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49
cannot really overfill a can, for you cannot put a quart into a pint pot. What the
merchant really means is that sufficient head space has not been left with the result
that the generation of a very small amount of gas inside the tin causes an obvious
loss of vacuum.
The food inspector should, therefore, regard all these "springers," "flippers,"
"puffy cans," "slack caps" and "overfilled cans" as early stages of blowing and
should deal with them on the principle that a "blown can is a bad can." When the
pressure in the can is slightly positive it is useful to pierce the can or slightly open
it with the key, under water. The demonstration of bubbles rising to the surface of
the Avater usually settles any argument as to whether the can is blown or not.
A loss of vacuum without development of positive pressure may, of course, be
due to a leak which may be very small and even be temporarily sealed by the contents.
It seems to be obvious, therefore, that we should regard loss of vacuum as justifying
rejection.
Now we must consider our great bug-bear, the "hydrogen swell." As you
know, the majority of foodstuffs tend to attack the iron and tin of which the can is
made, but this is particularly the case with certain fruits of low acidity. Apparently
it is very difficult to get an absolutely perfect coating of tin on the sheet iron and there
are, almost inevitably, some minute points where the iron is not covered. Moreover
the tin-coating is liable to damage in the making of a can and in the great and sudden
changes in the internal pressure during the processing and subsequent cooling of
the filled and sealed can. Where there is the slightest defect in the continuity of the
tin-coating a tin-iron couple is formed in the presence of an electrolyte, resulting in
corrosion of the metal and the production of hydrogen. The activity with which this
electrolytic action proceeds depends on the acidity of the contents and the presence
of depolarising agents. There is a certain range of acidity between pH 3.5 and pH 4.5
which is most favourable to the process and in such cases the addition of a small
quantity of citric acid to the syrup in the cans of fruit has been found to delay the
development of hydrogen swells.
Oxygen is a depolarising agent and this provides another reason for obtaining
a good vacuum and therefore a minimum of oxygen in the can. Certain fruit pigments
(anthocyanins) act as depolarisers and therefore such fruits as cherries, raspberries,
loganberries, black currants, plums and greengages are particularly liable to give
trouble. Lacquering of the inside of the cans curiously enough increases liability to
the formation of hydrogen swells. It is impossible to get an absolutely unbroken
coating of lacquer and apparently the action is concentrated at the exposed spots
and because of the rapid corrosion of the iron there is grave danger of perforation
of the can. Some coloured fruits become discoloured in unlacquered cans and therefore
efforts are being made to obtain a perfect coating of lacquer so absolutely protecting
the metal of the can from the contents. Promising results have been obtained by
spraying with lacquer the inside of the cans after they have been made.
The temperature of storage also influences the development of hydrogen swells,
electrolytic action being retarded if the cans are kept cool. Time, too, is a factor
and we probably should not see much of hydrogen swells if all canned fruits were
consumed within a year of packing.
The whole subject is very fully dealt with by Morris and Bryan, of the Cambridge
Low Temperature Research Station in Special Report No. 40 of the Department of
Scientific and Industrial Research ("The Corrosion of the Tin Plate Container by
Food Products") to which I would direct the attention of the Medical Officer of Health
and Pood Inspectors. However, what we are concerned with at the moment is not
the problems of the canner but the action we should take for the protection of the
consumer. It must be admitted that the contents of a can which is a genuine hydrogen
swell are not very likely to be injurious to health for only in exceptional cases will
there be a really serious degree of contamination of the food with tin; but on the other
hand we cannot be sure that a blown can is a hydrogen swell without opening it and
examining the contents and the inside of the can, and even if we find that one blown