The effect of this commencing fermentation is found to be, that the mass is rendered more porous by the disengagement of elastic fluid, which separates its parts from each other, and greatly increases its bulk. The operation of baking puts a stop to this process, by evaporating great part of the moisture which is requisite to favour the chemical attractions, and probably by still further changing the nature of the component parts. Bread thus made will not possess the uniformity which is requisite, because some parts may be mouldy, while others are not sufficiently changed from the state of dough. The same means are used in this case as have been found effective in promoting the uniform fermentation of large masses. This consists in the use of a leaven or ferment (as before mentioned), which is usually a small portion of dough of the same kind, but in a more advanced stage of the fermentation. To prepare an original leaven, take 8 oz. of flour, and two pints of blood-warm water, and as soon as the sponge begins to rise on the second day, add 1 lb. of flour, and four pints of water; and thus proceed for a day or two more, when a mixture will be obtained, which, being added to a quantity of flour and water intended for bread, will determine the fermentation to take place throughout the mass in three or four hours.

It is only under peculiar circumstances that a recourse to an original ferment is necessary; for this ferment having been once obtained, and the dough nearly ready for baking made from it, the fermentation of the next parcel of bread is readily put in action by reserving some of the fermented dough or leaven, as it is called, and using it for that purpose. To preserve this leaven from becoming sour, several methods are adopted. In the north of England, the leaven for the next week's baking is kept fit for use by being buried a few inches deep in a sack of flour. In Italy it is said to be kept fresh even for three months by being buried deep in flour. The French, if they intend to use the leaven in a few days, keep it in a warm place between two bowls, and add every day as much flour as the leaven weighs, and a sufficient quantity of water to restore the original consistence; but if it is not to be used for a week, or longer, the scrapings of the kneading trough are cut into small pieces, dried by a gentle heat, and when wanted, rubbed down with warm water.

It appears from the Scriptures, that the practice of making leavened bread is of extreme antiquity; but the addition of the scum that arises in the vinous fermentation of beer, called barm or yeast, seems to be of modern date, and is now in general use throughout the north of Europe. In this country this yeast is generally used in the proportion of a pint to a 100 lbs. of flour, and is dissolved in the first parcel of water with which the flour is mixed, and no leaven is used; but at Paris, and other great towns in France, the dough is made first with leaven, and a little yeast is added to the last parcel of water, merely to increase the sponginess of the bread. Although we have given under the head Barm an account of the nature of this useful ferment, and various modes of preparing it, we shall here add some further information which more immediately appertains to the manufacture of bread. If yeast is not to be purchased, original yeast may be obtained by boiling a quarter of a peck (3 1/2 lbs.) of meal for eight or ten minutes in three pints of water, and pouring off two pints, which is to be kept in a warm place; the fermentation will commence in about thirty hours, at which time four pints more of a similar decoction of malt are to be added, and when this ferments, another four pints are to be added, and so on, until a sufficient quantity of yeast is obtained.

In Edinburgh the bakers multiply their yeast daily, by mixing 10 lbs. of flour with two gallons of boiling water, and covering it up for about eight hours. Two pints of yeast, made the day before, are then stirred in, and in about six or eight hours as much new yeast will be generated as will suffice for 420 lbs. of flour. When original yeast is prepared from malt, the fermenting wort may be added to the flour as well as the yeast, according to Mr. Stock, whose patent substitute for yeast is merely wort in a state of fermentation. This wort is made from 2 lbs. of malt, 1/5 oz. of sugar, and 1 oz. of hops, to each gallon. Two gallons of this wort are sufficient for 12 bushels of wheaten flour. The Hungarians prepare a similar ferment for keeping all the year, by boiling in water in the summer wheat bran (obtained in grinding for household flour,) along with hops; the decoction soon ferments, and then a sufficient quantity of bran is flung in to drink up all the liquid, and allow it to be formed into balls, which are dried in a gentle heat. When wanted for use, some of these balls are broken, and boiling water poured upon them, which, after some time, is strained off and used to make up the dough.

In like manner, the Romans prepared their ferment by drawing off in vintage time a quantity of grape juice, while at the height of its fermentation, pouring into it a sufficient quantity of millet flour to absorb it all, and forming into small balls, which, when wanted, were broken, infused in boiling water, and the whole then mixed with the dough. A similar ferment may be prepared in this country from a decoction of raisins, which must subsequently be either pressed between boards with a heavy weight, or be mixed up with ground millet, as otherwise the strongest part of the must would remain amongst them. In summer, the leaven, yeast, and even dough, is apt to turn sour, and to communicate that taste to the bread; this is remedied by stirring a few tea-spoons-full of carbonate of magnesia into the ferment or dough.