Chapter 96

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.