The only substances for making loaf bread, by which term is meant, bread which is light, white, and porous, is the flour of wheat; and it is to the larger quantity of gluten, that wheat flour owes the property of being converted into loaf-bread. The average quantity of gluten contained in wheat flour, amounts to about one-fifth of the whole weight of the meal; but it varies in quantity in different kinds of wheat, according to the soil and season in which the corn has been reared, culture, and various other circumstances. Wheat kept in damp storehouses affords scarcely any gluten, and hence, in proportion as the flour of wheat is altered and deteriorated, which happens, as it is known, when it is kept too much compressed, without being occasionally stirred up and aired in hot and close granaries; in a word, as it undergoes a chemical change, its property of making good bread is diminished; and chemical analysis shows the quantity of gluten has become lessened under such circumstances; and when it is greatly diminished the meal forms no longer a tenaceous ductile dough. The spoiled flour produces a kind of bread which is heavy, harsh, and difficult of digestion.
The greater the proportion of gluten, the easier the panification of bread-flour is effected, and the better is the bread. The wheat of the South of Europe generally contains a larger quantity of gluten, and is therefore more excellent for the manufacture of Maccaroni, Vermicelli, and other alimentary substances, requiring a glutenous paste.
Sir H. Davy found the flour of the wheat of this country to consist of from twenty to twenty-four per cent. of gluten. Barley contains six, and rye five per cent. of gluten.
We may now understand why potatoes, rice, beans, pease, buckwheat, millet, oats, and other nutritive cereal grains, abounding in starch, cannot be made into light and porous bread, although they are well calculated for being made into wholesome puddings, and why they only form crude, heavy, insipid cakes, when made into dough and baked, and not light porous loaf-bread.
In further confirmation of this statement it may be remarked, that if gluten of wheat, or only a portion of wheaten flour be incorporated by kneading with the before-named kinds of flour, a fermentable cohesive paste is produced, from which perfect bread may be made.
THEORY OF THE PANIFICATION OF BREAD FLOUR.
Bread, when chemically examined, is very different from flour; it no longer forms with water a tenaceous ductile mass, nor can starch, gluten, and saccharine mucilage be separated from it.
The chemical changes that take place in the panification of bread-flour, are by no means well understood. The saccharine mucilage, it appears, commences the fermentative chemical action that takes place in the dough, for without this substance, a mixture of flour, yeast, and water, cannot be made into true bread. The fermenting process when once commenced, is kept up by the gluten, forming the body of the paste through which the fecula and saccharine matter are diffused; and when the slight fermentation which it suffers, from changes in the saccharine matter, and supported by the presence of the gluten, has commenced, the paste becomes spongy and porous, from the disengagement of carbonic acid gas, while it still retains in some measure its elasticity; hence the lightness and porosity of well-baked wheaten bread; and hence bread, possessing these qualities, cannot be prepared from the flour of oats, barley, rye, or rice, or from any of the nutritive roots, as in all of these the quantity of gluten is considerably less, or entirely wanting, and no gluey elastic dough can be formed. The starch, which was merely diffused through the gluey dough, combines, during the baking, with a portion of water, into a stiff jelly, which renders the bread more digestible, and the gluten wholly disappears. A portion of carbonic acid gas, which becomes disengaged during the fermenting process, enlarges the bulk of the dough, which is thus rendered light, porous, and full of eyes, or cavities, in consequence of the extraction of the air bubbles, in the viscid glutenous matter; and the porosity of the bread is in proportion to the extent to which the rising of the dough is suffered to proceed.
Some chemists persuade themselves that the fermentation of the flour dough differs materially from the fermentation of saccharine substances; namely, that the vinous, acetous, and putrefactive stages of the fermenting process take place simultaneously in the dough. They imagine the vinous fermentation to take place in the saccharine mucilage, the acetous in the starch, and the putrefactive in the gluten at the same time, and from the modification of each by the others, they consider that peculiar action to originate which converts paste into bread. Against this opinion, however, the following objections may be urged. In the first place, the quantity of saccharine mucilage is so extremely small as to produce no sensible effect alone on the whole mass, and what little there is probably passes speedily into the acetous fermentation. Secondly, the temperature that is required for bread-making is considerably lower than that at which starch dissolves in water, and where this is the case no alteration will take place, even in a long course of time: this is clearly shown by the usual process of starch-making, in which the bruised wheat is fermented for several days in large vats, in order to destroy the gluten, after which the starch is procured by simple deposition from the washings of the residue; and thirdly, no vestige whatever of the products evolved during the putrefactive fermentation of gluten, can be traced in any stage of the panification of bread flour.