208. We are principally indebted to the industry of man for this valuable addition to our materia alimentaria. The vegetables which yield it may be said to owe their nutritive qualities to cultivation. The art of feeding mankind on so small a grain as wheat, says Dr. Darwin, seems to have been discovered in Egypt, by the immortal name of Ceres; but it is probable, that it has risen to its present advanced state progressively, and is indebted to the labour of many generations for its perfection. The flour of wheat contains three distinct substances; a mucilaginous saccharine matter, starch, and a peculiar substance, possessing many of the properties of animal matter, termed gluten. It is to the quantity of this latter ingredient that wheat flour possesses so decided a superiority over that of barley, rye, or oats, for from these latter grains much less gluten can be extracted. It furnishes by far the best ingredient for making that important article of diet, Bread; although it may also be made of all the various sorts of grain, as well as of chestnuts, of several roots, and of the potatoe. I shall first describe the nature of wheaten bread, and then compare it with that produced from other substances.

The first process for rendering farinaceous seeds esculent, is to grind them into powder, between mill-stones, which Dr. Darwin aptly terms the "artificial teeth of society." The meal thus produced is purified from the husk of the seed, or bran, by the operation of sifting or bolting; and it is then denominated flour. This, when mixed with water or milk, undoubtedly possesses the power of nourishing the body; but it will evidently follow, from the observations which have been so frequently made in the progress of this work, that in this raw state it would not be sufficiently digestible: it would clog the stomach, and oppose those actions which are essential to chymification. The application, however, of heat renders the compound more easy to masticate as well as to digest; whence we find, in the earliest history, a reference to some process instituted for the purpose of producing this change, although the discovery of the manufacture of bread, simple as it may appear to us, was probably the work of ages. It has been just stated, that wheaten flour is the best adapted for making bread, that is to say, loaf bread; this depends upon the superior quantity of gluten which it contains, and which operates in a manner to be presently explained.

The first stage of this process of panification consists in mixing the flour with water, in order to form a paste, the average proportion of which is two parts of the latter to three of the former; but this will necessarily vary with the age and quality of the flour: in general, the older and the better the flour, the greater will be the quantity of water required. This paste may be regarded as merely a viscid and elastic tissue of gluten, the interstices of which are filled with starch, albumen, and sugar. If, then, it be allowed to remain for some time, its ingredients gradually re-act upon each other, the gluten probably performing an important part; by its action on the sweet principle, a, fermentation is established, and alcohol, carbonic acid, and lastly acetic acid, are evolved. If the paste be now baked, it forms a loaf full of eyes like our bread, but of a taste so sour and unpleasant that it cannot be eaten. If a portion of this old paste, or leaven, as it is called, be mixed with new-made paste, the fermentation commences more immediately, a quantity of carbonic acid is given off, but the gluten resists its disengagement, expands like a membrane, and forms a multitude of little cavities, which give lightness and sponginess to the mass.

We easily, therefore, perceive why flour, deficient in the tenacity which gluten imparts to it, is incapable of making raised bread, notwithstanding the greatest activity may be given to the fermentative process by artificial additions. Where, however, leaven has been employed, the bread will be apt to be sour, in consequence of the great difficulty of so adjusting its proportion, that it shall not, by its excess, impart an unpleasant flavour, nor, by its deficiency, render the bread too compact and heavy. It is for such reasons, that, in this country, we employ barm; a ferment which collects on the surface of fermenting beer. It appears that we are indebted to the ancient Gauls for this practice. In Paris it was introduced about the end of the seventeenth century; the Faculty of Medicine, however, declared it to be prejudicial to health, and it was long before the bakers could convince the public that bread baked with barm was superior to that of leaven. A great question arose among chemists, as to the nature of this barm that could produce such effects, and elaborate analyses were made, and theories deduced from their results; but all these ingenious speculations fell to the ground, when it was found that barm dried, and made into balls, would answer every purpose: the bakers imported it in such a form from Picardy and Flanders, and when again moistened, it fermented bread as well as the recent substance.

