PESTICIDE RESIDUES IN FOOD - 1984
Sponsored jointly by FAO and WHO
Data and recommendations of the joint meeting
of the FAO Panel of Experts on Pesticide Residues
in Food and the Environment and the
WHO Expert Group on Pesticide Residues
Rome, 24 September - 3 October 1984
Food and Agriculture Organization of the United Nations
Maleic hydrazide was evaluated in 1976, 1977 and 1980 1/. A
temporary ADI was estimated for the sodium and potassium salts in 1980
and previously recorded Guideline Levels were converted to temporary
An extensive review of the biological and economic features of
maleic hydrazide in the USA was available which, together with other
information, enabled the meeting to deal with questions raised by the
1976 and 1980 JMPRs. (US Department of Agriculture, 1979).
Purity of the Technical Product
It was stated in the 1980 evaluation that the temporary ADI
estimated at that meeting referred to maleic hydrazide containing up
to 15 mg/kg hydrazine. This was a typographical error. This was the
upper limit of the concentration of hydrazine present in maleic
hydrazide used in animal feeding studies that did not give rise to the
development of cancer in laboratory animals. The figure should have
been 1.5 mg/kg. The figure was correctly recorded as 1.5 mg/kg in the
report of the 1980 meeting.
Liu et al. (1974) reported that commercial maleic hydrazide
formulations may contain amounts of hydrazine varying from 0.14 to
870 mg/kg. Bakker et al. (1983) reported the results of a survey
of maleic hydrazide formulations including several stored at 50°C for
10 weeks. Fourteen commercial formulations with maleic hydrazide
concentrations ranging from 180-360 g/l were investigated. The
hydrazine content of the maleic hydrazide in these formulations ranged
from less than 0.05 to 53 mg/kg. During the storage of two samples at
50°C for 10 weeks, the hydrazine contents increased from 2.2 to 120
and 0.4 to 54 mg/l. Thirteen of the formulations were diethanolamine
salts, the other a potassium salt. The hydrazine content of the
potassium salt was less than 0.05 mg/kg. In later studies (personal
communication) it was found that all samples of the potassium salt
contained less than 1 mg/kg of hydrazine and that the potassium salt
was stable to storage at 50°C.
1/ See Annex 2 for FAO and WHO documentation
The withdrawal by the principal US manufacturer of the
diethanolamine salt in 1980 has reduced the concern over the possible
presence of hydrazine as an impurity in commercial formulations. The
meeting was not convinced that all sources of the diethanolamine salt
had yet disappeared from European markets but recommended that the FAO
Specification for maleic hydrazide should be adopted and enforced
world-wide. This incorporates a limit of 1 mg/kg for hydrazine as an
impurity in maleic hydrazide.
RESIDUES IN FOOD AND THEIR EVALUATION
Two formulations of maleic hydrazide (MH), the diethanolamine
(DEA-MH) and potassium (K-MH) salts, were marketed for many years but
the DEA-MH formulation was withdrawn by the principal manufacturer in
1980 when a means was found to increase the efficacy of the K-MH
formulation. Since then the K-MH form has mainly been used. The rate
of application is 3.3 kg/ha. The systemic action of MH makes it
important for the control of tobacco suckers, and of sprouting in
stored potatoes and onions.
About 80% (1.45 million kg a.i.) of the total MH marketed in the
USA is used to control tobacco suckers. Sprout inhibition in potatoes
for storage accounts for 16.3% (300,000 kg), and about 1.5%
(30,000 kg) is used to inhibit sprouting of stored onions. K-MH
formulations are also registered for use on tobacco, potatoes and
onions in Austria, Australia, Canada, Czechoslovakia, Italy, Kenya,
The Netherlands and Taiwan among others. It is registered for use on
tobacco only in Bulgaria, Colombia and Mexico. MH is used on other
root crops in Canada.
Use on potatoes
Stored potatoes will sprout at temperatures above 5°C unless
sprouting is inhibited. At temperatures below 7°C however the starch
in potatoes is converted to sugar, a change which gives rise to an
unacceptable dark brown colour in cooked potato products such as
crisps and chips. The use of an inhibitor is therefore virtually
The USA produces about 1.37 million tonnes of fall-crop potatoes
annually, 1.2 million tonnes of which are stored for 1 month or
longer. More than half of the total US production of potatoes is grown
in three Pacific Northwest States, Idaho, Washington and Oregon, which
are also the major processing States. Thirty-one percent of the total
potato production used for food is processed into some type of frozen
product, 13% is processed into potato chips, 7% is dehydrated, 2% is
canned, and the remaining 47% is marketed fresh. Some kind of sprout
inhibitor is used on about 60% of fall-produced potatoes.
