Now, having discussed the subject of the flour, the next step in order is the different ways of making it into bread. These may all be included under two divisions, - those made by fermentation, and those without fermentation.

Fermentation, what is it? - Fermentation is that change in organic substances by which their sugar, starch, gluten, etc., are decomposed or recombined into new compounds. This change may be spontaneous under favorable conditions of air, moisture, and warmth; or it may be hastened by the presence of a ferment. A ferment is some albuminous substance in a state of decomposition, and, when introduced into any other albuminous substance, in however minute a quantity, causes a change which pervades the whole mass. These fermenting substances are in great variety, and the germs of some of them are always present in the air. There are different kinds of fermentation.

The lactic fermentation is the change in milk when it sours. The casein, or albuminous part of the milk, by exposure to the air and warmth, begins to decompose, becomes a ferment, and changes the sugar of the milk into an acid called lactic acid. This reacts upon the remainder of the milk, as any acid would, and causes it to coagulate or harden, and gives it a sour taste.

The alcoholic fermentation is that which is produced in substances rich in sugar or starch, as the fruits and grains from which wines and beer are made. Some of these ferment germs are present in the juice of grapes; and under the influence of air, moisture, and warmth, they seize upon the sugar already present in the natural fruit juices, and any that may be added, and convert it into carbonic acid gas and alcohol. In the grains, a portion of the gluten ferments and changes the starch into sugar, and then the sugar into carbonic acid and alcohol. In converting the starch into sugar there is no change evident to the eye; but as soon as the sugar is decomposed into alcohol and carbonic acid gas, large bubbles of gas appear, which swell the whole mass.

Acetic fermentation is caused by allowing alcoholic fermentation to go on beyond a certain limit, or in a temperature above 90°. A familiar illustration of this is the change of wine or cider into vinegar.

Now, bread-dough contains gluten, sugar, and starch; and if the dough be kept warm for a certain time, lactic fermentation will be developed spontaneously, and the bread made from such dough will be sour and heavy. Alcoholic fermentation can also be spontaneously produced in dough, by making first a batter (as the semi-fluid state is more favorable to rapid chemical change), and subjecting it to a temperature of 110° for five or six hours; then, adding more flour, allowing it to rise again, and then baking it. Bread made in this way is called salt or milk-rising's bread. But it does not keep well, and is not generally liked.

It is not always convenient to wait for dough to be raised in this manner, so we hasten the process by the addition of some active ferment. Leaven, or a piece of old dough, left to sour, and then mixed with the new dough was formerly used; this produced lactic as well as alcoholic fermentation, and though the bread was light and spongy in texture, it had an unpleasant sour taste. But since the chemistry of yeast fermentation has been understood, yeast has come to be considered the best ferment for producing alcoholic fermentation in bread rapidly, and with no objectionable result.

Yeast, what is it? - Yeast is a plant or germ of the fungus tribe. Under the microscope it is found to consist of numberless minute rounded or oval bodies which are true vegetable cells. Yeast is therefore one of the simplest and smallest of vegetable organisms. Each little cell consists of an enveloping skin or membrane, containing a liquid or sap. They grow or expand from the minutest microscopic points, and seem to bud off from each other and multiply into many millions to the cubic inch. These cells are easily propagated in any medium where they find congenial food, particularly in the juice of grapes. If grape-juice be filtered and left to stand in a warm place two or three hours, it becomes first cloudy, then thick, and gives off bubbles of gas, showing there has been some change in its composition. In a short time a grayish-yellow froth, or layer of yeast cells, collects on the surface.

"Whether the germs or spores of the yeast plant exist already in the juices of the living grape, or whether they are always floating in the air, and cling to the exterior of the fruit, and only become mixed with the juice in the winepress, is not known;" neither is it known just how they decompose the sugar of the grape. But it is enough for our purpose to know that they grow in the juice and expand there, and that an active ferment may be dissolved out of these yeast cells, sufficient to cause alcoholic fermentation.

The natural development of yeast through the agency of plants is too slow and inconvenient a process to rely upon; therefore we manufacture it from various substances rich in starch and sugar. Brewer's yeast is made from malt, or sprouting grain, usually barley; home-made yeast, from flour and potatoes.