating to residues of fumigants, to which reference
should be made. That of 1978 referred to problems caused in member
countries of FAO and WHO by the use of certain fumigants and the
re-evaluation of methyl bromide was recommended.
Recent concern about the many sources of inorganic bromide entering
the human diet, some of which arise from methyl bromide usage, both
before and after harvest, has led to the need for an assessment of
these contributions in this new context. In addition the availability
of more sensitive methods for determining residual methyl bromide in
foodstuffs allows a re-appraisal of existing guideline residue levels
for the unchanged fumigant in fumigated commodities.
USE PATTERN
Pre-harvest Use
Methyl bromide is applied to soil plots in glasshouses or in the open
for control of nematodes and other pests, weeds and micro-organisms.
It may be applied to soil under sheeting in the vaporised form or
injected as a liquid and allowed to vaporise in situ. Aeration of the
soil before planting is necessary to avoid phytotoxicity from the
fumigant. Leaching of plots with water is sometimes undertaken to
reduce soil bromide contents. Its use in field plot treatments has
been increasing in recent years. It may be used as the pure compound
or with 2% chloropicrin added as a warning (lachrymatory) agent.
Post-harvest Use
Methyl bromide is widely applied in the vapour phase to many types of
stored dry foodstuffs and other products such as tobacco, and to a
more limited extent to fresh fruit and vegetables in plant quarantine
operations. Little change in the pattern of post-harvest cereal grain
or other stored product uses of methyl bromide has taken place since
the 1971 evaluation. However an awareness that bromide residues
occurring in food as a result of fumigation with methyl bromide are
accumulative in the food, together with attempts to minimise the
occurrence of resistance in insect populations, has led to a reduction
in the intentional repeated fumigations of parcels of dry stored
commodities with the same fumigant which were an earlier feature of
long-term tropical storage. In international trading conditions
though, the possibility of re-infestation and subsequent re-treatments
with methyl bromide cannot be discounted. Some quarantine schedules
provide for fumigation of fresh produce with methyl bromide (e.g.
Hargreaves et al., 1978) but because of possible damage to fruit and
vegetables such treatments are likely to be at low dosage levels and
unlikely to be repeated.
RESIDUES ARISING FROM SUPERVISED TRIALS
Pre-harvest Use
When methyl bromide is applied to soil, much of the gas eventually
escapes by diffusion to the atmosphere. A proportion is however
hydrolysed, resulting in the deposition of inorganic bromide (bromide
ion) in the soil. This bromide may be leached downwards, away from
the areas from which plants feed, by rainfall or by irrigation. That
which remains in the root area may be taken up by the plant and
concentrated in the leaves and to a lesser extent in fruits (Maw and
Kempton 1973). So far as can be ascertained, no evidence has been
published of any organo-bromine compounds occurring in plants as a
result of growth on methyl bromide treated soil.
Common rates of application of methyl bromide to soil vary between 50
and 125 g per m2 (1-2´ lb per 100 ft2). Kempton and Maw (1972)
found that soil applications at 50-100 g per m2 gave soil bromide
levels of 10-60 mg/kg, in turn leading to bromide ion levels in
lettuce leaves of 100-700 mg/kg (wet weight). Parallel tests on
plants grown in soil containing known amounts of potassium bromide
confirmed a relationship between bromide content of soil and of plant
tissue gown in it (Maw and Kempton 1973). However Coosemans and Van
Assche (1978) showed that the relationship between soil bromide
content and plant bromide content was affected by the presence of
other anions in the soil. They reported 50-250 mg/kg bromide ion in
the lettuce following methyl bromide soil application at levels of
60-120 g per m2 with no leaching. Coosemans and Van Assche (1977)
also determined bromide ion in the fruit of strawberries and tomatoes
grown in fumigated soil, up to 80 mg/kg and 4 mg/kg respectively, but
much more in the leaves of both plants.
