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
HYDROGEN PHOSPHIDE
Hydrogen phosphide, there referred to as phosphine, was evaluated at
the 1965 Joint Meeting (FAO/WHO 1965c), reviewed and re-evaluated in
1966 and 1967 (FAO/WHO 1967b, 1968b) and the recommendations for
tolerances revised in 1969 (FAO/WHO 1970b). Reference should be made
to Appendix IV which includes Section 3 of the report of the 1971
meeting (FAO/WHO 1972a) where general principles concerning fumigant
residues are discussed.
IDENTITY
The data here considered refer to hydrogen phosphide from tablets,
pellets or paper packets containing aluminium phosphide from which the
gas is released on exposure to the atmosphere. These preparations
contain other materials formulated to control the evolution of the
hydrogen phosphide and to prevent combustion.
RESIDUES IN FOOD AND THEIR EVALUATION
Use Pattern
Post-harvest use on dry foodstuffs
The fumigant is used in many countries for controlling insects in many
types of dry stored foods. The main use is for cereal grains in bins,
in boxcars or on floors when the preparations are added during loading
or are inserted into the bulk. Stacks of bagged or packaged
commodities can also be fumigated in sealed stores or under a covering
of gas-proof sheets. In the United States uses with processed food for
human consumption is subject to recommendations that no unreacted
aluminium phosphide come into contact with such food and that none of
the food will be offered to the consumer before 48 hours airing. In
the United Kingdom it is also recommended that the fumigant
preparation will be applied in a manner that permits all powder
residues to be removed after the treatment.
Residues in foods
The types of residues may be considered as follows:
Unreacted aluminium phosphide. The powdery residue consists mainly
of aluminium hydroxide but typically contains about 5% of the original
content of aluminium phosphide, This appears to be occluded within the
particles of aluminium hydroxide, liberating the fumigant very slowly
into the atmosphere unless it is disturbed. By following recommended
practices, these residues are avoided.
Residues of unreacted phosphine. The information previously reviewed
indicated that all traces of sorbed phosphine are eliminated very
rapidly from fumigated food by normal aeration.
Non-volatile residues. Early studies, as reported by Dieterich et
al. (1967), concluded that there was virtually no loss of phosphine in
or reaction with exposed food. It was claimed that practically 100% of
the phosphine applied in a closed system could be recovered.
In similar experiments, Berck (1968) was unable to recover all the
phosphine applied. The losses appeared to be related to the nature of
the food, its moisture content, the temperature and the exposure time.
Berck inferred that there was some breakdown on, or reaction with, the
food. Other experiments of the same type have provided evidence
supporting the likelihood that there is a small amount of reaction in
foods, typically of the order of 1 ppm (Heseltine, 1970).
The high background of phosphorus compounds naturally present in food
makes it difficult to determine phosphorus-containing reaction
products in food by conventional chemical analysis. This difficulty
can be overcome in experiments using radioactively-labelled phosphine.
Several laboratories are at present active in such investigations.
Robinson and Bond (1970) used 32P-labelled phosphine derived from
labelled aluminium phosphide to demonstrate the presence of phosphorus
residues after treatment of wheat, flour, insects and cystine. The
radioactive residue on wheat and flour could not be removed by
thorough aeration or by heating at baking temperature. It was shown to
be largely water soluble and paper chromatography was used to identify
the main products as hypophosphite and phosphite. It was concluded
that the oxidation of phosphine to the lower oxyacids of phosphorus
was mainly a surface phenomenon and in the normal course of air
oxidation these residues would eventually all appear as
ortho-phosphate. Deposition of oxidation products of phosphine was
found also to occur on glass and other surfaces. Reduction of cystine
in vitro was found to take place but at a very slow rate. In these
investigations no attempt was made to ensure that all of the
radioactivity was accounted for.
Disney and Fowler (1971a, 1971b) have also provided interim reports on
a study of the reaction of 32P-labelled phosphine with whole wheat.
Their results are in agreement with Robinson and Bond. They also
demonstrate the important effect of moisture content as well as
exposure period on the amount of the residue which cannot be removed
by prolonged aeration. After exposure of wheat to a phosphine
concentration of 3 mg per litre at 25% the residue (calculated as
phosphorus) varied between 1.2 ppm for a five-day exposure of wheat of
10% moisture content and 18.4 ppm for a 14-day exposure at a moisture
content of 19.7%. These treatment levels are several times higher than
encountered in practical fumigations when it can be inferred that the
total residue is unlikely to exceed 1 or 2 ppm. Autoradiograms of
sections of whole wheat showed most of the radioactivity in the outer
layers and in the crease. It was also found that approximately 70% of
the residue is extractable from whole grain by hot water.