MH is applied in the field to the green vines of fall crop
potatoes destined for storage. The treatment prevents sprouting and
maintains the original high quality. The best figures available
indicate that 220,000 tonnes of potatoes are treated each year with
MH, using 272,000 to 320,000 kg of MH. This is applied to 80,000 to
100,000 hectares of potatoes. Yields are not reduced if the
recommended rate of one application at 3.3 kg ai/ha is applied to the
vines 2 to 3 weeks after full bloom. Application of MH too early in
the growing season, or application of more than double the recommended
rate, can cause a cracking of the bud end, and in some cases an
internal brown spot flecking on the apical end of the potato tuber. MH
is absorbed through the leaves and translocated into the tubers, where
it suppresses external and internal sprouting.
Because of its internal mode of action, MH is the only chemical
that will prevent tubers left in the field at harvest from growing
into volunteer plants the next year. If allowed to grow, these
volunteer plants serve as a source of leaf roll virus, which reduces
yields and causes an internal discolouration in many varieties, called
"net necrosis." This often renders the tubers unusable for table stock
and many forms of processing. Inhibition of volunteer growth of
potatoes reduces the potential source of leaf roll virus inoculum by
as much as 98% (Sparks, 1978). Volunteers can also interfere with
harvesting machinery and contaminate or compete with other vegetables.
There is no alternative sprout inhibitor that will prevent volunteer
potatoes, since all other inhibitors are applied after the tubers have
been harvested and removed from the field. Other methods of control
are only partially effective, and are more expensive.
In addition to controlling sprouting of potato tubers, MH has
been found by some workers to increase the food value and quality of
tubers. Wittwer and Patterson (1951) state that MH-treated tubers held
at low temperatures accumulate less sugar and produce lighter-coloured
potato chips than the non-treated control. Patterson et al. (1952)
indicated that treatment with MH appeared to result in lower contents
of reducing sugars (see also the 1976 evaluation).
Use on onions
Maleic hydrazide is used to prevent sprouting in stored onions.
It is applied to onions in the field at a rate of 2.2 kg/ha when the
bulbs are mature. Five to seven green leaves are essential for
adequate absorption of the compound. The crop is harvested after 2-4
weeks and the tops are removed. The onions are then cured before
In addition to its direct effects, an indirect benefit of MH
Treatment is the elimination of volunteer onions, which often carry
disease, in the following season.
FATE OF RESIDUES
MH is absorbed into plant leaves and readily translocated.
Movement occurs from xylem to phloem and vice versa, and the chemical
is distributed throughout plants after application to leaves (Crafts,
1959, 1967; Crafts and Yamaguchi, 1958). In the early work on
translocation, the translocated compound(s) containing the 14C from
14C-MH were not identified, but generally assumed to be MH. More
recent studies with tobacco show that 14C-labelled MH moves as the
intact molecule from one leaf to another (Frear & Swanson, 1978).
Burley tobacco was grown and treated with 14C-labelled MH under
controlled conditions (Davis and Grunwald, 1974). More 14C-MH was
absorbed at 100% than at 75% relative humidity. Light stimulated the
uptake of MH in leaf disks, and the increased uptake was not due to an
increase in transpiration. 14C-MH was translocated and most of the
translocated radioactivity was recovered from the top leaves.
In other work by Davis and Atkinson (1976), the concentration in
the untreated lower leaves of tobacco reached about 100 mg/kg within
24 hours after treating the upper leaves. Concentrations in the upper
leaves declined rapidly from 300 mg/kg initially as MH was transported
away from the treated area. In tests conducted in North Carolina,
residues in mid-stalk flue-cured tobacco averaged 514 mg/kg
immediately after application and decreased to 344 mg/kg over a 10-day
harvest period of dry weather. The decrease was not statistically
significant, but suggests that residues decline with time. Further, in
a companion test at another location where 5.7 cm of rain fell on the
third day after treatment, residues had decreased by 66% by the fourth
day after treatment.
Pendergrass (1969) applied 14C-MH to the green foliage of 22
onion plants near maturity. The 14C-MH had a specific activity of
0.3 MCi/m mole and was combined with 1.000 mg/l of Dupanol WAQ wetting
agent. Volumes of 0.1 ml of this solution were injected into the
internal cavity of four leaves per onion for a total dosage of 10 uCi
per onion. In a separate treatment, the onions were sprayed with
unlabelled MH at the normal rate of application for sprout control.
