Mercury, cadmium and lead content of canned Tuna fish

Abstract

Mercury levels in canned tuna fish were determined by cold vapour atomic absorption spectrophotometry while cadmium and lead levels were determined by flame atomic absorption spectrophotometry. The metal contents in the samples, expressed in μg g−1 wet weight, varied from 0.20 to 0.66 with an average value of 0.29 for mercury, from 0.09 to 0.32 with an average value of 0.18 for cadmium and from 0.18 to 0.40 with an average value of 0.28 for lead. The results of this study indicate that tuna fish from the Mediterranean coast of Libya have concentrations well below the permissible levels for these toxic metals. Their contribution to the body burden can therefore be considered negligible.

Full text

Mercury, cadmium and lead content of canned tuna ®sh
R.B. Voegborlo*, A.M. El-Methnani, M.Z. Abedin
Environmental Science Department, Faculty of Engineering and Technology, P.O. Box 68, Brack-Alshati, Libya
Received 26 March 1996; received in revised form 19 November 1997; accepted 19 November 1997
Abstract
Mercury levels in canned tuna ®sh were determined by cold vapour atomic absorption spectrophotometry while cadmium and
lead levels were determined by ¯ame atomic absorption spectrophotometry. The metal contents in the samples, expressed in mgg
ÿ1
wet weight, varied from 0.20 to 0.66 with an average value of 0.29 for mercury, from 0.09 to 0.32 with an average value of 0.18 for
cadmium and from 0.18 to 0.40 with an average value of 0.28 for lead. The results of this study indicate that tuna ®sh from the
Mediterranean coast of Libya have concentrations well below the permissible levels for these toxic metals. Their contribution to the
body burden can therefore be considered negligible. #1999 Published by Elsevier Science Ltd. All rights reserved.
1. Introduction
Toxicological and environmental studies have
prompted interest in the determination of toxic elements
in food. While mercury, cadmium and lead can be tol-
erated only at extremely low levels, at certain con-
centrations they are exceptionally toxic to humans.
Fish accumulate substantial concentrations of mer-
cury in their tissues and thus can represent a major
dietary source of this element to humans. With the
exception of occupational exposure, ®sh are acknowl-
edged to be the single largest source of mercury for
man. In some instances ®sh catches were banned for
human consumption because their total mercury con-
tent exceeded the maximum limits recommended by the
Food and Agriculture/World Health Organisation
(FAO/WHO, 1972). Takizawa (1979) cited the case
where several major incidents of human poisoning in
Japan (at Minamata) were implicated in the ingestion of
methylmercury-contaminated ®sh in large quantities.
The likelihood of mercury toxicity from ®sh consump-
tion has been identi®ed in Peru and some coastal
regions of the Mediterranean (Inskip & Piotrowski,
1985; Piotrowski & Inskip, 1981).
Tuna was recognised as a predator able to con-
centrate large amounts of heavy metals. For example,
Enomoto and Uchida (1973) reported mercury con-
centrations ranging from 50 to 120 mgg
ÿ1
in internal
organs of Japanese tuna. The ingestion of food is an
obvious means of exposure to metals, not only because
many metals are natural components of foodstus, but
also because of environmental contamination and con-
tamination during processing. Solder used in the manu-
facture of cans is a recognised source of contamination
of food by lead during canning. The presence of heavy
metals, and particularly mercury, in the environment
has been a matter of concern since their toxicity has
been clearly documented (Uchida, Hirakawa & Inoue,
1961). The presence of mercury in the environment was
reviewed (Holden, 1973; Krenkel, 1973). Extensive sur-
veys have been carried out, in a number of countries, to
evaluate the presence of heavy metals in the aquatic
biota, including ®sh, which can often be considered as
indicators of marine pollution. Levels of heavy metals
including mercury, lead and cadmium, in ®sh, have been
widely reported (Hellou, Warren, Payne, Belkhode &
Lobel, 1992; Joseph & Srivastava, 1993; Kowalewska &
Korzeniewski, 1991; Sharif, Mustafa, Mirza & Sa®ul-
lah, 1991; Sharif, Mustafa, Hossain, Amin & Sa®ullah,
1993; Winchester, 1988). The toxic nature of certain
metals and the major contribution made to the total
body burden of these metals by food consumption are
well documented (Bonner & Bridges, 1983; Browning,
1969; Department of Health and Social Security, DHSS,
1980). Hence the levels of these metals in foodstus are
under frequent review.
Canned tuna ®sh are frequently and largely eaten in
Libya, so their toxic metal content should be of some
concern to human health. The present study was, there-
fore, carried out in view of the scarcity of information
Food Chemistry 67 (1999) 341±345
www.elsevier.com/locate/foodchem
0308-8146/99/$ - see front matter #1999 Published by Elsevier Science Ltd. All rights reserved.
PII: S0308-8146(98)00008-9
* Corresponding author.