A brief summary is available of a third investigation in which
32P-labelled phosphine was used to treat wheat, flax and rapeseed
(Tkachuk, 1971). Approximately 50% of the added 32PH3 formed
non-PH3 residues. In wheat the distribution of residues was
approximately 85, 12 and 4% in the bran, endosperm and germ fractions.
In wheat bran approximately 56 and 11% of the residues are
water-soluble and appear to be hypophosphite and pyrophosphate; the
remaining 33% are water-insoluble and have not been identified.
Robison and Hilton (1971) have developed a method to determine traces
of phosphine released from zinc phosphide in sugar-cane by
gas-chromatography. Part of the released phosphine reacted
irreversibly with sugar-cane and, in a separate investigation with
32P-labelled phosphine added to sugar-cane in aqueous acid, about
30% of the phosphine reacted irreversibly to form water-soluble
compounds of phosphorus while another 10% remained irreversibly bound
in fibre. It was suggested that reaction occurred to phosphorus
oxyacids which would be water-soluble and that a portion of the acid
may have formed insoluble iron or aluminium salts in the fibre.
The general problem of residues in cereals after the use of phosphine
has been discussed by Robinson (1971a).
The experimental work with 32P-labelled phosphine has been
criticized (Rauscher, 1971) on the grounds that this behaves
differently from inactive phosphine. Robinson and Bond (1970) and also
Disney and Fowler (1971a, 1971b) had already considered this
possibility and were satisfied that there was no significant
difference. In view of the criticisms, however, carefully designed
experiments to establish the validity of the work with the labelled
phosphine were performed in both laboratories. Accounts have been
presented by Robinson (1971b) and by Disney and Fowler (1971c) to a
Joint FAO/IAEA Panel (FAO/IAEA 1971) which endorsed the conclusion
that the data obtained by the use of 32P-labelled phosphine were not
invalidated by the use of the radioactive material.
Evidence of residues in food in commerce or at consumption
The methods used to examine samples of grain in commerce for phosphine
usually also determine that evolved from any residual aluminium
phosphide present and the small amounts reported, usually below 0.1
ppm, must normally be derived from the latter source, thus accounting
for the occasional higher sample in a consignment of grain.
Methods of residue analysis
The method developed by Bruce, Robbins and Tuft (1962) hydrolyses
aluminium phosphide in the presence of dilute sulfuric acid to form
phosphine. The liberated phosphine is driven out by nitrogen gas into
scrubbers, and the contents of the scrubbers are analysed by bromine
oxidation of phosphine to phosphoric acid which is determined
colorimetrically. A limit of detection in grain below 0.005 ppm is
claimed.
The method by Heseltine (1963), based in part on the above method,
depends upon reaction with acid potassium permanganate and a
colorimetric determination of the phosphate as the blue reduction
product of the phosphomolydate. The limit of determination for
phosphine residues in grain is 0.01 ppm.
Gas-chromatography has been used to determine concentrations of
phosphine in air; but it is difficult to apply such methods to the
determination of residues. Robinson and Hilton (1971) described a
procedure for determining zinc phosphide in sugar-cane. Phosphine was
released by suspension of the sample in aqueous acid and toluene in a
sealed flask and the amount in the toluene layer was determined by
gas-chromatography using a photometric detector. A considerable loss
of phosphine occurred and calibration appeared difficult. A correction
factor of 1.7 was applied to the results to give an accuracy of ± 10%.
Absolute sensitivity of the gas-chromatographic determination was very
high (20 pg of phosphine) and minimum amounts of phosphine equivalent
to about 0.005 ppm in sugar-cane were reported.
National tolerances
Hydrogen phosphide (as reported to the meeting)
Several countries have 0.1 ppm for raw cereals but West Germany has
adopted a figure of 0.05 ppm. For grain immediately before milling
Belgium and the Netherlands reduce this figure to 'zero'.
The United States of America has a tolerance in or on processed foods
of 0.01 ppm. The regulation requires that the finished food should be
aerated for 48 hours before it is offered to the consumer and the
formulation containing aluminium phosphide must on no account be used
so that it or its unreacted residues will come into contact with any
processed food.
Appraisal
Preparations of aluminium phosphide which evolve hydrogen phosphide by
reaction with moisture in the surrounding atmosphere are used for the
post-harvest fumigation of a wide range of produce including processed
foods.