Results indicated that distribution was fairly uniform, with 69% of
the applied MH translocated to the bulb. At this point in onion
growth, about 30 to 35% of the total weight of the plant is in the
foliage. Of the total MH, 49% was found in the outer bulb leaves, 16%
in the inner shoot leaves, and 4% in the root plate. The concentration
in the root plate, however, was six times that found in the outer bulb
leaves. In culinary preparation most of the root plate of an onion
bulb is discarded as it is discoloured or appears soiled. It is not
included in such preparations as, for example, frozen fried onion
The results of Crafts (1959) showed that MH diffused readily into
potato tuber tissue within 2 days after application to the cut
surface. In tests conducted in Canada with MH as a 2500 mg/l spray
applied to potato foliage 3 weeks after full bloom, sufficient was
absorbed in 24 hours to inhibit sprouting (Franklin, 1959), and
residues of 6 mg/kg were found in the tubers. A 48-hour absorption
period provided complete inhibition of sprouting. MH residues in
tubers increased to a maximum of 36 mg/kg 1 week after application.
Although some early work on translocation suggested that MH was
not readily held in storage tissues along the translocation path
(Crafts, 1959), other research conducted during the same period
(Towers et al., 1958) indicated that the ß-D-glucoside of MH is
formed in Nicotiana rustica, N. sanderae, and representatives of
three other plant species. About 15% of the MH in leaf tissues was
present as the glucoside. Recent studies on the fate of MH in tobacco
plants confirmed the formation of the ß-D-glucoside in N. tabacum
(Frear & Swanson, 1978).
Although MH residues decline with time, the growth regulator is
only slowly degraded within plant tissues and residues may persist for
extended periods. From 17 to 22% of the MH remained unchanged in the
tobacco plant four weeks after application to leaves, and was
extractable with methanol (Frear & Swanson, 1978). Methanol-soluble
metabolites were present in amounts ranging from 14 to 18% of the
applied 14C. In fresh tissue after short periods (Davis & Grunwald,
1974), 99% of the 14C-label could be extracted with trichloracetic
acide-acetone and perchloric acid. In cured tobacco, however, all the
radioactivity was recovered from the RNA, DNA and proteins. Frear and
Swanson (1978) also reported that 27 to 33% of the applied 14C was
present as a methanol-insoluble fraction four weeks after leaf
treatment. A large amount of this fraction was found in the roots, and
30 to 40% of the foliar-applied MH was excreted into the growing
medium unchanged. Davis and Grunwald (1974) showed that relative
humidity affected the translocation of MH to the stem and to the root.
Translocation, excretion and glucoside formation were similar in
selected flue-cured and burley varieties of tobacco (Frear & Swanson,
Morphological and physiological responses of plants to MH have
been investigated by workers since its growth-regulating properties
were first recognized. Summaries of the literature on MH (Zukel, 1957,
1963) include many references and abstracts as well as a synopsis of
the major observations in the abstracts cited. MH was discussed
briefly in reviews by Crafts (1953), Woodford et al. (1958), and
Shaw et al. (1960).
Because MH has an exceedingly low vapour pressure (essentially
zero at 50°C), it was concluded that losses of MH from soil and plant
surfaced by volatilization would be negligible. Observations on loss
rates of MH from leaf surfaces support this conclusion (Smith
et al ., 1959).
Published data on loss rates from soil show that MH disappears
rapidly. In a study reported by Levi and Crafts (1952), oats planted 2
months after treating the soil were not injured by 80 mg/kg of MH in
any of 11 California soils. When oats were planted immediately after
application, however, seedling plants were injured by 5 mg/kg of MH in
the soil. In some soils, initial concentrations as high as 490 mg/kg
were not toxic to oats planted 2 months after application. In later
research with more refined detection methods and under laboratory
conditions the level of MH in soil dropped from 100 to 5 mg/kg in 3
weeks (Helweg-Anderson, 1971). Another experiment reported by the same
author showed that about 90% of 4.5 and 9 kg/ha applications
disappeared in 12 days. Only traces were present after 80 days. In
work reported by Hoffman et al. (1962), an application rate of
2.25 kg ai/ha resulted in 1 mg/kg of residue in the surface 15 cm of
soil immediately after application. The disappearance of MH was very
rapid from sand and muck and somewhat less rapid from clay soils.
Following a farmer application of 15 kg ai/ha of MH on burley tobacco,
it was not possible to detect any residue in the soil 12 months later.
