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48
or a period of heating that would make his product unmarketable. By commercial
sterilisation is meant such degree of heating as is necessary to prevent the development
of bacterial spoilage of the food under the conditions in the container. It is, indeed,
the failure to obtain absolute sterilisation that makes the maintenance of a good vacuum
in the can so important.
If we adopt these criteria we shall not go far wrong; we may occasionally be
tripped up by the survival of the toxins of food poisoning organisms of the salmonella
group, but we have no means of safeguarding ourselves against that contingency.
Having decided to adopt the good canner's own criteria, let us stick to them
through thick and thin. Let us not be put off by the suggestion that "though the
can is all wrong the food is all right." We know that it is practicable to market in
this country canned foods of all kinds from all parts of the world in such condition
that they do not occasion the slightest anxiety to the most careful food inspector.
If the cans give rise to suspicion as to the soundness of the contents there is something
wrong somewhere; either in the nature, quality or condition of the product canned
or in the quality of the can or in the process of canning or in the subsequent transport
or storage and the obligation is on the manufacturers and merchants to ascertain
what is wrong and to correct it, stimulated by the certain knowledge that they will
not be allowed to sell their unsatisfactory cans for human consumption, and that
their product will remain suspect and therefore subject to special examination, until
they have established a reputation for reliability. By adopting this standard we
are not in danger of prejudicing the nutrition of any section of the community;
by departing from it we shall put the food inspector in an impossible position, we shall
put the careful expert canner at a disadvantage in competition with the careless,
inexpert one and, sooner or later, we shall be badly let down.
I have here a number of cans illustrating the various conditions which the food
inspector may encounter but before I show them to you I want to refer to a few special
points.
First of all the production and maintenance of a good vacuum. The canner has
long recognised the importance of reducing to a minimum the amount of air enclosed
in the can, whether in the liquid or solid contents or in the head space. This used to
be achieved by venting the cans immediately they were removed from the retorts
in which they were sterilised or "processed" and sealing the vent with solder when the
air and steam had blown off. Now it is done before the processing by passing the
filled but open cans through an exhaust box where the contents are heated by steam.
The cans are immediately capped and sealed with the result that the contraction of
the food and of the remaining air and the condensation of steam, on cooling, creates
a partial vacuum. The degree of vacuum depends, of course, on the temperature
at which the cans are sealed; thus sealing at 200°F. will give 16.5 inches of vacuum,
while sealing at 70°F. will give only 1 inch. A well exhausted can shows 8 inches
or more of vacuum when the can is cold. With some products, particularly meat,
an exhaust box is not used but the cans which are already sealed except for a single
vent-hole, are passed into a vacuum chamber in which 20 to 25 inches of vacuum are
obtained by an exhaust pump. In this chamber the vent is closed with solder and
the cans are then ready for processing.
The food inspector, therefore, has the right to expect a negative pressure in every
can and if there is no vacuum he can conclude that either air has leaked in or there
has been gas production inside. There is never any difficulty with the obviously
blown can with the ends definitely convex and as tight as a drum; but loss of vacuum
is progressive and the tightly blown can has gone through the stages from a negative
pressure of 8 inches or more, through the neutral point to slight positive and on to
strongly positive pressure. If there is a negative pressure of any degree the food
inspector will pass the can unless it is unsatisfactory in some other respect, but it is
when the internal pressure has reached the neutral point or is slightly positive that
he may be uncertain as to the action he should take. For cans in this condition the
merchant has a number of terms such as "springer," "flipper," "puffy can" and
"slack cap" and he considers that he is entitled to market them, albeit at a reduced
price. He also talks of "overfilling," a term which is definitely misleading. You