about heavy metals in marine organisms from this
region. In this paper, the levels of mercury, cadmium
and lead in samples of canned tuna ®sh are reported.
It is hoped that the results of this study will help in
generating data needed for the assessment of toxic metal
intake from this source.
2. Materials and methods
2.1. Apparatus
All glassware was soaked overnight in 10% (v/v)
nitric acid. Glassware, for the analyses of lead and cad-
mium was rinsed thoroughly with deionised distilled
water and dried before use, and that for mercury ana-
lyses was rinsed with distilled water. This was to control
the possible mercury contamination of water from the
resins used in deionisers.
A Perkin-Elmer Model 2380 atomic absorption spec-
trophotometer equipped with a deuterium background
corrector was used for the determination and the mer-
cury/hydride generator was a Perkin±Elmer Model
MES-10 with an open quartz tube. The signals were
obtained on a Perkin-Elmer PRS-10 Printer Sequencer.
2.2. Reagents
All reagents used were of analytical reagent grade
(BDH Chemicals Ltd, Poole, England). Standard stock
solutions of lead, cadmium and mercury were prepared
by diluting concentrated solutions to obtain solutions of
1000 mg l
ÿ1
(E. Merck).
The working solutions were freshly prepared by
diluting an appropriate aliquot of the stock solutions
through intermediate solutions using 5% HNO
3
for
diluting lead and cadmium solutions, and 1 MHCl for
diluting mercury solution. Stannous chloride was pre-
pared fresh by dissolving 10 g in 100 ml of 6 MHCl. The
solution was boiled for about 5 min, cooled, and
nitrogen bubbled through it to expel any mercury
impurities. Diluting solution for mercury determination
was prepared by diluting 100 ml of conc HNO
3
and
25 ml of conc H
2
SO
4
to 1000 ml with distilled water.
2.3. Sample preparation and digestion
Tuna ®sh caught by commercial vessels from the
Mediterranean coast of Libya are canned as chunks at
a commercial factory on land. Fifty cans of tuna
(5 kg each) obtained from the Tuna Canning Factory in
Misurata, Libya were used for this study. After opening
each can, oil was drained o and the meat was
homogenized thoroughly in a food blender with stain-
less steel cutters. Samples were then taken and digested
promptly.
The homogenised sample (10.01 g) was weighed
into a 100 ml Erlenmeyer ¯ask and 1 ml of conc HCl
was added. After about 10 min, 5 ml of conc HNO
3
was
added slowly. After swirling gently, 2 ml of (1+1)
H
2
SO
4
was added. It was then covered with a watch
glass and left at room temperature until most of the
sample had dissolved. The ¯ask was then placed on top
of a steam bath until complete dissolution. It was then
removed from the steam bath, cooled and the solution
transferred carefully into a 20 ml volumetric ¯ask and
diluted to the mark with distilled water. For each run, a
duplicate sample, spiked samples, and two blanks were
carried through the whole procedure.
For the determination of lead and cadmium, about
100.01 g was weighed into a 150 ml beaker and 10ml of
freshly prepared 1:1(v/v), H
2
O
2
(30%):HNO
3
(conc) was
added per gram of sample, slowly, in portions. The bea-
ker was covered with a watch glass and, after most of the
sample had dissolved, heated on a hot plate until the
solution was clear. Heating was continued until the
volume was reduced to about 5 ml. The solution was
allowed to cool, transferred into a 20 ml volumetric ¯ask
and diluted to the mark with deionised distilled water.
For each run, a duplicate sample, spiked samples and
two blanks were carried through the whole procedure.
2.4. Determination of recovery
The recoveries of the metals were determined by add-
ing increasing amounts of mercury, cadmium and lead to
samples which were then taken through the digestion
procedure. The resulting solutions were analysed for the
metal concentrations. The results are reported in
Tables 1±3. The mean recoveries for lead, cadmium and
mercury were 99.8, 99.3 and 97.2, respectively, with
coecients of variation 8.7, 3.8 and 3%, respectively.
2.5. Chemical analysis
Lead and cadmium were determined by direct
aspiration of the sample solutions into the air/acetylene
¯ame. The blanks and calibration standard solutions
were also analysed as the sample solutions and calibra-
tion curves constructed. Mercury was determined by the
MES-10 Mercury/Hydride system with a modi®cation
in the operation. The manufacturer's operating proce-
dure involves continuous addition of sodium borohy-
dride solution from a reluctant reservoir with the aid of
argon gas until maximum absorbance is produced. This
procedure was, however, found to give poor reproduci-
bility because the volume of sodium borohydride added
each time varies. In this study, the reluctant reservoir
was left empty. An aliquot of the sample solution (5 ml)
was diluted to 30 ml in the reaction ¯ask with the dilut-
ing acid solution and 2 ml of the stannous chloride
solution was added. The reaction ¯ask was immediately
342 R.B. Voegborlo et al. / Food Chemistry 67 (1999) 341±345

