CHAPTER LIX. CHEMISTRY OF FERMENTATION.

328. Ferments.--A large number of chemical changes are brought
about through the direct agency of bodies called ferments; their
action is called fermentation. Ferments are sometimes lifeless
chemical products found in living bodies; but in other cases they
are humble plants.

329. Yeast is one of the most common of living ferments, wild
yeast being a microscopic plant found on the ground near apple-
trees and grape-vines, and often in the air. The cultivated
variety is sold by grocers. The temperature best suited to the
rapid multiplication of the germs forming the ferment plant is 25
degrees to 35 degrees.

330. Alcoholic and Acetic Fermentation.--The changes which the
juice of the apple undergoes in forming cider and vinegar are a
good illustration of fermentation by a living plant. Apple-juice
contains sucrose. Yeast germs from the air, getting into this
unfermented liquor, cause it to "work." This process changes
sucrose to glucose, and glucose to alcohol and CO2, and is known
as alcoholic fermentation. The latter reaction, C6H12O6 = 2 C2H6O
+ 2 CO, is only partially correct, as other products are formed.
The juice has now become cider; the sugar alcohol. After a time,
if left exposed, another organism finds its way to the alcohol,
and transforms it into acetic acid, HC2H8O2, and H2O. This
process is called acetic fermentation. C2H6O + O2 = HC2H3O2 +
H2O. For this fermentation, a liquor should not have over ten per
cent of alcohol. Mother of vinegar consists of the germs that
caused the fermentation. Still a third species of ferment may
cause another action, changing acetic acid to H2O and CO2. The
vinegar then tastes flat. HC2H3O2 + 4 O = 2H2O + 2 CO2.

Some mineral acids, as H2SO4 and HCl, and some organic acids, are
regarded as lifeless ferments. To this class are thought to
belong the diastase of malt and the pepsin of the stomach. This
variety of ferments exists in the seeds of all plants, and
changes starch to glucose.

331. Bread which is raised by yeast is fermented, the object
being to produce CO2, bubbles of which, with the alcohol, cause
the dough to rise and make the bread light.

Grapes and other fruits ferment and produce wines, etc., from
which distilled liquors are obtained.

332. Lactic Fermentation changes the sugar of milk, lactose, to
lactic acid, i.e. sour milk. In canning fruit, any germs present
are killed by heating, and those from the air are excluded by
sealing the can. Milk has been kept sweet for years by boiling,
and tightly covering the receptacle with two or three folds of
cotton cloth.

333. Putrefaction is fermentation in which the products of decay
are ill-smelling. Saprophytes attack the dead matter, feed on it,
and cause it to putrefy. This action, as well as that of ordinary
fermentation, used to be attributed solely to oxygen. Germs bring
back organic matter to a more elementary state, and so have a
very important function. By some scientists, digestion is
regarded as a species of fermentation, probably due to the action
of lifeless ferments; e.g. sucrose cannot be taken into the
system, but is first fermented to glucose.

334. Most Infectious Diseases are now thought to be due to
parasites of various kinds, such as bacteria, microbes, etc.,
with which the victim often swarms, and which feed on his
tissues, multiplying with enormous rapidity. Such diseases are
small-pox, intermittent and yellow fevers, etc. Consumption, or
tuberculosis, is believed to be caused by a microbe which
destroys the lungs. In some diseases not less than fifteen
billions of the organisms are estimated to exist in a cubic inch.
These multiply so rapidly that from a single germ in forty-eight
hours may be produced nearly three hundred billions. These germs
do not spring into life spontaneously from inorganic matter, but
come from pre-existent similar forms. Parasites are not so rare
in the system even of a healthy person as is generally supposed.
They are found on our teeth and in many of the tissues of the
body.

Several infectious diseases are now warded off or rendered less
virulent by vaccination, the philosophy of which is that the
organisms are rendered less dangerous by domestication; several
crops, or generations, are grown in a prepared liquid, each less
injurious than its parent. Some of the more domesticated ones are
introduced into the system, and the person has only a modified
form of the disease, often scarcely any at all, and is for a more
or less limited time insured against further danger.

Dust particles and motes floating in the air are in part germs,
living or dead, often requiring only moisture and mild
temperature for resuscitation. Most of these are harmless.