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living bodies can develop in it. Such food is sterile food and it will
keep almost indefinitely.
The problem of preserving food therefore resolves itself simply into
putting a stop to the activities of microbes, by killing them or by
preventing them from carrying on their usual decomposition-causing
functions.
The application of heat is one non-objectionable method of preserving
food, and it is used widely in the pasteurisation of milk, which
is raised to a temperature of 60-70° C. for half an hour, the result
being an enormous reduction in the number of bacteria it contains and
consequently an increase in the length of time during which the milk
will remain sweet.
Drying is another useful method of preserving food for the reason
that bacteria cannot develop without water. All micro-organisms,
bacteria, yeasts, and moulds, nourish themselves by taking in through
their outer coverings water in which is dissolved whatever they require
as food. Solid matter cannot pass through the cell wall of a microorganism,
thus germs cannot exist in anything that is dry.
Preservation by cold is another method which has a great deal to
recommend it, and it is a method that is becoming more widely adopted
since the coming into force of new regulations governing the use of
preservatives in food, which will be dealt with in subsequent paragraphs.
Cold paralyses bacterial activity, but does not kill micro-organisms.
At 10° C. few bacteria grow; and below 0°C. no bacteria grow, for the
simple reason that water freezes at this temperature and the bacteria
are unable to get the liquid nourishment they require.
Cold storage and refrigeration do not alter the chemical composition
of food, and it is safe to say that the introduction of these two methods
is revolutionising the distribution and storing of food, and has brought
within the reach of the housewife in this country, foreign foods which
would otherwise be nothing more or less than expensive luxuries.
Indeed, cold storage is the only solution to the problem of feeding this
country which produces only enough food to feed its inhabitants for
about one and a half days a week.
Having mentioned briefly the three more or less natural and nonobjectionable
methods of keeping food, we pass to the less desirable
preservation by chemicals. Such chemicals as boric acid, nitrous acid,