connected to the system and the plunger actuated
immediately, allowing argon to bubble through the
solution after ¯owing through the empty reservoir.
During this period, any mercury vapour generated is
swept into the absorption quartz cell aligned in the light
path of the mercury hollow cathode lamp where the
absorption is measured. Aliquots of the calibration
standard solutions and blanks were analysed in the same
way as the samples.
3. Results and discussion
Fifty samples of canned tuna ®sh from Misurata
canning factory were analysed for lead, cadmium and
mercury. Good recoveries of spiked samples demon-
strate the accuracy of the methods used (Tables 1±3).
Of the 50 samples analysed, mercury was detected in
20 samples, while lead and cadmium were detected in
only 12 samples. The concentrations of lead, cadmium
and mercury are presented in Table 4 as means with
standard deviation and coecient of variation. The
results of the analysis indicate that the concentration of
cadmium varied from 0.09 to 0.32 with a mean of
0.18 mgg
ÿ
1; for lead it ranged from 0.18 to 0.40 with a
mean of 0.28 mgg
ÿ1
. Good agreements were observed
when our results were compared with those reported by
other authors (Committee for Inland Fisheries of Africa
CIFA, 1992). The cadmium concentrations were low
compared to ®sh from the coast of Philippines and the
Northern Indian Ocean (CIFA, 1992). Woidich and
Pfanhauser (1974) reported a concentration range of
cadmium in tuna ®sh (0.050±0.970 mgg
ÿ1
) within which
our values fell. Muller and Forstner (1973), however,
reported higher levels of cadmium (10±40 mgg
ÿ1
)in
®shes from Necker and Ems. Teherani, Stehlik, Tehrani
and Schada (1979) reported 0.1±0.13 mgg
ÿ1
cadmium in
several ®sh types caught in upper Austrian waters,
Table 4
Mean contents of mercury, cadmium and lead (g g
ÿ1
) in canned tuna samples
Metal No. of samples Range Mean Standard deviation Coecient of
variation (%)
Mercury 20 0.20±0.66 0.29 0.12 40.7
Lead 12 0.18±0.40 0.28 0.07 24.3
Cadmium 12 0.09±0.32 0.18 0.08 42.2
Table 1
Recovery of lead from canned tuna samples
Sample no. Sample
weight (g)
Concentration
of lead (mgg
ÿ1
) added
Concentration
of lead (mgg
ÿ1
) recovered
% Recovery
9 10 0.20 0.18 90
9 10 0.40 0.41 103
9 10 1.00 0.96 96
9 10 2.00 2.20 110
Table 2
Recovery of cadmium from canned tuna samples
Sample no. Sample
weight (g)
Concentration of
cadmium (mgg
ÿ1
) added
Concentration of
cadmium (mgg
ÿ1
) recovered
% Recovery
9 10 0.10 0.10 100
9 10 0.20 0.19 95
9 10 0.50 0.52 104
9 10 1.00 0.98 98
Table 3
Recovery of mercury from canned tuna samples
Sample no. Sample
weight (g)
Concentration of
mercury (mgg
ÿ1
) added
Concentration of
mercury (mgg
ÿ1
) recovered
% Recovery
10 1 0.02 0.0202 101
10 1 0.05 0.0490 98
10 1 0.10 0.0980 98
10 1 0.20 0.1920 96
R.B. Voegborlo et al. / Food Chemistry 67 (1999) 341±345 343

