[Footnote: Read at an evening meeting of the Pharmaceutical Society,
March 5, 1884.]
By PROFESSOR ATTFIELD, F.R.S., ETC.
From time to time, during the past twelve years, paragraphs have
appeared in newspapers and other periodicals tending in effect to warn
the public at least against the indiscriminate use of canned foods. And
whenever there has been any foundation in fact for such cautions, it has
commonly rested on the alleged presence and harmfulness of tin in the
food. At the worst, the amount of tin present has been absurdly small,
affording an opportunity for one literary representative of medicine to
state that before a man could be seriously affected by the tin, even if
it occurred in the form of a compound of the metal, he would have to
consume at a meal ten pounds of the food containing the largest amount
of tin ever detected.
But the greatest proportions of tin thus referred to are, according to
my experiments, far beyond those ever likely to be actually present in
the food itself in the form of a compound of tin; present, that is to
say, on account of the action of the fluids or juices of the food on the
tin of the can. Such action and such consequent solution of the tin, and
consequent admixture of a possibly assimilable compound of tin with the
food, in my opinion never occurs to an extent which in relation to
health has any significance whatever. The occurrence of tin, not as a
compound, but as the metal itself, is, if possible, still less
important.
During the last fifteen years I have frequently examined canned foods,
not only with respect to the food itself as food, and to the process of
canning, but with regard to the relation of the food to, or the
influence if any of the metal of, the can itself. So lately as within
the past two or three months I have examined sixteen varieties of canned
food for metals, with the following results:
Decimal parts of
a grain of tin
(or other foreign
metal) present in
Name of article a quarter of a lb.
examined.
Salmon none.
Lobsters none.
Oysters 0.004
Sardines none.
Lobster paste none.
Salmon paste none.
Bloater paste 0.002
Potted beef none.
Potted tongue none.
Potted "Strasbourg" none.
Potted ham 0.002
Luncheon tongue 0.003
Apricots 0.007
Pears 0.003
Tomatoes 0.007
Peaches 0.004
These proportions of metal are, I say, undeserving of serious notice. I
question whether they represent more than the amounts of tin we
periodically wear off tin saucepans in preparing food--a month ago I
found a trace of tin in water which had been boiled in a tin kettle--or
the silver we wear off our forks and spoons. There can be little doubt
that we annually pass through our systems a sensible amount of such
metals, metallic compounds, and other substances that do not come under
the denomination of food; but there is no evidence that they ever did or
are ever likely to do harm or occasion us the slightest inconvenience.
Harm is far more likely to come to us from noxious gases in the air we
breathe than from foreign substances in the food we eat.
But whence come the much less minute amounts of tin--still harmless, be
it remembered--which have been stated to be occasionally present in
canned foods? They come from the minute particles of metal chipped off
from the tin sheets in the operations of cutting, bending, or hammering
the parts of the can, or possibly melted off in the operations necessary
for the soldering together of the joints of the can. Some may, perhaps,
be cut, off by the knife in opening a can. At all events I not
unfrequently find such minute particles of metal on carefully washing
the external surfaces of a mass of meat just removed from a can, or on
otherwise properly treating canned food with the object of detecting
such particles. The published processes for the detection of tin in
canned food will not reveal more than the amounts stated in the table,
or about those amounts; that is to say, a few thousandths or perhaps two
or three hundredths of a grain, if this precaution be adopted. If such
care be not observed, the less minute amounts may be found. I did not
detect any metallic particles in the twelve samples of canned food just
mentioned, but during the past few years I have occasionally found small
pieces of metal, perhaps amounting in some of the cases to a few tenths
of a grain per pound. Now and then small shot-like pieces of tin, or
possibly solder, may be met with; but no one has ever found, to my
knowledge, such a quantity of actual metallic tin, tinned iron, or
solder as, from the point of view of health, can have any significance
whatever.
The largest amount of tin I ever detected in actual solution in food was
in some canned soup, containing a good deal of lemon juice. It amounted
to only three-hundredths of a grain in a half pint of the soup as sent
to table. Now, Christison says that quantities of 18 to 44 grains of the
very soluble chloride of tin were required to kill dogs in from one to
four days. Orfila says that several persons on one occasion dressed
their dinner with chloride of tin, mistaking it for salt. One person
would thus take not less than 20 to 30 grains of this soluble compound
of tin. Yet only a little gastric and bowel disturbance followed, and
from this all recovered in a few days. Pereira says that the dose of
chloride of tin as an antispasmodic and stimulant is from 1/16 to ½ a
grain repeated two or three times daily. Probably no article of canned
food, not even the most acid fruit, if in a condition in which it can be
eaten, has ever contained, in an ordinary table portion, as much of a
soluble salt of tin as would amount to a harmless or useful medicinal
dose.