In a survey in the U.K. of lettuce stated to have been grown on either
unfumigated soil or on soil fumigated at 1´ - 2 lb per ft2 (Roughan
1979) ranges of inorganic bromide contents of lettuce leaves were as
follows:
Fumigated soil
mg/kg Br- wet weight No. of samples within range
0 - 5 0
5 - 10 1
10 - 50 3
50 - 100 12
100 - 200 12
200 - 500 19
500 - 1000 6
1000 - 2000 1
Soil said to have been untreated
mg/kg Br- wet weight No. of samples within range
0 - 5 26
5 - 10 2
10 - 50 4
50 - 100 2
100 - 200 1
200 - 500 5
From a plot treated with methyl bromide in July 1977, lettuce planted
in July 1977 and harvested in September contained 140-450 mg/kg Br-
while spring cabbage planted in October 1977 and harvested in May 1978
contained 300-800 mg/kg. In the same plot, lettuce planted in July
1978 and harvested in September contained 14-26 mg/kg. The world data
baseline for crops grown on untreated soil was said to be
approximately 5 mg/kg.
FATE OF RESIDUES
Pre-harvest Use
A large proportion of a given amount of methyl bromide applied to soil
diffuses away either to the atmosphere or away from the point of
application. Soils of high organic matter content tend to retain more
fumigant and subsequently bear higher contents of inorganic bromide,
probably as a result of hydrolytic decomposition. Tonic bromide is
taken up by some plants and concentrates in the leafy parts, e.g. in
lettuce, cabbage, tomatoes. So far as is known, no residues other
than bromide ion occurring in growing plant material as a result of
soil fumigation with methyl bromide have been reported.
Post-harvest Use
A major proportion of a dose of methyl bromide, generally applied by
exposure of stored commodities to the vapour, is removed still in the
vapour phase on ventilation and desorption. Foods of high oil or fat
content retain free methyl bromide for longer periods but El Lakwah
(1978) considered the fineness of division of extracted or expeller
products more important than the fat content in determining the amount
originally absorbed.
Scudamore and Houser (1970) reported detailed studies on the rate of
loss of free methyl bromide from a range of stored fumigated
commodities. These studies suggested that after normal storage and
pressing, free methyl bromide would be unlikely to be detected at the
point of sale. The 1971 Meeting reviewed earlier work on the
reactions of methyl bromide with protein components of cereals
(Bridges 1955, Winteringham 1955, Winteringham et al. 1955). Several
groups of workers have since studied the reactions of the methyl
radical with proteins and amino-acids in various foods (e.g. Adomako
1974; Asante-Poku et al. 1974; Pest Infestation Control Laboratory
1978) but no analytical methods suitable for determining
quantitatively such residues for monitoring or regulatory purposes
have been published to date. Recently isoelectric-focussing studies
on 14C-labelled protein fractions from fumigated cereals coupled with
HPLC analyses on fractions separated by gel-permeation chromatography
have shown broad range methylation of protein and free amino-acids
(Pest Infestation Control Laboratory 1980).
In certain foods, for example navy beans heavily treated with methyl
bromide, the formation of methyl methionine sulphonium bromide and its
subsequent spontaneous decomposition gives rise to dimethyl sulphide
(Fishwick 1978), which taints the commodity and can be determined
analytically. Dimethyl sulphide volatilises fairly rapidly though
and, as it occurs naturally in a number of foodstuffs, it can probably
be regarded as of little or no toxicological significance.
Fate in Storage and Processing
A large proportion of any free methyl bromide disappears fairly
rapidly from most fumigated stored products by volatilisation and due
to the reactions with plant constituents already described (Scudamore
and Heuser, 1970). No free methyl bromide is known to be taken up by
plants grown on pre-fumigated soil and none should therefore be
present unless produce is subsequently fumigated. If, after stored
product fumigation, small amounts of free methyl bromide persist until
the products are milled or otherwise processed, or cooked, it is
expected that further reductions in residue levels would then occur,
but analytical methodology sensitive enough to determine such expected
levels, i.e. below 0.1 mg/kg, has only recently become available
(Fairall and Scudamore, 1980). Any inorganic bromide produced as a
result of decomposition of methyl bromide remains at a constant level,
with levels likely to be highest in the high protein fraction of
milled products.
EVIDENCE OF RESIDUES IN FOOD IN COMMERCE OR WHEN OFFERED
FOR CONSUMPTION
Free Methyl Bromide and Dimethyl Sulphide
Although the studies of Scudamore and Heuser (1970) showed that after
storage of fumigated oily or sorptive commodities at very low
temperatures it was possible to detect low levels of free methyl
bromide, only negative results on free methyl bromide in products at
the point of sale have been recorded (Singh et al., 1976; Johansson,
1976). The presence of dimethyl sulphide in methyl bromide fumigated
navy beans imported into the U.K. was reported by Fishwick (1978), but
normal processing for the baked bean trade is thought to reduce this
to insignificant amounts.