which are lower than values reported here. The con-
centration of lead was found to be less than 0.35 mgg
ÿ1
in most of the samples, which agrees well with values
reported by other authors (CIFA, 1992; Woidich and
Pfanhauser, 1974).
The concentration of mercury in the tuna ®sh samples
analysed varied from 0.2 to 0.66 mgg
ÿ1
. Apart from two
samples which have concentrations of 0.55 and
0.66 mgg
ÿ1
mercury, all the samples have concentrations
below the 0.5 mgg
ÿ1
limit recommended by the FAO/
WHO (1972) and adopted by many countries (CIFA,
1992). The levels of the toxic metals in the tuna samples
are not high when compared to some other areas of the
world. The mercury content of tuna ®sh has variously
been reported as ranging from 0.8 to 1.20 mgg
ÿ1
with an
average content that is between 0.3 and 0.4 mgg
ÿ1
(Holden, 1973), below which our values fall. Mean
mercury levels reported here are lower by an order of
magnitude compared to values reported for mullets in
the Tyrrhenian Sea, an area close to naturally occurring
mercury deposits (CIFA, 1992). However, they were
similar to levels in other tropical, less industrialised
areas like Indonesia, Thailand and Papua New Guinea
(CIFA, 1992). A similar trend was observed when our
values were compared with values (0.04±0.44 mgg
ÿ1
)
reported for canned salmon and tuna and values (0.009±
0.73 mgg
ÿ1
) reported for canned sea food (Fricke, Rob-
bins & Caruso, 1979; Kaiser & Tolg, 1980).
With respect to the heavy metal content of marine
organisms taken from other Mediterranean coastal
areas, very little comparison data appear to be avail-
able. However, our results compare well with values
reported for ®sh from the Mediterranean coast of Israel
(Hornung & Kress, 1989; Roth & Hornung, 1977) and
that of Morocco (El-Hraiki, Kessabi, Sabhi, Benard &
Buhler, 1992).
Because of the bioaccumulation of mercury by ®sh
and shell®sh, these food items can be a rich source
of metal (Buzina, Suboticanec, Vukusi'c, Sapunar,
Anton'ic & Zorica, 1989; Piotrowski & Inskip, 1981).
As a consequence of its known toxicity, as well as that
of lead and cadmium and of the serious contamination
of foods that occurs from time to time during commer-
cial handling and processing, most countries monitor
the levels of toxic elements in foods. The Joint Food
and Agriculture Organisation/World Health Organisa-
tion (FAO/WHO) Expert Committee on Food Addi-
tives has suggested a provisional tolerable intake of
400±500 mg cadmium per week for man; the quantity of
mercury to be tolerated in human food is 0.3 mg per
week and, for lead, a weekly intake of 3 mg (FAO/
WHO, 1972). The maximum concentration of lead
which is permitted in prepared foods speci®cally intended
for babies or young children is 200mgkg
ÿ1
(FAO/WHO).
Although, marine food does not signi®cantly con-
tribute to the chronic lead body burden, the monitoring
of lead concentration in the diet is essential since ®sh of
various types were found to be contaminated with lead
in addition to cadmium and mercury. Lead concentrations
could be high in marine animals that live on sediment.
Though estimates of the amount of toxic metals con-
sumed in the diet are dicult to obtain and a discussion
of metal tolerances in the diet is beyond the scope of this
paper, it can be concluded from the results so far
obtained that mercury, lead and cadmium content of
the canned tuna ®sh is unlikely to constitute a sig-
ni®cant health hazard.
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