8. CANNING consists in sealing foods in receptacles, such as cans or
jars, in such a way that they will remain sterile for an indefinite
period of time. Several methods of canning are in use, and the one to
adopt will depend considerably on personal preference and the money that
can be expended for the equipment. In any case, successful results in
canning depend on the care that is given to every detail that enters
into the work. This means, then, that from the selection of the food to
be canned to the final operation in canning not one thing that has to do
with good results should be overlooked.
9. SELECTION OF FOOD FOR CANNING.--A careful selection of the food that
is to be canned is of great importance. If it is in good condition at
the time of canning, it is much more likely to remain good when canned
than food that is not. The flavor of the finished product also depends a
great deal on the condition of the food. Fruits have the best flavor
when they are ripe, but they are in the best condition for canning just
before they have completely ripened. Immediately following perfect
ripeness comes the spoiling stage, and if fruits, as well as vegetables,
are canned before they are completely ripe, they are, of course, farther
from the conditions that tend to spoil them. This, however, does not
mean that green fruits or vegetables should be canned.
Whenever possible, any food that is to be canned should be perfectly
fresh. The sooner it is canned after it has been gathered, the more
satisfactory will be the results. For instance, it is better to can it
12 hours after gathering than 24 hours, but to can it 2 hours after is
much better. Fruits, such as berries, that are especially perishable
should not be allowed to stand overnight if this can be prevented; and
it is absolutely necessary to can some vegetables, such as peas, beans,
and corn, within a very few hours after gathering. Unless this is done,
they will develop a bad flavor because of _flat sour_, a condition that
results from the action of certain bacteria. Imperfect fruits should
not be canned, but should be used for making jam, marmalade, or jelly.
10. WHY CANNED FOODS SPOIL.--Canned foods spoil because of the action of
micro-organisms that cause fermentation, putrefaction, and molding. The
reasons for the spoiling of food are thoroughly discussed in _Essentials
of Cookery_, Part 2, and in that discussion canning is mentioned as one
of the means of preserving food or preventing it from spoiling. However,
when canning does not prove effective, it is because undesirable
bacteria are present in the food. Either they have not been destroyed by
the canning process or they have been allowed to enter before the jar
was closed, and have then developed to such an extent as to cause the
food to spoil. Odors, flavors, and gases result from the putrefaction,
fermentation, or molding caused by these bacteria, and these make the
foods offensive or harmful, or perhaps both.
11. PREVENTING CANNED FOODS FROM SPOILING.--From what has just been
said, it will be seen that the success of canning depends entirely on
destroying harmful micro-organisms that are present in the food and
preventing those present in the air from entering the jars in which the
food is placed.
Some foods are more difficult to keep than others, because bacteria act
on them more readily and the foods themselves contain nothing that
prevents their growth. Among such foods are meat, fish, peas, corn,
beans, and meat soups. On the other hand, some foods contain acids that
prevent the growth of bacteria, and these keep easily. Among these are
rhubarb, cranberries, and green gooseberries. However, foods that keep
easily are few, and in most cases extreme care in the process of canning
must be exercised.
12. While warmth is necessary for bacterial growth, very high
temperatures will destroy or retard it. In canning, a temperature as
high as 212 degrees Fahrenheit, or boiling point, retards the growth of
active bacteria, but retarding their growth is not sufficient. They must
be rendered inactive. To do this requires either a higher temperature
than boiling point or long continued cooking at 212 degrees. _Spores_
are a protective form that many kinds of bacteria assume under
unfavorable conditions. They are very difficult to kill, and unless they
are completely destroyed in the canning process, they will develop into
active bacteria when conditions again become favorable. The result of
the spore development is the spoiling of the food.
13. Other things besides the application of heat assist in the keeping
of canned food, as, for example, the acids of the fruits and vegetables
themselves, as has been mentioned. The use of sugar also assists; the
greater the quantity of sugar in solution the easier it will be to keep
the food. This is proved in the case of jams and jellies, which will
keep without being sealed tight or put into jars immediately after
cooking. Salt helps to keep vegetables that are canned, and, in making
butters, conserves, and pickles, the spices and vinegars used help to
protect the foods from bacterial action. However, none of these things
are essential to the keeping of any _sterile food_, by which is meant
food in which all bacteria or sources of bacteria have been rendered
inactive by the application of sufficient heat.
14. CANNING PRESERVATIVES.--Numerous compounds, usually in the form of
powders, are advertised as being useful for keeping canned foods from
spoiling. None of them should be used, however, because they are
unnecessary. If the work of canning is carefully and effectively done,
good foods will keep perfectly without the addition of a preservative.
The pure-food laws of the United States and of many of the states
themselves forbid the use of some preservatives because of their harmful
effect on the human system. For this reason, to say nothing of the extra
expense that would be incurred in their use, such preservatives may well
be left alone.