Metallic particles of tin are without any effect on man. A thousand
times the quantity ever found in a can of tinned food would do no harm.
Food as acid as say ordinary pickles would dissolve tin. Some
manufacturers once proposed using tin stoppers to their bottles of
pickles. But the tin was slowly dissolved by the acid of the vinegar.
These pickles, however, had a distinctly nasty "metallic" flavor. The
idea was abandoned. Probably any article of food containing enough tin
to disagree with the system would be too nasty to eat. Purchasers of
food may rest assured that the action taken by this firm would be that
usually followed. It is not to the interest of manufacturers or other
venders to offend the senses of purchasers, still less to do them actual
harm, even if no higher motive comes into force.
In the early days of canning, it is just possible that the use of
"spirits of salt" in soldering may have resulted in the presence of a
little stannous, plumbous, or other chloride in canned food; but such a
fault would soon be detected and corrected, and as a matter of fact,
resin-soldering is to my knowledge more generally employed--indeed, for
anything I know to the contrary, is exclusively employed--in canning
food. Any resin that trained access would be perfectly harmless. It is
just possible, also, that formerly the tin itself may have contained
lead, but I have not found any lead in the sheet tin used for canning of
late years.
In conclusion: 1. I have never been able to satisfy myself that a can of
ordinary tinned food contains even a useful medicinal dose of such a
true soluble _compound_ of tin as is likely to have any effect on man.
2. As for the metal itself, that is the filings or actual metallic
particles or fragments, one ounce is a common dose as a vermifuge;
harmless even in that quantity to man, and not always so harmful as
could be desired to the parasites for whose disestablishment it is
administered. One ounce might be contained in about four hundredweight
of canned food. 3. If a possibly harmful quantity of a soluble compound,
of tin be placed in a portion of canned food, the latter will be so
nasty and so unlike any ordinary nasty flavor, so "metallic," in fact,
that no sane person will eat it. 4. Respecting the globules of solder
(lead and tin) that are occasionally met with in canned food, I believe
most persons detect them in the mouth and remove them, as they would
shots in game. But if swallowed, they do no harm. Pereira says that
metallic lead is probably inert, and that nearly a quarter of a pound
has been administered to a dog without any obvious effects. He goes on
to say that as it becomes oxidized it occasionally acquires activity,
quoting Paulini's statement that colic was produced in a patient who had
swallowed a leaden bullet. To allay alarm in the minds of those who fear
they might swallow pellets of solder, I may add that Pereira cites
Proust for the assurance that an alloy of tin and lead is less easily
oxidized than pure lead. 5. Unsoundness in meat does not appear to
promote the corrosion or solution of tin. I have kept salmon in cans
till it was putrid, testing it occasionally for tin. No trace of tin was
detected. Nevertheless, food should not be allowed to remain for a few
days, or even hours, in saucepans, metal baking pans, or opened tins or
cans, otherwise it _may_ taste metallic. 6. Unsound food, canned or
uncanned, may, of course, injure health, and where canned food really
has done harm, the harm has in all probability been due to the food and
not to the can. 7. What has been termed idiosyncrasy must also be borne
in mind. I know a man to whom oatmeal is a poison. Some people cannot
eat lobsters, either fresh or tinned. Serious results have followed the
eating of not only oatmeal or shell fish, but salmon and mutton;
_hydrate_ (misreported _nitrate_) of tin being gratuitously suggested as
being contained in the salmon in one case. Possibly there were cases of
idiosyncrasy in the eater, possibly the food was unsound, possibly other
causes altogether led to the results, but certainly, to my mind, the tin
had nothing whatever to do with the matter.
In my opinion, given after well weighing all evidence hitherto
forthcoming, the public have not the faintest cause for alarm respecting
the occurrence of tin, lead, or any other metal in canned foods.--_Phar.
Jour, and Trans., March 8, 1884, p. 719_.
[In reference to Prof. Attfield's statement contained in the closing
paragraph, we remark: It is well known that mercury is an ingredient of
the solder used in some canning concerns, as it makes an easier melting
and flowing solder. In THE SCIENTIFIC AMERICAN for May 27, 1876, in a
report of the proceedings of the New York Academy of Science, will be
seen the statement of Prof. Falke, who found metallic mercury in a can
of preserved corn beef, together with a considerable quantity of
albuminate of mercury.--EDS. S.A.]
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