The powder remaining after the use of the fumigant preparation is
mainly aluminium hydroxide, but may contain a small part, up to about
5%, of the original content of aluminium phosphide. This presents no
hazard if good practice is followed. The normal cleaning of cereals
before milling is effective in eliminating almost all of this powder
and it has been widely accepted that a residue in a raw cereal of 0.1
ppm, determined and expressed as hydrogen phosphide would yield a
residue in bread and other food ready for consumption of a level at or
below that which can be determined by current methods of analysis
(0.01 ppm). For other foods which cannot be so cleaned before
processing good practice requires that the fumigant preparation
residue does not come into contact with the food. In these
circumstances any residue of unreacted hydrogen phosphide is rapidly
reduced below 0.01 ppm. Nevertheless tolerances are required to ensure
that good practices are observed.
It has been established that under normal fumigation conditions there
is some, albeit a very small amount of, reaction of phosphine on or in
cereals, leaving a non-volatile product which will probably not exceed
a few parts per million. On present evidence a large part of this
small residue consists of the lower oxyacids of phosphorus. which, in
the normal course of air oxidation, can be expected to appear as
ortho-phosphate. The nature of the reaction or breakdown products in
food which has been exposed to phosphine requires further
investigation and studies using isotopically labelled phosphine are in
progress.
1. It is recommended that the present tolerance of 0.1 ppm in raw
cereals be confirmed.
2. On the understanding that, if good practice is followed, any
residue of hydrogen phosphide, present as such or derived from
any aluminium phosphide, is reduced below the limit of
determination by present methods (0.01 ppm), it is recommended
that the tolerance of 0.01 ppm in flour, other milled cereal
products, breakfast cereals, dried vegetables and spices should
be confirmed and extended to include nuts, groundnuts, dried
fruit, cocoa beans and other similar foods known to be fumigated
with hydrogen phosphide.
Further work desirable
1. Further elucidation of the nature of the reaction or breakdown
products of hydrogen phosphide in grain.
2. Further data on the residues, if any, determined and expressed as
hydrogen phosphide, in products known to be fumigated
commercially with hydrogen phosphide.
REFERENCES
Berck, B. (1968) Sorption of phosphine by cereal products. J. Agr.
Food Chem., 16: 419-425
Bruce, R. B., Robbins, A. J. and Tuft, T. O. (1962) Phosphine residues
from Phostoxin-treated grain. J. Agr. Food Chem., 10: 18-21
Dieterich, W. H., Mayr, G., Hildt K., Sullivan, J. B. and Murphy, J.
(1967) Hydrogen phosphide as a fumigant for foods, feeds and processed
food products. Residue Reviews 19: 135-149
Disney, R. W. and Fowler, K. S. (1971a) Phosphorus-32-labelled
phosphine in the determination of fumigation residues in grain. Proc.
2nd Int. Congr. Pestic. Chem., Tel Aviv, Israel, (in press)
Disney, R. W. and Fowler, K. S. (1971b) Residues in cereals exposed to
hydrogen phosphide. Working paper for FAO/IAEA Panel, Vienna
Disney, R. W. and Fowler, K. S, (1971c) The possibility of isotope
exchanges or other interfering reactions occurring during the
determination of residues from fumigation with 32P-labelled
phosphine. Working paper for FAO/IAEA Panel, Vienna
FAO/IAEA (1971) Tracer aided studies of the fate and significance of
foreign substances in food and agricultural environment. Proceedings
of a combined panel and research coordination meeting. Vienna
Heseltine, H. K. (1963) Determination of phosphine. Pest Infestation
Research, Agricultural Research Council, London
Heseltine, H. K. (1970) Pest Infestation Research, Agricultural
Research Council, London
Robinson, J. R. (1971a) Hydrogen phosphide residues in cereals.
Working paper for FAO/IAEA Panel, Vienna
Robinson, J. R. (1971b) Phosphorous residues from 32-P-phosphine: an
artifact? Working paper for FAO/IAEA Panel, Vienna
Robinson, J. R. and Bond, E. J. (1970) The toxic action of phosphine
Studies with 32-PH3: terminal residues in biological materials.
J. stored Prod. Res., 6: 133-146
Robison, W. H. and Hilton, H. W. (1971) Gas-chromatography of
phosphine derived from zinc phosphide in sugar-cane. J. Agr. Food
Chem., 19: 875-878
Tkachuk, R. (1971) Phosphine fumigation of wheat - residue formation.
Cereal Sci. Today, 16: 307. (Abstract of paper presented at Amer,
Ass. Cereal Che., Ann. Met Meeting, Dallas)