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