Man has discovered various methods of preserving for a long time foods liable to putrefy. The simplest method is to freeze the food or at least keep it very cold, a principle applied on a large scale in the refrigerator cars which convey meat from Western States to cities in the East. On a smaller scale this plan is used in the refrigerator found in most homes.

Another simple method of preserving meat and fish from putrefaction is to dry them rapidly and keep them dry, since the Bacteria of putrefaction thrive only in the presence of moisture: but the most common methods of preserving foods are two; one of which depends on the killing of all Bacteria present and preventing the entry of fresh ones, the other on adding substances which prevent the growth of the putrefactive organisms.

All the great " canning" industries of the United States depend on the fact that putrefactive Bacteria are killed by exposure to the temperature of boiling water; and that cans of meat and vegetables so exposed can be sealed while hot, so as to prevent the entry of living Bacteria.

Well-known instances of the use of substances which prevent the development of putrefactive organisms are the employment of salt to preserve meats and fish; and of strong vinegar or plenty of sugar in the case of pickles and preserves.

How is it that Bacteria are found in dust ? Why is it harder to keep meat in summer ?

Describe a plan of preserving meat from decomposition; a second plan; a third; a fourth.

Fermentation

Many microscopic plants, some of them Bacteria and some not, decompose complex dead organic matters into simpler ones without any accompanying emission of foul smells: when this is the case the process is usually called a fermentation instead of a putrefaction. There is, however, no real scientific distinction between a putrefaction and a fermentation: whether the products smell well or ill is a very secondary matter; the essential fact is the destruction, by certain kinds of minute living things, of the complex and elaborated products of other plants or animals. For this reason biologists make no distinction between putrefactions and fermentations: they name all such processes fermentations.

Formed And Unformed Ferments

It is still usual to speak of certain dead substances as ferments; for example, the substance (ptyalin) in our saliva which turns starch into sugar (p. 197), and the pepsin (p. 177) of the gastric juice, are frequently called ferments. It is very desirable to draw a sharp line between such dead things as pepsin (which can be chemically extracted from the lining of a dead stomach and still act in breaking down or changing organic substances) and such living things as Bacteria, causing fermentations as a side result of their own life-actions, and quite inactive when killed.

It is now customary among physiologists to distinguish the two classes of ferments, the living and the dead respectively, as formed and unformed.*

What distinction is made in ordinary language between a putrefaction and a fermentation? What is the essential nature of both processes? What is an " unformed " ferment? Name one or two. How do such ferments differ from " formed " ferments?

Every living ferment has its own field of work. Some act on albumens and decompose them ; others break up fats and oils and make them rancid. Still others destroy starches and sugars: the souring of milk, for example, is due to the alteration of the milk-sugar by a ferment which turns it into an acid (lactic acid). It might, at first sight, seem that all true ferments were merely mischievous; for they destroy much to make little. But if they did not destroy dead animals and plants the earth would now be so covered by the dead bodies of its former inhabitants, that neither animal nor plant could live on it: the land would be covered by a layer of mummies. Ferment organisms break up all this dead and useless matter and resolve much of it into simpler substances, which, in turn, serve to nourish succeeding living things. For example, when animal or vegetable substances putrefy, much of their carbon is given off to the air as carbon dioxide gas, which green plants then use in making food for animals (see p. 132).