FERMENTATION
=58. Souring of fruit.=--When a little fruit is set away in a warm
place for a day or two it sours or ferments. Anything sweet will do
the same thing. Little bubbles rise up through the juice and a foam
comes on top. Then the juice has a sharp taste or is sour. Canned and
preserved fruit becomes sour soon after the jar is opened, and cider
soon turns to vinegar. All fruit juice does this even in cold weather.
But in cold weather it keeps for a longer time.
[Illustration: =Fermentation in a jar of cherries.=]
=59. Preserving fruit.=--If your mother wishes to keep fruit all winter
she boils it and at once puts it into tight jars. This shuts out the air
and then the fruit keeps good all winter. Boiling kills all living
things, and no more can get in through the tight jars. Does a living
thing have anything to do with making the fruit juice turn sour?
=60. Yeast.=--Yeast will make all sweet things ferment. Bakers make
yeast grow in bread sponge. Yeast is alive. It is made of millions of
tiny round cells. New cells sprout out from the side of the old cells
like young lilies on an old lily bulb. Soon each new cell breaks off
and lives all by itself. In a single night enough new cells will form
to fill the whole loaf of bread.
[Illustration: =Yeast plant cells (x500).=]
=61. How yeast makes alcohol.=--Yeast will grow only where sugar is.
When it has grown for some time there is no more sugar, and instead of
a sweet taste there is a sharp or sour taste. The yeast has changed
the sugar to alcohol. All alcohol is made from sugar by yeast.
The seeds of the yeast plant are everywhere in the air. Some are on
the skins of fruit and so are found in the juice when it is squeezed
out. There they begin to grow at once and soon change the sugar to
alcohol. They do this by taking a gas away from the sugar. The gas
rises in little bubbles, and makes a froth upon the top of the juice.
Boiling kills the yeast plant. If the juice is at once put into tight
jars no new yeast plants can get in, and so the juice keeps.
=62. Vinegar.=--Sometimes fruit juice turns sour. The sourness is due
to vinegar. Besides yeast, other little living plants fall into the
juice and turn the sugar to vinegar. But if there is much alcohol in
the juice, the vinegar plants will not grow.
=63. Yeast in bread.=--Growing yeast plants always make alcohol. They
change some of the sugar of bread dough to alcohol and a gas. The gas
bubbles through the bread and makes it light. When bread is baked, the
heat of the oven drives off the alcohol, and so we do not eat any in
bread.
=64. Alcohol.=--Alcohol is a clear liquid and looks like water. It has
a sharp taste and smell. It burns very easily and makes a very hot
flame. Its smoke cannot be seen, and its flame will not make anything
black, as a match flame will do.
=65. Use of alcohol.=--Alcohol will dissolve more things than water
will dissolve. It is used to dissolve drugs, varnishes, perfumery, and
many other things. It will dissolve even oil and fat. Tailors clean
grease spots from clothes with it. It takes water away from flesh and
makes it dry, hard, and tough. It will keep anything from rotting. In
museums we pour alcohol over pieces of flesh or plants in glass jars.
Then they will keep and we can look at them at any time. Thus alcohol
is a very useful thing, and we could hardly do without it.
=66. Strong drink.=--Some men use alcohol in a wrong way. They swallow
it as a drink. But men cannot drink pure alcohol, for it would burn
their mouths. They always drink it mixed with some water. Alcohol in
water is called _strong drink_.
=67. Why men use strong drink.=--Some men take strong drink to make
themselves warm, and some to make themselves cool. Some drink to keep
themselves awake, and some to make themselves sleep. Some drink to
keep themselves still, and some to make themselves stir around faster.
Men use strong drink really because it seems to make them feel strong
for a while. It does not make them stronger, but it harms the body and
the mind. Its alcohol does the harm.
WHAT WE HAVE LEARNED
1. Sugar in fruit or in water turns to alcohol or vinegar, and a
gas.
2. The change to alcohol is caused by the cells of the yeast plant.
3. The change to vinegar is caused by another small plant.
4. Boiling fruit juice kills the yeast plants and then the juice
will keep without change.
5. Alcohol looks like water. It has a sharp and burning taste.
6. Alcohol takes water from flesh and hardens it.
7. Alcohol burns with a great heat and no smoke.
8. Alcohol is used to dissolve things, and to keep things from
spoiling.
9. Alcohol in water forms _strong drink_.