Also, in tobacco grown on soil where the crop had been treated with
the recommended rate (170 mg/plant) of MH the previous year, leaf
residues of MH were not detected (Davis & Massie, 1977).
In later studies, Helweg (1975a, 1975b) showed that the
decomposition of MH at concentrations of 20 mg/kg or less followed
first-order kinetics, whereas at 120 mg/kg, zero-order kinetics more
closely described the loss. The addition of activated carbon to soil
retarded decomposition. With time in the soil, the capacity of carbon
to retard degradation decreased.
The rapidity with which phytotoxic effects of MH disappeared
from soil in the studies by Levi and Crafts (1952) strongly suggested
that micro-organisms were involved. Lembeck and Colmer (1957) reported
the isolation of two soil bacteria (Alcaligenes faecalis and
Flavobacterium diffusum) capable of using the diethanolamine salt of
MH as an energy source. They also showed that the phytotoxic effects
of MH were reduced by the action of the bacteria.
The results of Helweg-Anderson (1971), showing that sterilizing
the soil by autoclaving or gamma radiation prevented breakdown of MH,
confirmed the earlier report of Lembeck and Colmer (1957). Kaufman and
Kalayanova (1977) obtained similar results by the use of potassium
azide as a soil sterilant. In another publication by Helweg (1975b) on
the degradation of MH, 45% of the 14C added to soil in a 26 mg/kg
treatment was liberated in 20 days and 56% was liberated in 255 days
of incubation. However, micro-organisms capable of utilizing MH as a
sole source of carbon were not isolated. It was concluded that the
initial decomposition, if microbial, was by co-metabolism. Recent work
by Kaufman and Kalayanova (1977) suggested that cleavage of the MH
ring occurred by chemical mechanisms with subsequent degradation by
soil micro-organisms. The degradation pathway of MH in soil was
established in this work, with no evidence for the formation of
Limited information is available on the movement of MH into soil.
In a leaching experiment, Levi and Crafts (1952), through the use of a
bioassay, showed that MH was displaced downward when relatively large
quantities of water were added to the surface of soil columns. The
fact that it is rapidly degraded generally eliminates movement below a
few centimetres into the soil, except when rates of application are
excessive and when rain occurs soon after application. At recommended
rates, significant movement below the usual 15-cm plough layer would
seldom, if ever, occur.
RESIDUES IN FOOD IN COMMERCE OR AT CONSUMPTION
The MRL for MH residues on potatoes is 50 mg/kg. The average
residue is 15 to 25 mg/kg, and most residues would fall in a range of
10 to 40 mg/kg (Sparks, 1978). Routine monitoring of MH on potatoes is
not known to be carried out at any agency concerned with residue
analysis, but the potatoes themselves serve as a useful bioassay for
MH residues at or above the MRL because tuber injury occurs at the 45
to 50 mg/kg level (Sparks, 1978). Potatoes damaged by MH account for
an extremely small part of total production and are rejected for
normal food use.
On the basis of estimated consumption rates for potatoes and potato
chips, the average daily exposure for a 60 kg person is about
0.019 mg/kg per day.
The MRL for MH in onion bulbs in 15 mg/kg. Experience has shown
that the amount required for total sprout inhibition is 5 to 7 mg/kg.
At the recommended application rate the amount of MH absorbed is
usually within this range.
Residues of 2 to 7 mg/kg occurred in New York in 1962 when MH was
applied to onions at different stages of maturity (Isenberg, 1977). In
the United Kingdom residues of 4.1 to 11.0 mg/kg were found in 1968
and 2.8 to 3.1 mg/kg in 1969 (Whitewell, 1977).
The principle manufacturer of MH undertook a survey throughout
the USA and Canada (Uniroyal, 1984). 39 samples of potatoes from the
major potato-growing areas were analysed for MH residues by the method
of Lane et al. (1958) with the results shown in Table 1.
Table 1. Residues of maleic hydrazide in onion (survey data)
Location Residue, mg/kg
New York 2, 6, 10
Ohio 5, 3, 4, 7
Michigan 3, 2, 2
Washington State 2, 2, 3, 3, 12, 5, 15, 7, 12, 5, 3, 3, 6
Canada 9, 10, 11, 8, 8, 10, 10, 6, 10, 9, 4, 10,
7, 10, 5, 6
Mean of 39 results 6.5 mg/kg
At recommended application rates of 2.2 kg/ha, residues are
generally within the 5 to 7 mg/kg range. Per caput consumption of
treated onions in 1976 was roughly 2 kg. On the basis of 2 kg and
6 mg/kg residues, this equals about 0.00055 mg/kg per day for a 60-kg
METHODS OF RESIDUE ANALYSIS
The 1976 meeting required that the GLC method should be further
developed to make it suitable for regulatory purposes.
