HOW PLANTS SOUR OR SPOIL FOOD
=Germs, Microbes, or Bacteria.=--The dust and dirt of all sorts
contain thousands of tiny plants too small to be seen by the eye
without help. An instrument called a _microscope_ makes them appear so
large that their form and growth are easily studied. These little
plants are called _germs_ or _microbes_. They are also named
_bacteria_. They are so small that a million laid side by side would
not cover the head of a pin.
[Illustration: FIG. 16.--Bacteria or microbes found in water, dust,
and waste. They help change straw and other dead matter into food for
plants. Much enlarged.]
There are hundreds of different kinds of germs. Some are round like
little balls and others are the shape of tiny rods. Many of them which
look just alike act very different in growing. There are more than
twenty different kinds that grow in our bodies and cause diphtheria,
tuberculosis, and other diseases. We have measles and scarlet fever
because we have gotten these disease germs from some one else in whom
they were growing.
[Illustration: FIG. 17.--Mold which grew on moist bread in two days.
5, seed bodies breaking out of the sack; 1, 2, and 4, one of the seed
bodies after one, two, and four hours' growth. Much enlarged.]
Most germs feed on dead matter instead of our living bodies and make
it melt away or change into another form. An apple or a piece of meat
thrown out on the ground will soon change and become like the earth on
which it lies. The change, called decay, is caused by millions of
germs. The farmer's best friends are certain germs which help make the
ground rich, so that the crops will grow.
=Mold.=--The dust raised in sweeping contains tiny living seedlike
bodies. If these fall on bread, cheese, or fruit, and this food is
afterward kept moist in a warm room for a day or two, they will grow
into grayish fluffy spots. These spots are mold. The greenish white
growth on the top of some canned fruit and on berries left in the
warm kitchen over night is also mold.
Mold is a plant which grows from tiny round bodies acting like seeds
(Fig. 17). These seed bodies of mold are common in all dust and often
fly through the air. On this account food should be kept covered when
possible and especially when one is sweeping. Some mold gives bread,
cheese, and other food a bad taste, but it will not make one sick.
=How Germs Grow.=--Germs will not grow where it is very cold, but
freezing the germs does not kill them. Boiling one minute kills most
germs. Drying will stop the germs from growing, but will not kill all
of them. Sunlight kills many of them.
Moisture and warmth make germs grow rapidly. A germ in growing
lengthens out a little and then divides in the middle. It does this so
quickly that one germ may become two in fifteen minutes. Each of these
will then divide. In this way one germ can make many million germs in
a single day (Fig. 18).
=The Spoiling of Meat.=--Fresh meat will not remain good even one day
if left in a warm place. A large greenish blue fly seen buzzing about
in warm weather will sometimes lay its eggs on meat. These will hatch
the next day into little worms, called maggots. They grow rapidly and
a few days later change into flies.
[Illustration: FIG. 18.--Drawing of the germ at the top every ten
minutes, showing how it grew into two germs in a half hour. Much
enlarged.]
Germs will also spoil meat not kept cold. They feed on the meat and
give off a poison, making it unfit to eat. The bad odor tells when the
germs are at work. Every home should have a cold cellar or an ice box
to keep food from spoiling.
=Saving Food from Souring.=--The souring of milk and of cooked food of
any kind is due to the germs always present in the air and clinging by
the thousands to unwashed dishes and hands. If meat or fruit is cooked
and kept tightly covered, it will remain good for years. Many persons
save fruit and vegetables for use in winter by putting them in jars,
which are heated to kill the germs, and sealed tight to keep out other
germs.
=Yeast or the Alcohol Plant.=--Sweet cider and other fruit juices are
sometimes spoiled by a plant named yeast. This plant has the form of a
football and is so small that a million of its kind together would not
make a mass as large as the head of a pin. It floats about in the air
and is present on the skins of fruits.
Yeast is also called the alcohol plant because whenever it grows in a
sweet substance like fruit juice it changes part of it into a biting
substance called alcohol. At the same time it gives off a gas. It is
this gas which forms the bubbling or frothing in beer.
[Illustration: FIG. 19.--Yeast plants used in making bread and beer.
Those on the right are growing new plants. Much enlarged.]
The millions of yeast plants in the yeast cake bought at the store,
when put into the dough for bread, grow and form gas. This pushes the
bits of dough apart and makes it light. The little alcohol formed is
all driven off in the baking.
The alcohol which yeast forms by growing in sweet cider is in a few
weeks changed to vinegar by other germs called the vinegar plants.
Sour cider may make those who use it sick and drunk because it
contains alcohol. Yeast makes wine out of grape juice.
PRACTICAL QUESTIONS
1. Where are germs found?
2. What is the form of microbes?
3. Name some diseases caused by germs.
4. What is mold?
5. Why should food be kept covered when not in use?
6. What causes meat to spoil?
7. How may fruit be kept from spoiling?
8. Where is yeast found?
9. What effect has yeast on fruit juice?
10. Why should you not drink sour cider?