Starch. Starch is a substance found in wheat, corn, oats, and in fact in all grains, in potatoes, in the roots and stems of many plants, and in some fruits. In a pure state it is a white powder such as is seen in arrowroot and corn-starch. Examined by a microscope this powder is found to be made up of tiny grains of different shapes and sizes, some rounded or oval, others irregular. Those of potato-starch are ovoid, with an outside covering which appears to be folded or ridged, and looks somewhat like the outside of an oyster-shell, although its similarity extends no further than appearance, as the little ridges are true folds, and not overlapping edges.

Size of Starch Grains. Starch grains vary in size according to the source from which the starch is obtained. Those of ground rice are very small, being about 1/3000 of an inch in diameter; those of wheat are 1/1000 of an inch, and those of potato 1/300 of an inch.

Starch is a carbohydrate, being composed of six parts of carbon, ten of hydrogen, and five of oxygen. Its symbol is C6H10O5. It is insoluble in water, but when the water is heated, the grains seem to absorb it; they increase in size, the ridges or folds disappear, and when the temperature reaches 140° Fahr. or a little over, they burst, and the contents mingle with the liquid forming the well-known paste.

Test for Starch. Mix a teaspoon of starch with a cup of cold water and boil them together for a few minutes until a paste is formed ; then set it aside to cool. Meanwhile make a solution of iodine by putting a few flakes into alcohol, or use that which is already prepared, and which may be obtained at any pharmacy. Add a drop of this solution to the paste mixture; it will immediately color the whole a rich dark blue. This is known as the "iodine test," and is a very valuable one to the chemist, for by means of it the slightest trace of starch can be detected.

Exp. with Arrowroot. Make a thin paste by boiling a little arrowroot and water together. When cool test it with a drop of the iodine solution. The characteristic blue color will be very strong, showing that arrowroot is rich in starch.

Similar tests may be made with grated potato, wheat-flour, rice-flour, tapioca, and other starch-containing substances. Also powdered sugar, cream of tartar, and other substances may be tested, when it is suspected that they have been adulterated with starch.

Although starch grains burst and form a paste with water at 140° Fahr., that is not the temperature at which, it should be cooked for food, and the thickening which then takes place should not be confounded, as often happens, with the true cooking of starch. In order to understand the difference between the proper cooking of starch and the simple bursting of the grains, let us consider the changes which take place in starch when it is subjected to different degrees of heat, and also those which are produced in it during the process of digestion. All starch in food is changed into dextrine and then into sugar (glucose, C6 H12 O6) in the process of digestion. Glucose is a kind of sugar, resembling cane-sugar, but it is not so sweet.

Dextrine. Dextrine is a substance having the same chemical nature as starch, but differing in many of its properties. It may be described as a condition which starch assumes just before its change into glucose.

Exp. to show Dextrine. Carefully dry and then heat a little starch to about 400° Fahr. Keep it at this temperature until it turns brown, or for ten minutes. Then mix it with water, when it will dissolve, forming a gummy solution. Starch will not do this. Test it with iodine; it will not change color. The remarkable thing about the relation of dextrine to starch is that although they differ so much in properties they have the same chemical composition.

The change of starch into dextrine is an important point in cooking, because starch cannot be assimilated until the conversion has taken place, either before or after it is eaten. Now it will be seen that unless this change is either produced or approached in the cooking of starch-containing foods, they are not prepared as well as it is possible to prepare them; also, that it is not possible to cause this change at a low temperature; therefore 140° (the temperature at which the grains burst) should not be regarded as the cooking temperature of starch. It should be such a temperature as shall actually convert it into dextrine, or at least change it to such an extent that it will be more easily converted into dextrine, and ultimately into sugar, by the digestive fluids. This should be as near 401° Fahr. as practicable, - not that a potato, or a loaf of bread, or a pudding will have all the starch in it changed when it is put into an oven of that temperature. It would not be possible, on account of the water contained in each; but that in the outside may be, and the preparation of the remainder will be better than at a lower temperature.

There are other means of changing starch into dextrine than by heat, one of the most remarkable of which is diastase, a substance found in sprouting grains, which has the power to transform the starch stored in the grain by nature into soluble dextrine, in which form it can be taken up by the young plant for food. The crude starch could not thus be absorbed. The starch which we use as food is of no more value to us than it is to the young plant until it has been changed into dextrine or sugar. Now, if art outside of the body can accomplish what nature is otherwise forced to do in the alimentary canal, the body will be saved a certain amount of force, - a point of great importance, especially in the case of the sick or invalid, who can ill afford to waste energy.

Starch constitutes half of bread, our "staff of life"; nearly all of rice, the staff of life in the East; and the greater part of corn-starch, sago, arrowroot, tapioca, peas, beans, turnips, carrots, and potatoes.