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, C6H12O6) 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.
Arrowroot is the purest form of starch food known. Rice is richest in starch of all the grains. Tapioca is prepared from the root of a tropical plant; it is first crushed and the grains washed out with water, then the whole is heated and stirred, thus cooking and breaking the starch grains, which on cooling assume the irregular rough shapes seen in the ordinary tapioca of commerce. Probably a part of the starch is converted into dextrine, which accounts for the peculiarly agreeable flavor which tapioca possesses. Mixed with the grains, as they are taken from the plant, is a very dangerous poison which, being soluble in water and volatile, is partially washed away and partially driven out by the heat,—in fact the heating is done for this purpose. Sago is principally starch. It is obtained from the pith of the sago-palm. Imitations of both tapioca and sago are sometimes made from common starch.
Starch may be converted into grape-sugar by treating it with acids; that of corn is generally used for the purpose. Much of the glucose of commerce is made in this way. In the United States it is estimated that $10,000,000 worth is manufactured every year. It is used for table syrup, in brewing beer, in the adulteration of cane-sugar, and in confectionery. Honey is also made from it. The nutritive value of vegetables is due largely to the starch and sugar which they contain.
In the economy of the body starch is eminently a heat producer. Pound for pound it does not give as much heat as fat, but owing to its great abundance and extensive use it, in the aggregate, produces more. (Atwater.)