Over one hundred years ago François Appert was the first to make
practical application of the method of preserving food by putting it in
cans or bottles, which he hermetically sealed. He then put the full
bottles or cans in water and boiled them for more or less time,
depending upon the kinds of food.
In Appert's time and, indeed, until recent years it was generally
thought that the oxygen of the air caused the decomposition of food.
Appert's theory was that the things essential to the preservation of
food in this manner were the exclusion of air and the application of
gentle heat, as in the water bath, which caused a fusion of the
principal constituents and ferments in such a manner that the power of
the ferments was destroyed.
The investigations of scientists, particularly of Pasteur, have shown
that it is not the oxygen of the air which causes fermentation and
putrefaction, but bacteria and other microscopic organisms.
Appert's theory as to the cause of the spoiling of food was incorrect,
but his method of preserving it by sealing and cooking was correct, and
the world owes him a debt of gratitude.
In their investigations scientists have found that if food is perfectly
sterilized and the opening of the jar or bottle plugged with sterilized
cotton, food will not ferment, for the bacteria and yeasts to which such
changes are due can not pass through the cotton. This method can not be
conveniently followed with large jars.
Bacteria and yeasts exist in the air, in the soil, and on all vegetable
and animal substances, and even in the living body, but although of such
universal occurrence, the true knowledge of their nature and economic
importance has only been gained during the last forty years.
There are a great many kinds of these micro-organisms. Some do great
harm, but it is thought that the greater part of them are beneficial
rather than injurious.
Bacteria are one-celled and so small they can only be seen by aid of a
microscope. The process of reproduction is simple and rapid. The
bacterium becomes constricted, divides, and finally there are two cells
instead of one. Under favorable conditions each cell divides, and so
rapid is the work that it has been estimated that one bacterium may give
rise, within twenty-four hours, to seventeen millions of similar
organisms. The favorable conditions for growth are moisture, warmth, and
proper food.
Yeasts, which are also one-celled organisms, grow less rapidly. A bud
develops, breaks off, and forms a new yeast plant. Some yeasts and some
kinds of bacteria produce spores. Spores, like the dried seeds of
plants, may retain their vitality for a long time, even when exposed to
conditions which kill the parent organism.
Yeasts and nearly all bacteria require oxygen, but there are species of
the latter that seem to grow equally well without it, so that the
exclusion of air, which, of course, contains oxygen, is not always a
protection, if one of the anaerobic bacteria, as the kinds are called
which do not require oxygen, is sealed in the can.
Spoiling of food is caused by the development of bacteria or yeasts.
Certain chemical changes are produced as shown by gases, odors, and
flavors.
Bacteria grow luxuriantly in foods containing a good deal of nitrogenous
material, if warmth and moisture are present. Among foods rich in
nitrogenous substances are all kinds of meat, fish, eggs, peas, beans,
lentils, milk, etc. These foods are difficult to preserve on account of
the omnipresent bacteria. This is seen in warm, muggy weather, when
fresh meat, fish, soups, milk, etc., spoil quickly. Bacteria do not
develop in substances containing a large percentage of sugar, but they
grow rapidly in a suitable wet substance which contains a small
percentage of sugar. Yeasts grow very readily in dilute solutions
containing sugars in addition to some nitrogenous and mineral matters.
Fruits are usually slightly acid and in general do not support bacterial
growth, and so it comes about that canned fruits are more commonly
fermented by yeasts than by bacteria.
Some vegetable foods have so much acid and so little nitrogenous
substance that very few bacteria or yeasts attack them. Lemons,
cranberries, and rhubarb belong to this class.
Temperature is an important factor in the growth of bacteria and yeasts.
There are many kinds of these organisms, and each kind grows best at a
certain temperature, some at a very low one and others at one as high as
125° F., or more. However, most kinds of bacteria are destroyed if
exposed for ten or fifteen minutes to the temperature of boiling water
(212° F.); but, if the bacteria are spore producers, cooking must be
continued for an hour or more to insure their complete destruction.
Generally speaking, in order to kill the spores the temperature must be
higher than that of boiling water, or the article to be preserved must
be cooked for about two hours at a temperature of 212° F., or a shorter
time at a higher temperature under pressure. Yeasts and their spores
are, however, more easily destroyed by heat than bacteria spores. Hence,
fruits containing little nitrogenous material are more easily protected
from fermentation than nitrogenous foods in which in general
fermentation is caused by bacteria. Of course, it is not possible to
know what kinds of organisms are in the food one is about to can or
bottle; but we do know that most fruits are not favorable to the growth
of bacteria, and, as a rule, the yeasts which grow in fruits and fruit
juice can be destroyed by cooking ten or fifteen minutes at a
temperature of 212° F. If no living organisms are left, and the
sterilization of all appliances has been thorough, there is no reason
why the fruit, if properly sealed, should not keep, with but slight
change of texture or flavor, for a year or longer, although canned
fruits undergo gradual change and deterioration even under the most
favorable conditions.
When fruit is preserved with a large amount of sugar (a pound of sugar
to a pound of fruit) it does not need to be hermetically sealed to
protect it from bacteria and yeasts, because the thick, sugary sirup
formed is not favorable to their growth. However, the self-sealing jars
are much better than keeping such fruit in large receptacles, from which
it is taken as needed, because molds grow freely on moist, sugary
substances exposed to the air.