Popular Science Monthly/Volume 23/September 1883/The Granule of Starch
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The Granule of Starch
|Sketch of Sir William E. Logan, LL.D., F.G.S.→|
THERE may not seem much in a grain of starch, and in point of bulk there is very little; but we shall endeavor to show that there is a good deal of interesting and valuable information to be derived from a careful study of the little granule.
We are all familiar with such commodities as flour, potatoes, Indian corn, sago, peas, and arrowroot, and are consequently to some extent acquainted with what starch is; for all these substances consist essentially of starch, along with water and some minor admixtures. If we take a slice of a potato, for instance, and rub it on a grater of any sort in a basin of cold water, the water will soon become turbid; and a drop of it examined with a microscope will be found to contain a number of minute oval granules, which would in time sink to the bottom of the basin, forming a white deposit. These are grains of starch; and so minute are some varieties that three thousand of them laid end to end would barely make an inch. The starch of every plant differs from its neighbors both in size and shape, and this has a considerable influence on the character of the vegetable organ in which it is stored up; the hardness of rice, for instance, being due to the fact that rice-granules are extremely minute, with angular corners which fit closely and firmly together; whereas potato-starch is large and round, with considerable interspaces filled with water, and so forms a comparatively soft mass. But, notwithstanding their outward points of difference, in chemical composition the starches are all identical, consisting of carbon, hydrogen, and oxygen—exactly the same materials as sugar is composed of, and better known as the component elements of coal and water. Leaving the many varieties of starch in the mean while, let us consider one species, namely, that of wheat, because it is the most important in this country, forming the basis of our daily bread.
An ordinary grain of wheat, if sliced through the middle and examined as to its structure, will be found to consist of several layers, the outer a hard coating, which contains mineral salts, lime, sand, etc. Beneath this is a zone of matter very rich in gluten, the flesh-forming constituent of the wheat; while the central portion of the grain is occupied by a white, powdery mass, which is nearly pure starch. In manufacturing flour, the two outer layers, which together form the bran, are usually removed, leaving the white starchy flour of the central portion.
Let us now briefly consider the chief points in the chemistry of bread-making. If flour be worked up with water, it forms a sodden, insipid, indigestible mass; but, if heated to the temperature of boiling water, the starch-granules burst; and it is thereby rendered a little more digestible, although still forming a close, stiff, and not very palatable cake. Such is the character of unleavened bread, and of sea biscuits, a slightly different form of the same thing. To be fit for digestion, starch must be dissolved or softened by boiling or baking; hence the reason why raw nuts are so indigestible as compared with the favorite roasted chestnuts; and hence one reason for cooking food, which mankind has been taught by experience, ages before chemistry could give a scientific explanation of the reason why. Cooking is, in fact, a partial digestion; and the same is the case with baking, both being preliminary aids to the changes which take place in the mouth and stomach before the food is in a fit state for the preparation of the blood. Accordingly, we bake our bread; and we bake it in the way we do because a soft, spongy loaf is more readily moistened and acted on by the saliva and the juices of the stomach.
There is a good deal in the chemistry of bread-making; and our bread might be much improved if bakers had a more intelligent understanding of the science involved in their business; for, although several improvements have been introduced of late years, the most of our bread is still prepared in the old fashion. The necessary quantity of flour is 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 the mouth by three pairs of glands to the extent of some twenty ounces a day. It consists in great part of water, with a little salt and a peculiar substance called ptyaline, which possesses the property of changing starch into sugar, the change being accomplished most completely when the starch is dissolved or baked, at a temperature of about 98 Fahr., the normal temperature of the body. Although this ptyaline is present in the saliva to the extent of only one part in five hundred, yet, on its presence and action, the heat, and consequently the life of the body, is largely dependent; hence the importance of avoiding any unnecessary waste of it, such as frequently and unnecessarily accompanies smoking. Hence, likewise, we see the importance of chewing the food slowly and thoroughly, that it may be all brought under the influence of the ptyaline; and thus we can understand how indigestion or dyspepsia may be caused by hasty chewing or by excessive spitting, the starchy portion of the food in either case lying in the stomach as an undissolved mass.
Bread-making we have already stated is a form of cooking. The heat of the oven has converted the outside of the bread into sugar, and the starch in the inside has in fact been boiled in the steam of the water which the dough contained, so that it has become capable of being readily converted into sugar. The porous nature of the bread favors this conversion; for the saliva easily penetrates through the whole of the spongy mass; and the change is still further assisted by the water which the bread contains to the extent of some forty per cent. Biscuits, on the other hand, being as a rule dry and non-spongy, are less suitable for ordinary use, although containing in the same weight far more food-material than bread.
It may surprise some of our readers to be told that the starch of bread has not the slightest nutritive property. Its sole office is a heat producer; and, just like the coal of the engine, the starch or sugar is burned up inside us to keep up the temperature of the machine. It is the gluten, the sticky, tenacious matter in the grain, which is the nutritive, flesh-forming material; but in the present article we have no space to follow the changes which it undergoes in the system, for we are simply treating of starch at present; and we trust we have made it clear how it is changed into sugar, and thus made soluble and fit for absorption into the juices which keep the body at a uniform temperature and in good repair.
It is a common but mistaken notion that sago and tapioca are very nutritious. On the contrary, they consist almost wholly of starch, with only about three per cent of gluten, so that, unless cooked with milk or eggs, they form a very insufficient food. The same is the case with Indian-corn flour and arrowroot, which have scarcely a particle of nutritious matter in them, so that it is a great mistake to feed an invalid or a child on such materials. They are no doubt useful, as easily digested heat-producers; but they must be cooked with milk or eggs before they are of much use for actual nutriment; and many a child has been starved to death through its parents' ignorance of this fact. It is true, medical men often recommend arrowroot for those in delicate health, as it is of great importance to keep up the natural heat of the body with the least exertion of the digestive organs; but it can not be too widely known that arrowroot pure and simple is a mere heat-producer; and milk, beef-tea, soup, or other suitable flesh-forming food, must be given with it, if the child or invalid is to be kept alive. On the other hand, semolina, hominy, lentil-meal, pea-flour, etc., not being prepared by washing, contain a much greater amount of flesh-forming material than sago, arrowroot, etc.
The starches are largely used in several important manufactures. Dextrine or British gum is prepared by heating starch to a temperature of about 400° Fahr., and is preferred to gum-arabic because it is not so liable to crack or curl up the stamps or other paper prepared with it. Immense quantities of starch are used, too, in the manufacture of glucose or grape-sugar, which has exactly the same composition as starch, and is prepared by acting on the starch with sulphuric acid (oil of vitriol), which has the same effect as the ptyaline of the saliva. Linen rags are largely used for the same purpose, too; and, indeed, it is wonderful how few things are altogether useless at the present day. Old boots and horns provide some of our most brilliant colors; while dye-colors innumerable are made from the refuse of out gas-works; and the wash-heaps of our factories are proving mines of wealth, instead of mounds of rubbish.—Chambers's Journal.