The fermentation which occurs in bread making is similar to that which is responsible for the transformation of plant juices into intoxicating drinks.  The former process is not so old, however, since the use of alcoholic beverages dates back to the very dawn of history, and the authentic record of raised or leavened bread is but little more than 3000 years old.

214.  The Bread of Antiquity.  The original method of bread making and the method employed by savage tribes of to-day is to mix crushed grain and water until a paste is formed, and then to bake this over a camp fire.  The result is a hard compact substance known as unleavened bread.  A considerable improvement over this tasteless mass is self-raised bread.  If dough is left standing in a warm place a number of hours, it swells up with gas and becomes porous, and when baked, is less compact and hard than the savage bread.  Exposure to air and warmth brings about changes in dough as well as in fruit juices, and alters the character of the dough and the bread made from it.  Bread made in this way would not seem palatable to civilized man of the present day, accustomed, as he is, to delicious bread made light and porous by yeast; but to the ancients, the least softening and lightening was welcome, and self-fermented bread, therefore, supplanted the original unleavened bread.

Soon it was discovered that a pinch of this fermented dough acted as a starter on a fresh batch of dough.  Hence, a little of the fermented dough was carefully saved from a batch, and when the next bread was made, the fermented dough, or leaven, was worked into the fresh dough and served to raise the mass more quickly and effectively than mere exposure to air and warmth could do in the same length of time.  This use of leaven for raising bread has been practiced for ages.

Grape juice mixed with millet ferments quickly and strongly, and the Romans learned to use this mixture for bread raising, kneading a very small amount of it through the dough.

215.  The Cause of Fermentation.  Although alcoholic fermentation, and the fermentation which goes on in raising dough, were known and utilized for many years, the cause of the phenomenon was a sealed book until the nineteenth century.  About that time it was discovered, through the use of the microscope, that fermenting liquids contain an army of minute plant organisms which not only live there, but which actually grow and multiply within the liquid.  For growth and multiplication, food is necessary, and this the tiny plants get in abundance from the fruit juices; they feed upon the sugary matter and as they feed, they ferment it, changing it into carbon dioxide and alcohol.  The carbon dioxide, in the form of small bubbles, passes off from the fermenting mass, while the alcohol remains in the liquid, giving the stimulating effect desired by imbibers of alcoholic drinks.  The unknown strange organisms were called yeast, and they were the starting point of the yeast cakes and yeast brews manufactured to-day on a large scale, not only for bread making but for the commercial production of beer, ale, porter, and other intoxicating drinks.