The presence, therefore, of carbonic acid, water, acetic acid, and alcohol, could not be essential, for these ingredients were separated by the process of its preparation. At length it was discovered, that gluten, mixed with a vegetable acid, produced all the desired effects; and such is the nature of leaven, and such is the compound to which barm is indebted for its value as a panary ferment. After the dough has sufficiently fermented, and is properly raised, it is put into the oven previously heated, and allowed to remain till it is baked. The mean heat, as ascertained by Mr. Tillet, is 448°. When the bread is removed, it will be found to have lost about one-fifth of its weight, owing to the evaporation of water; but this proportion will be varied by the occurrence of numerous circumstances, which it is not easy to appreciate. Newly-baked bread has a peculiar odour as well as taste, which are lost by keeping: this shows, that some peculiar substance must have been formed during the operation, the nature of which is not understood.

Bread differs very completely from the flour of which it is made, for none of the ingredients of the latter can be discovered in it; it is much more miscible with water than dough; and on this circumstance its good qualities, most probably, in a great measure depend. It is not easy to explain the chemical changes which have taken place. It appears certain, that a quantity of water, or its elements, is consolidated and combined with the flour; the gluten, too, would seem to form a union with the starch and water, and thus to give rise to a compound, upon which the nutritive qualities of bread depend 1.

1 The late novel researches of M. Raspail, with regard to the true nature of fecula, have thrown a new light upon this subject. The proximate principle, fecula, has been generally described as composed of small grains of a white crystalline appearance, insoluble in cold, but combining easily with boiling water, forming a hydrate which is known by the name of starch. M. Raspail, however, by a microscopic examination of these grains, and by numerous experiments conducted with great care, has very satisfactorily shown that these grains are "organic vesicles," consisting of an investing membrane filled with gummy matter. When put into water, heated to 122°, this membrane, which is impervious to cold water, expands; in boiling water it bursts, and then its contents dissolve in the water. So that fecula is not actually nutritive to man until it has been boiled, for the heat of the stomach is not sufficient to burst all the grains of the feculent mass which is subjected to the rapid action of this organ. Panification, therefore, must be regarded as a process, the object of which is to burst all the grains of fecula.

The finest and best-baked bread is that which is made of farina abounding in gluten; for this latter body, rising in large blisters by the dilatation of the gases imprisoned within it, allows each feculent grain to participate in the communication of the heat, and to burst as it would by boiling. Hence, after panification, if the paste has been well kneaded, we do not find a single grain of fecula entire. - Raspail's Organic Chemistry, by W. Henderson, M.D.

209. Unleavened bread consists of a mixture of meal and water, formed into a firm and tough cake, made as thin as possible, to favour its drying, and sometimes with the addition of butter, to render it more soluble, friable, and porous; but it renders it sourer, and more apt to produce acidity on the stomach. Of the unleavened sorts of bread, biscuit is by far the best; and in all cases where leavened or fermented bread does not agree, its use cannot be too strongly advocated. I shall have occasion, hereafter, to relate cases in which the use of common bread did not agree, and in which acidity of the stomach was cured by the substitution of biscuit.

210. The different sorts of bread to be met with in this country may be considered under three classes: viz. white, wheaten, and household. In the first, all the hran is separated; in the second, only the coarser; in the third, none at all: so that fine bread is made only of flour, wheaten bread of flour and a mixture of the finer bran, and household of the whole substance of the grain, without taking out either the coarse bran or fine flour. It is necessary for the medical practitioner to understand these distinctions; for it will be proved that an important dietetic fact is connected with them. The tendency of starch upon the bowels is astringent. Bread, therefore, which is made of the whitest flour, is apt to render them costive; but this is counteracted by the presence of bran, the scales of which appear to exert a mechanical action upon the intestines, and thus to excite them into action. I have already stated, in the Pharmacologia, that there are many bodies which have the power of thus acting upon the inner coats of the intestinal canal, and of increasing its peristaltic motion; and it is not improbable, that the harsh and coarse texture which certain grasses assume in moist situations may be a wise provision in nature to furnish an increased stimulus to the intestines of the animals who are destined to feed upon them, at a time when their diminished nutritive qualities may render such an effect salutary.

The practical application of such views is obvious; and experience has sanctioned the propriety of the practice that may be founded upon them. By changing the quality of the bread, I have frequently succeeded in regulating the alvine discharges.