The present meeting was advised that considerable time and effort
had been given to this task but the outcome was still not
satisfactory. The original method of Wood (1953) as modified by Lane
et al. (1958) was later developed by the Naugatuck Chemical
Division of the United States Rubber Company. This development was
subjected to collaborative study (Lane, 1963, 1965), which showed that
a wide range of concentrations of maleic hydrazide could be accurately
determined in any likely substrate. The method recovers both free and
conjugated maleic hydrazide.
The meeting had available an extensive review of the biological,
agricultural and economic implications of the use of maleic hydrazide
in agriculture, which provided some of the information required by the
1976 JMPR. Further information was received from the principal
manufacturer. The diethanolamine salt is no longer available, the
potassium salt being the only form now in use.
Maleic hydrazide is essential to the production, storage and
marketing of both potatoes and onions to prevent sprouting and
subsequent deterioration of their acceptability and food value. It is
applied to the growing crop and is distributed uniformally throughout
the tuber or bulb by systemic transfer. It is therefore possible to
obtain a long-lasting effect (9 - 12 months) on the whole crop with
minimal rates of application. This avoids the need for grading and
rejection of some of the crop prior to marketing.
An additional advantage derived from spraying potato crops with
maleic hydrazide is that it prevents the growth of volunteer potato
plants from tubers left in the field, thus preventing the carry-over
of potato virus diseases from one crop to the next.
Valuable information on the fate of maleic hydrazide in various
components of the environment was available. This confirmed that there
was no risk of carry-over into subsequent rotational crops.
Approved treatments of potatoes lead to residues in the tubers
which, under some conditions, can be in the range 40-50 mg/kg.
However, the average is between 15 and 25 mg/kg and most residues fall
within the range 10-40 mg/kg. The potatoes themselves serve as a
useful bioassay for maleic hydrazide residues at or above the MRL
because tuber injury occurs at the 45-50 mg/kg level. Potatoes damaged
in this way account for an extremely small proportion of the total
production and are rejected for normal food use.
The MRL for maleic hydrazide in onion bulbs is 15 mg/kg.
Experience has shown that the amount required for total sprout
inhibition is 5 to 7 mg/kg. Following the use of maleic hydrazide at
the approved rate the amount absorbed is usually within this range.
Monitoring studies indicated residues in the range 2-11 mg/kg in both
potatoes and onions.
The distribution of maleic hydrazide in onion bulbs has been
studied with the labelled compound. Following approved treatments
approximately 70% of the applied maleic hydrazide is transferred to
the bulb, which represents about 65-70% of the total weight of the
onion plant. Of the 70% in the bulb, 48% was found in the outer part,
16% in the inner shoot leaves and 4% in the root plate. The root plate
is discarded in culinary practice.
It had been established that the diethanolamine salt was
unstable, giving rise to hydrazine. Information available confirmed
that the commercial formulation of the potassium salt contained less
than 1 mg/kg of hydrazine. Hydrazine does not occur as a metabolite in
plants, animals or soil because strong reducing conditions are needed
to convert maleic hydrazide to hydrazine (e.g. sodium hydroxide and
Maleic hydrazide residues occur as a mixture of the parent
compound and various conjugates. The method of analysis, which
involves heating at 160°C in a 600g/l solution of sodium hydroxide,
effectively converts these conjugates into free maleic hydrazide. The
meeting agreed that the residue should therefore be defined as "sum of
free and conjugated maleic hydrazide."
Attempts to develop alternative methods of analysis based on GLC
procedures have so far been only partially successful.
Reliable information indicated that there were no significant
uses on crops other than tobacco, potatoes and onions. The use on
trees, grass and other ornamentals is small and does not lead to
residues in food.
There does not appear to be any possibility that maleic hydrazide
residues on waste commodities such as potato skins could give rise to
residues in foods of animal origin when such wastes are used as animal
The meeting was satisfied that all the information and further
work listed as required in 1976 had now been supplied and that there
were no further questions concerned with residues evaluation to be
pursued. It recommended that the maleic hydrazide used in agriculture
be confined to the potassium salt which should be 99.9% pure and
contain not more than 1 mg/kg of hydrazine.
The meeting agreed that the TMRLs previously recommended were
appropriate. As an ADI was estimated, the limits were converted to
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