Inorganic Bromide
Reports too numerous to list refer to the presence of inorganic
bromide in products or commodities fumigated with methyl bromide, but
in commerce it is impossible to determine, except perhaps by
inference, whether such residues arose as a result of storage
fumigation or from the presence in the plant of bromide from another
source such as soil fumigation with a bromine compound, or
contamination of soil with sea water prior to growth. There is
however strong inferential evidence that increased incidence at retail
outlets of lettuce and cabbage plants with raised inorganic bromide
content e.g. greater than 100 mg/kg (wet weight) may be related to the
increasing use of methyl bromide sterilization of soil, especially in
glasshouses. A few examples of lettuce plants bought at retail
outlets in the United Kingdom in 1978 contained residues exceeding 500
mg/kg (wet weight) but 92% of plants purchased in the South East of
the country contained less than 6 mg/kg whilst of samples received for
analysis from other areas, only 8% exceeded 150 mg/kg (Roughan, 1979).
METHODS OF RESIDUE ANALYSIS
(a) Free methyl bromide
Determination of free methyl bromide in commodities can be carried out
by cold extraction with an acetone/water mixture followed by gas
chromatography with flame-ionisation or electron capture detection
(Heuswer and Scudamore 1968, 1969). The limit of determination of
this method is about 0.5-1 mg/kg. Great care must be taken in the
determination of free methyl bromide not to incur losses due to
volatilisation or increased substrate interactions during recovery.
Recently a method has been published (Fairall and Scudamore, 1980) in
which extracted methyl bromide is reacted to form methyl iodide which
is then determined by gas chromatography, with a claimed limit of
sensitivity of 0.01 mg/kg. Grave and Hogendoorn (1979) report a
claimed sensitivity of 0.05 mg/kg for a head-space method for residual
methyl bromide in grain.
(b) Inorganic (bromide ion)
Bromide ion may be determined selectively by selective ion electrode
(Banks et al. 1976) or by reacting the extracted inorganic bromide
with ethylene oxide to form ethylene bromohydrin, which is then
determined by gas chromatography using electroncapture detection
(Heuser and Scudamore, 1970; Panel on Fumigant Residues in Grain,
1976; Stijve, 1977). Bromide ion may also be determined by total
bromide chemical assay (Mapes and Shrader 1957), by X-my fluorescence
analysis (Getzendaner et al. 1968) or by neutron activation analysis
(Lindgren et al. 1962) providing that other bromine compounds have
first been removed selectively with a solvent such as methylene
chloride. In the absence of such pre-extinction those total bromide
assay methods will include any organo-bromine compounds present, which
may be of a totally different order of toxicity to mammals.
NATIONAL LIMITS REPORTED TO THE MEETING
No national residue limits for free methyl bromide in commodities are
known to be in force. An EEC draft directive in 1976 proposed a limit
of 0.5 mg/kg in cereals to be put into circulation for human
consumption.
Limits for inorganic bromide in a wide range of commodities exist in
United States and Canadian regulations and in other national
schedules. A figure of 50 mg/kg in raw cereals is widely adopted. At
present, however, there is no way of determining the source of the
residue, which could for example also be present in plant-derived
foods as a result of uptake from soil.
APPRAISAL
Methyl bromide continues to be a fumigant of major importance
world-wide, in stored product insect and mite disinfestation, in the
control of fungi and to a more limited extent in partial sterilization
of feeding stuffs such as poultry food. It is increasingly being used
as a soil sterilant, both in glasshouses and on open agricultural
land.
In the soil sterilization role the fumigant is rapidly dispersed, by
seepage, by volatilization and by decomposition, leaving a residue of
inorganic bromide. The amount of residue appears to be greater, as a
proportion of the dose applied, in soils of higher organic matter
content. This soil inorganic bromide is reduced by irrigation or by
rainfall.
