yeasts_, and _molds_, a knowledge of which is of the utmost importance
to the physician and the farmer, as well as the housewife. Just in what
ways these are beneficial to the farmer and the physician is beyond the
scope of the subject of cookery, but in the household their influence is
felt in three ways: They are the cause of the decay and spoiling of
foods; they are of value in the preparation of certain foods; and they
are the cause of contagious diseases. It will thus be seen that while
some microbes are undesirable, others exert a beneficial action.

40. It is only within comparatively recent years that the action of
micro-organisms has been understood. It is now definitely known that
these minute living things seize every possible chance to attack
articles of food and produce the changes known as fermentation,
putrefaction, souring, and decay. Micro-organisms that cause
fermentation are necessary in bread making and vinegar making, but they
are destructive to other foods, as, for example, those which are canned
or preserved. Organisms that cause putrefaction are needed in the making
of sauer kraut, salt rising bread, and cheese. Molds also help to make
cheese, but neither these nor putrefactive organisms are desirable for
foods other than those mentioned. It should be remembered, however, that
even those foods which require micro-organisms in their making are
constantly in danger of the attacks of these small living things, for
unless something is done to retard their growth they will cause food to
sour or decay and thus become unfit for consumption.

Some foods, of course, withstand the attacks of micro-organisms for
longer periods of time than others. For example, most fruits that are
protected by an unbroken skin will, under the right conditions, keep for
long periods of time, but berries, on account of having less protective
covering, spoil much more quickly. Likewise, vegetables without skins