Many different agents for lightening the dough have been used at various times. The ancient leaven was made by allowing flour and water to stand in a warm place till it fermented. Part of this dough was used to start the fermentation in a new mixture of flour and water. In some sections of our own country "salt rising" bread is commonly used. In England aerated bread, made by forcing carbon dioxide under pressure into the dough, has been advocated and used to some extent.
The most common method of lightening the loaf, in this country at least, is by means of yeast. Yeast comes into the household in three forms, that of liquid yeast, compressed, and dried yeast. The last is most often used by those too far from the source of supply to obtain compressed yeast in good condition. It makes satisfactory bread, but the process is a long one, as time must be allowed for the dry yeast to take up water and renew its life processes. Liquid, or home brewed yeast, prepared usually from potato with the water from a few hops, frequently with the addition of sugar and flour, and the whole fermented by means of the addition of a "pitching" yeast, is much less used than formerly. Aside from the trouble of preparation, it is open to the disadvantage of usually containing many bacteria and wild yeasts. Many think, however, that the fine texture and delicious flavor of old fashioned home made bread was due in part to the use of this yeast.
Compressed yeast is a by product of the distillery or the brewery. It is skimmed from the top of the fermeriting liquor, is washed, strained, mixed with a small amount of starch and pressed into large cakes. At the distributing centers it is cut and wrapped in foil and sold for one or two cents, according to locality. It is, on the whole, the most satisfactory yeast to use in bread making, though it is rarely, if ever, free from the bacteria that cause the souring of bread when conditions are right for their growth.
The changes that take place in the process of bread-making are largely those of fermentation. Some of the starch of the flour is changed to sugar, and the sugar is broken up into alcohol and carbon dioxide. If the fermentation goes too far the alcohol is changed to acetic and other acids and the bread becomes sour. Yeast is not the sole agent working; bacteria and not yeast are responsible for the souring, while the change of starch into sugar is probably accomplished by bacteria or some enzyme (ferment) present in the flour.
Chemical changes, such as the change of some of the starch into dextrin and some of the sugar into caramel, which takes place especially in the crust of the bread, are caused by the heat of the oven, while the same agent is responsible for the driving off of the alcohol and carbon dioxide present.
A few years ago bread was almost invariably made by what is called the long process. A small amount of yeast was used and the bread was allowed to rise over night. Now more often the bread is set in the morning and the whole process is carried through in six hours.
The advantage of the latter method is that it makes it possible to watch the process and regulate the temperature more carefully than can be done if the bread is set at night. As temperature is an important factor in the growth of the yeast, too low a temperature hindering its growth, and too high a temperature favoring the growth of the acid producing bacteria, this is a distinct advantage. The most favorable range of temperature is from 75 degrees to 90 degrees F.
On the other hand, the long process produces a loaf of a texture preferred by many, and some experiments tend to show that it may be slightly more digestible.