orable 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 deg. F., or more. However, most kinds of bacteria are destroyed if
exposed for ten or fifteen minutes to the temperature