Fermentation.--It is of common knowledge to country boys or girls that the
juice of fresh apples, grapes, and some other fruits, if allowed to stand
exposed to the air for a short time will _ferment_. That is, the sweet
juice will begin to taste sour and to have a peculiar odor, which we
recognize as that of alcohol. The fermenting juice appears to be full of
bubbles which rise to the surface. If we collect enough of these bubbles of
gas to make a test, we find it to be carbon dioxide.
Evidently something changed some part of the apple or grape, the sugar,
(C{6}H{12}O{6}), into alcohol, 2(C{2}H{6}O), and carbon dioxide, 2(CO{2}).
This chemical process is known as _fermentation_.
[Illustration: Apparatus to show effect of fermentation. _N_, molasses,
water and yeast plants; _C_, bubbles of carbon dioxide.]
Yeast causes Fermentation.--Let us now take a compressed yeast cake, shake
up a small portion of it in a solution of molasses and water, and fill a
fermentation tube with the mixture. Leave the tube in a warm place
overnight. In the morning a gas will be found to have been collected in the
closed end of the tube (see Figure on page 138). The taste and
odor of the liquid shows alcohol to be present, and the gas, if tested, is
proven carbon dioxide. Evidently yeast causes fermentation.
What are Yeasts?--If now part of the liquid from the fermentation tube
which contains the settlings be drawn off, a drop placed on a slide and a
little weak iodine added and the mixture examined under the compound
microscope, two kinds of structures will be found (see Figure below),
starch grains which are stained deep blue, and other smaller ovoid
structures of a brownish yellow color. The latter are yeast plants.
[Illustration: Yeast and starch grains. Notice that the starch grains
around which are clustered yeast cells have been rounded off by the yeast
plants. How do you account for this?]
Size and Shape, Manner of Growth, etc.--The common compressed yeast cake
contains millions of these tiny plants. In its simplest form a yeast plant
is a single cell. The shape of such a plant is ovoid, each cell showing
under the microscope the granular appearance of the protoplasm of which it
is formed. Look for tiny clear areas in the cells; these are vacuoles, or
spaces filled with fluid. The nucleus is hard to find in a yeast cell. Many
of the cells seem to have others attached to them, sometimes there being
several in a row. Yeast cells reproduce very rapidly by a process of
budding, a part of the parent cell forming one or more smaller daughter
cells which eventually become free from the parent.
Conditions favorable to growth of Yeast.--_Experiment._--Label three pint
fruit jars A, B, and C. Add one fourth of a compressed yeast cake to two
cups of water containing two tablespoonfuls of molasses or sugar. Stir the
mixture well and divide it into three equal parts and pour them into the
jars. Place covers on the jars. Put jar A in the ice box on the ice, and
jar B over the kitchen stove or near a radiator; pour the contents of jar C
into a small pan and boil for a few minutes. Pour back into C, cover and
place it next to B. After forty-eight hours, look to see if any bubbles
have made their appearance in any of the jars. If the experiment has been
successful, only jar B will show bubbles. After bubbles have begun to
appear at the surface, the fluid in jar B will be found to have a sour
taste and will smell unpleasantly. The gas which rises to the surface, if
collected and tested, will be found to be carbon dioxide. The contents of
jar B have fermented. Evidently, the growth of yeast will take place only
under conditions of moderate warmth and moisture.
Carbohydrates necessary to Fermentation.--Sugar must be present in order
for fermentation to take place. The wild yeasts cause fermentation of the
apple or grape juice because they live on the skin of the apple or grape.
Various peoples recognize this when they collect the juice of certain
fruits and, exposing it to the air, allow it to ferment. Such is the _saki_
or rice wine of the Japanese, the _tuba_ or sap of the coconut palm of the
Filipinos and the _pulque_ of the Mexicans.
Beer and Wine Making.--Brewers' yeasts are cultivated with the greatest
care; for the different flavors of beer seem to depend largely upon the
condition of the yeast plants. Beer is made in the following manner.
Sprouted barley, called malt, in which the starch of the grain has been
changed to grape sugar by digestion, is killed by drying in a hot kiln. The
malt is dissolved in water, and hops are added to give the mixture a bitter
taste. Now comes the addition of the yeast plants, which multiply rapidly
under the favorable conditions of food and heat. Fermentation results on a
large scale from the breaking down of the grape sugar, the alcohol
remaining in the fluid, and the carbon dioxide passing off into the air. At
the right time the beer is stored either in bottles or casks, but
fermentation slowly continues, forming carbon dioxide in the bottles. This
gives the sparkle to beer when it is poured from the bottle.
In wine making the wild yeasts growing on the skin of the grapes set up a
slow fermentation. It takes several weeks before the wine is ready to
bottle. In sparkling wines a second fermentation in the bottles gives rise
to carbon dioxide in such quantity as to cause a decided frothing when the
bottle is opened.
Commercial Yeast.--Cultivated yeasts are now supplied in the home as
compressed or dried yeast cakes. In both cases the yeast plants are mixed
with starch and other substances and pressed into a cake. But the
compressed yeast cake must be used fresh, as the yeast plants begin to die
rapidly after two or three days. The dried yeast cake, while it contains a
much smaller number of yeast plants, is nevertheless probably more reliable
if the yeast cannot be obtained fresh.
[Illustration: _a_ _b_ _c_]
The cut illustrates an experiment that shows how yeast plants depend upon
food in order to grow. In each of three fermentation tubes were placed an
equal amount of a compressed yeast cake. Then tube _a_ was filled with
distilled water, tube _b_ with a solution of glucose and water, and tube
_c_ with a nutrient solution containing nitrogenous matter as well as
glucose. The quantity of gas (CO{2}) in each tube is an index of the amount
of growth of the yeast cells. In which tube did the greatest growth take
place?
Bread Making.--Most of us are familiar with the process of bread making.
The materials used are flour, milk or water or both, salt, a little sugar
to hasten the process of fermentation, or "_rising_," as it is called, some
butter or lard, and yeast.
After mixing the materials thoroughly by a process called "kneading," the
bread is put aside in a warm place (about 75 deg. Fahrenheit) to "rise." If we
examine the dough at this time, we find it filled with holes, which give
the mass a spongy appearance. The yeast plants, owing to favorable
conditions, have grown rapidly and filled the cavities with carbon dioxide.
Alcohol is present, too, but this is evaporated when the dough is baked.
The baking cooks the starch of the bread, drives off the carbon dioxide and
alcohol, and kills the yeast plants, besides forming a protective crust on
the loaf.
Sour Bread.--If yeast cakes are not fresh, sour bread may result from their
use. In such yeast cakes there are apt to be present other tiny one-celled
plants, known as _bacteria_. Certain of these plants form acids after
fermentation takes place. The sour taste of the bread is usually due to
this cause. The remedy would be to have fresh yeast, to have good and fresh
flour, and to have clean vessels with which to work.
Importance of Yeasts.--Yeasts in their relation to man are thus seen to be
for the most part useful. They may get into canned substances put up in
sugar and cause them to "work," giving them a peculiar flavor. But they can
be easily killed by heating to the temperature of boiling. On the other
hand, yeast plants are necessary for the existence of all the great
industries which depend upon fermentation. And best of all they give us
leavened bread, which has become a necessity to most of mankind.