Plants grown in soils containing inorganic bromide from whatever
source take up bromide; this is particularly concentrated in the leafy
parts of plants such as lettuce and cabbage but smaller amounts are
found in fruits, e.g. strawberries, tomatoes. Residual inorganic
bromide from plant uptake appears to be the sole cause of concern for
food residues occurring as a result of using methyl bromide for soil
sterilization. In goods in commerce it cannot be specifically related
to pesticide usage and it must be borne in mind that inorganic bromide
is taken into the human diet from a number of food additive sources,
importantly in bread and beer manufacture, and in medication. Any
acceptable limits must therefore be set with a view to limitation of
the total dietary intake of bromide, rather than relating to a
toxicological assessment of the consequences of pesticide usage in
isolation. However, further data on the possibly beneficial effects
of soil irrigation on bromide ion levels in crops are required.
Without post-fumigation leaching it may sometimes be difficult to
produce leafy vegetables containing bromide ion within legally
acceptable levels.
In stored product disinfestation, including animal feeding stuff
treatments, general methylation of proteins and free amino-acids takes
place with concurrent formation of an inorganic bromide residue.
Laboratory electrophoretic studies have indicated changed patterns of
enzymes and other proteins in fumigated cereals, but no routine
analytical methods are at present available to measure these reaction
products as residues. Inorganic bromide formed can be determined
specifically and the same considerations apply as to those discussed
in relation to bromide from plant uptake. The amounts of reaction
products and the bromide residues increase roughly in proportion to
the severity and the number of treatments.
Whilst small amounts of residual free methyl bromide can be determined
in fumigated stored products, these generally disappear relatively
quickly on storage and/or processing. Although no reports of free
methyl bromide in goods offered for human consumption have been
published, the dangerous nature of methyl bromide as an alkylating and
mutagenic agent indicates that extremely low limits at or about the
new lower limit of detection should be set. The meeting therefore
recommended that the guideline levels for free methyl bromide should
be reduced as given below.
From available information it appears that no MRLs can be set for
bromide ion (inorganic bromide) in foods arising solely from methyl
bromide fumigation because no distinction can be made from other
sources of bromide acquisition. However there is a considerable body
of evidence that cereals not grown on fumigated soil or subjected to
fumigation with bromine compounds in storage generally contain less
than 5 mg/kg bromide ion. The limit of 50 mg/kg already widely
adopted therefore indicates that raw cereals have not been subjected
to excessive treatments with methyl bromide and hence suffered major
changes in protein/amino acid content. Consequently the meeting
concluded that the previously recommended limit for bromide ion in raw
materials should remain unchanged. MRLs for bromide ion in lettuce
and cabbage were adopted as an aid to the reduction of bromide dietary
intake from all sources.
EVALUATION
Guideline levels are recorded for bromomethane and MRLs recommended
for bromide ion, as shown below.
Bromomethane
(Guideline Levels): Nuts, peanuts 10a
Cereal grains, cocoa beans 5a
Dried fruits 2a
Milled cereal products 1a
a To apply at point of entry into a country, and in the case of
cereal for milling, if product has been freely exposed to air for a
period of at least 24 hours.
Bromomethane Bread and other cooked cereal
(cont'd) products, milled cereal products
cocoa products, dried fruits,
nuts, peanuts 0.01b
Bromide ion (MRLs) Cereal grains, wheat flour
(wholemeal) 50c,d
(These MRLs replace "Raw cereals"
and "wholemeal flour" at the
same level)
Lettuce, cabbage 100d
FURTHER WORK OR INFORMATION
Required: (by 1982)
Further data on levels of bromide ion in crops grown in soil fumigated
with bromomethane with special attention to the effects of soil
irrigation on such levels.
Desirable:
1. Data on levels of free bromomethane in fumigated commodities and
processed products, using newly available more sensitive analytical
methods.
2. Further information on reaction products of bromomethane with
fumigated commodity constituents.
REFERENCES
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in fumigated cocoa. IAEA Vienna 1976. ISBN 92-0-111576-8.
Asante-Poku, S., Aston, W.P. & Schmidlt, D.E. Site of decomposition
of methyl bromide in cocoa beans., J.S. Fd. Ag. 25, 285.
a To apply at point of entry into a country, and in the case of
cereal for milling, if product has been freely exposed to air for a
period of at least 24 hours.
b To apply to commodity at point of retail sale or when offered for
consumption.
c Relates exclusively to bromide ion, not to bromine present as
unchanged fumigant.
d Relates to bromide ion from all sources.
Bridges, R.C. (The fate of labelled insecticide residues in food
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See Also:
Toxicological Abbreviations