Page:Popular Science Monthly Volume 23.djvu/707

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put into a trough with about half its weight of water, and sufficient salt and yeast or leaven, then thoroughly mixed up into what is known as the "sponge." (Here we may remark that the best flour takes up the largest quantity of water; and a rough test of the quality of two samples of flour may be made by comparing the quantity of water required to obtain a dough of similar consistency.) After the sponge is made, it is left for about five hours in a warm place to ferment, after which it is kneaded with the rest of the flour, and again left to rest some time. The dough is then weighed into lumps, which are put in tins, and set aside till they have risen to twice their previous bulk. It is to the yeast or leaven that the raising of bread is due, and the action is identical with that of the fermentation of beer. The flour contains a small amount of a nitrogenous substance which changes a portion of the starch into sugar; the yeast then attacks the sugar, splitting it into alcohol and carbonic-acid gas, the little bubbles of which try to escape from the mass of the dough, but get entangled by the gluten and gum which the flour contains; and thus every part of the bread becomes penetrated with little cavities. Eventually the fermentation would cease, and the bubbles of gas would find their way to the outside, thus leaving the dough much less light and spongy than we wish it to be; but the baker guards against this by putting it at the proper time into a hot oven, the heat of which at first increases the fermentation. In a few minutes, however, the temperature becomes sufficiently high to kill all the yeast-germs; the fermentation is thereby stopped; and, by continued heating, the starch-granules are burst and the mass is fixed in the porous form it has then attained. A little of the alcohol is retained in the bread; but practically almost the whole of it—in London amounting to some three hundred thousand gallons per annum—is driven off by the heat. During the baking, the starch of the outer portions of the bread has been browned by the heat, and thereby changed into a sugar known as dextrine or British gum; and perhaps this fact accounts for the fondness of some children and even grown-up people for crusts.

Of late years a system for making what is called aërated bread has proved very successful, and is free from the slightest objection. The dough is made by mixing the flour with water saturated with carbonic acid gas, which on heating is expelled from the water, and thus distends the dough, producing a light, spongy bread, with no loss of starch or sugar, and without any injurious or objectionable ingredient having been introduced.

Having dealt with the baking of the bread, let us now briefly consider its further progress in being adapted for the wants of the body. As soon as a piece of bread is put into the mouth, an abundant flow of saliva takes place; and in fact it needs no actual tasting to induce this flow, for even the sight or smell of anything nice is quite sufficient to "make the mouth water," as we express it. The saliva is poured into