•
Introduction
Leaching
of toxic elements from cooking utensils is a long-recognised problem,
and covers a wide range of elements, the traditional examples including
materials such as lead (mainly from earthenware) and copper (typically
from coffee pots with defective tin-plating). More recently, nickel
and aluminium from kitchenware have raised concerns. In this article,
literature on hazards caused by chromium leaching from stainless steel
kitchen utensils and other consumer items is discussed from the point
of view of toxicity.
•
Essentiality of chromium
Studies
of patients on total parenteral nutrition have indicated that a lack
of chromium may lead to disturbances in glucose metabolism(2).
In addition, the glucose balance of diabetic patients with a low chromium
intake was reported to be ameliorated by supplementation of 200 µg
chromium per day(7).
Thus, although no improvement in glucose tolerance
was observed among non-selected diabetics on a normal diet(27),
chromium is at present generally considered an essential element for
humans(1, 8, 18).
The US National Research Council has published a recommended daily
intake of chromium of 50 - 200 µg/day.
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Toxicity of chromium
The
toxicity of chromium depends on the valence state. Hexavalent chromium
compounds are considered to be more toxic than trivalent chromium
compounds. In stainless steel, chromium is in metallic form (valence
state 0). Specifically, occupational exposure to some hexavalent chromium
compounds by inhalation has been shown to result in an elevated incidence
of cancer in the respiratory system (nose and lungs) and several studies
have demonstrated that some hexavalent chromium compounds induce cancer
in experimental animals. Available data show no such association between
exposure to trivalent or metallic chromium and cancer induction(9).
It has also been reported that stainless steel cylinders, implanted
intramuscularly, did not induce local tumours in mice, whilst cylinders
made of a reference alloy were clearly carcinogenic(25).
Only very limited studies are available on the carcinogenicity of
chromium after oral administration; no carcinogenic response was described
after oral exposure to chromium (III) compounds, the only valence
state studied(9).
Hexavalent chromium compounds are mutagenic in a large variety of
test systems, whilst again, trivalent compounds are generally negative(9).
Some chromium compounds are also allergenic and induce notably allergic
dermatitis. Hexavalent chromium salts, and especially chromic acid,
are corrosive and may induce corrosive skin damage, the best known
of these being the perforation of the nasal septum.
•
Sources of dietary chromium
Concentration
of chromium in different dietary items is generally low: dairy products
especially are low in chromium (< 0.5 µg/serving), while
meat, poultry, grain products, fruit and vegetables generally have
between 1 - 10 µg/serving(5). Peeling
of apples decreased their chromium content to one third; this would
indicate that part of their chromium content is derived from surface
contamination. The chromium contents of separate samples of several
food items vary markedly e.g. 100-fold for different lots of beer
or breakfast cereals(3 - 5).
Concentration of chromium was higher in bread and fu (wheat gluten
product) and natto (fermented soybean product) than in wheat flour
and soybean
respectively(26).
Chromium was reported to leach into wine from bottles (coloured with
chromium-containing colours) and also during storage in stainless
steel tanks(16, 28)
- while a further study reported lower chromium levels in wine stored
in stainless steel vessels than in wine stored in redwood caskets(24).
Kumpulainen and co-workers reported that the mixing of meat in the
presence of orange juice in a standard kitchen mixer with stainless
steel blades almost doubled its chromium content in three minutes(15).
It thus appears that a large proportion, in some cases most, of the
chromium found in food does not originate from the raw materials but
arises as a consequence of processing. There are no reasons to believe
that this exogenous chromium behaves differently from endogenous inorganic
chromium.
•
Normal chromium intake in the diet
Daily
chromium intake varies between different geographic areas and is usually
between 20 to 85 µg/ day, although
values up to 130 µg/day have been reported(6,
14, 22). It would seem that at present the levels at the
low end of the scale occur mainly in Western countries.
•
Release of chromium from stainless steel kitchen utensils
Several
studies have investigated the leaching of chromium from stainless
steel utensils under various conditions. Cold 5 % acetic acid (acidity
chosen to simulate that of vinegar) did not cause chromium to leach
from 6 different saucepans. When the acid was boiled for 5 minutes
in the saucepans, the concentration of chromium observed was no different
to the analytical background (0.035 mg/l) in three of these, was 2-fold
over that in two others and 8-fold (0.3 mg/l) in the sixth. Since
the last saucepan also corroded visibly during the study, the authors
considered it likely that its chromium content was probably less than
11 %(12). All the
saucepans tested were more than one year old; the authors also tested
one new saucepan and report (no figures given) that it originally
leached more metals and only became similar to the others after two
months.
