43. Among the common methods of caring for foods that are to be used at
a future time are canning and drying. CANNING, which is discussed fully
in another Section, consists in preserving sterile foods in sealed cans
or jars. The aim in canning is to prevent the growth of micro-organisms,
and to do this the process known as _sterilizing_--that is, the
destroying of bacteria and other micro-organisms by means of heat--is
resorted to. Canning theories are different now from what they were in
former times. For example, housewives formerly made heavy, rich
preserves of available fruits because it was thought that sugar must be
used in large quantities in order to keep or prevent them from spoiling.
While it is true that the sugar assisted, science has since proved that
sterilizing is what must be done, so that now only the sugar desired for
sweetening need be used.
44. The other method of keeping food, namely, DRYING, depends for its
success on the fact that such micro-organisms as bacteria cannot grow
unless they have a considerable quantity of moisture or water. Molds
grow on cheese, bread, damp cloth or paper, or articles that contain
only a small amount of moisture, but bacteria need from 20 to 30 per
cent. of water in food in order to grow and multiply. This explains why
in high altitudes and dry climates foods keep for a long time without
artificial means of preservation. It also explains why the old-fashioned
housekeeper dried fruits and why the preservation of certain meats is
accomplished by the combined methods of smoking and drying, the creosote
of the smoke given off from the wood used in this process acting as a
preservative. All the grains, which are very dry, keep for long periods
of time, even centuries, if they are protected from the moisture of the
air. Peas, beans, and lentils, as well as dried biscuits and crackers,
are all examples of how well food will keep when little or no moisture
is present.