Three out of four tested stainless steel pots released small amounts
(< 0.2 mg/l) chromium in 4 % acetic acid in three consecutive half-hour
extractions at 100°C. The fourth pot released 20 times this amount
at the first extraction, but very little thereafter(21).
No leaching of chromium could be detected into tea, coffee, milk or
fruit juice from either old or new stainless steel bowls or tumblers
(chromium content 9.74 - 20.80 %), while the leaching was 0.04 to
0.4 µg/g to curd or lemon pickle from new utensils and 0.03
to 0.3 µg/g from old utensils. Leaching to 5 % sodium carbonate
and 5 % acetic acid decreased with successive experiments using new
utensils(13).
The chromium content of crayfish hepatopancreas cooked in a stainless
steel pan increased from approximately 0.05 to 0.15 mg/kg fresh weight.
When crayfish abdominal muscle was similarly cooked, the levels compared
to the raw fish were much less changed, if at all(11).
The cooking of peeled potatoes in new steel pots increased their chromium
content by 60 %, while a decrease was observed after cooking in old
steel kettles. Both in new and old kettles, a minor decrease in the
chromium content was observed in potatoes boiled in their jackets(23).
Preparation of meals increased their chromium contents such that,
of the total 82.6 µg of chromium present in three typical German
daily meals (breakfast, lunch, dinner), 45.3 µg was added from
the food preparation process(29).
Leaching
of chromium into 4 % acetic acid from 18-8 steel kitchenware (spoons,
ladles and knives) was less than 60 ng (0.5 ng/cm2), while it was
ten times higher from several steel items of unknown composition.
More chromium was dissolved from new than from used pudding cups(20).
Between
30 - 50 µg/l of chromium were released in HCl-acidified water
(pH 2.5), canned tomato juice, bottled pineapple juice and lemon juice
upon 1-hour boiling in a stainless steel pot; no chromium leaching
was observed in non-acidified water(19).
Chromium
is thus leached into food items, especially those with a marked acidity,
from stainless steel kitchen utensils.
Leaching is more pronounced at elevated temperatures, such as in boiling
water and is more extensive from steels containing lower proportions
of chromium. When the utensils have been in use, the amounts of chromium
leached
decrease. It is very unlikely that the total amount of chromium leached
from stainless steel utensils, even into acidic foods, exceeds 50
µg/day i.e. an amount considered beneficial to health.
•
Chromium speciation
Chromium
may occur in several valence states; among these, only valence states
(0), (III) and (VI) are stable. It has been postulated that nearly
all hexavalent chromium in nature is anthropogenic(10).
Little information is available on the valence state of chromium released
from stainless steel surfaces. Recently it was reported that the erythrocyte/plasma
chromium concentration ratio was elevated in patients with joint arthroplasty
with cobalt-chromium orthopaedic implants. Although no quantitative
interpretation is possible, this finding was interpreted to mean release
of at least some hexavalent chromium from the implants upon corrosion(17).
However, it would seem that as far as the chromium released from stainless
steel during cooking is concerned, this is unlikely to occur to any
significant extent in the hexavalent state. Acidic food items leach
more chromium from stainless steel than do neutral or basic food items.
Hexavalent chromium is unstable in acid solution and tends to be rapidly
reduced to the trivalent state. Furthermore, upon ingestion dietary
chromium ends up in the stomach, the contents of which are clearly
acidic, down to pH 1. This acidity would further enhance the rate
of the reduction of chromium (VI). It is thus likely that most chromium
from kitchen utensils in the gastrointestinal tract is as chromium
(III). This is not discrepant with more pronounced toxicity and faster
absorption of ingested chromium (VI) compounds in comparison to chromium
(III) compounds: in experiments with high doses, some chromium (VI)
will be absorbed before it has time to be reduced, whilst at low doses
reduction is likely to occur in the gastrointestinal tract.
•
Research needs
There
is still a lack of knowledge on the valence state of chromium in different
dietary items and especially of chromium
leaching from kitchen utensils and on the changes of valence states
during food storage.
•
Conclusions
Small
amounts of chromium will leach from stainless steel utensils into
food during its processing, storage and during
meal preparation. Although it is conceivable that some of this leached
chromium may be in hexavalent state, it is unlikely that this leaching
will result in actual absorption of hexavalent chromium by the organism.
No toxicity is to be expected from the chromium leached from kitchenware,
it may in fact be beneficial to health, since the amounts of chromium
in present Western-type diets are generally small in comparison to
amounts considered to be optimal.
•
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