EXPERIMENT 17: STIFFENING OF COOKED STARCH.—Place the test tube containing cooked starch from Experiment 16 in cold water. After ten minutes examine it. Can you pour it out of the tube? How does cooked starch change when cooled?
EXPERIMENT 18: THE STRUCTURE OF STARCH.—Examine starch under the microscope. While you are still looking through the microscope, make a drawing of several grains of starch. Insert this drawing in your notebook.
CELLULOSE.—Cellulose is a tough substance found in the fiber of wood. As previously mentioned the outside covering of vegetables and fruits and their interior framework contain much cellulose. The fibrous material found in rolled oats consists almost entirely of cellulose.
EXPERIMENT 19: SEPARATION OF CELLULOSE AND STARCH.—Place a heaping teaspoonful of rolled oats in a cup and add just enough water to cover it. Allow it to stand for at least 15 minutes. Pour the mixture into a cheese- cloth and press out the moisture and much of the starch, catching it in a saucepan. Rinse the starch out of the cloth as thoroughly as possible by holding it under running water. Examine the substance remaining in the cloth. Tear it into pieces. Is it tough? Does it suggest any common material? What is it? Heat the contents of the saucepan. What is this substance?
The tiny grains of starch shown under the microscope (see Figure 23) contain both starch and cellulose. The latter forms the outer covering of the microscopic grains. Starchy vegetables contain much cellulose: (a) in the outside covering; (b) in the interior framework; (c) in the covering of the starch grains.
[Illustration: From Household Chemistry, by J. M. Blanchard. Figure 23.—Grains of Starch. a, potato starch; b, corn-starch. (Much magnified.)]
Some plants rich in cellulose can be eaten in the raw state. But certain fibrous foods, especially cereals or grains, are irritating if eaten in the uncooked condition. It is necessary to soften them if used as food. Now cellulose itself is not soluble in cold or hot water nor is it softened by boiling in water. But other materials existing with cellulose are softened or changed by cooking. Hence changes in these substances in contact with the cellulose brought about by boiling water soften the food and separate cellulose fibers.
Heat and moisture applied to starchy foods serve three important purposes:
(a) They soften the food; (b) they change the starch to a paste or make it semisoluble; (c) they improve the flavor.
Cellulose is not a fuel material; it does not serve in the body as an energy-giver. Its value in diet is due to the fact that it is bulky and furnishes ballast for the alimentary canal. It stimulates the flow of the digestive juices as it brushes against the walls of the digestive tract, and thus aids in the digestion of foods and in the elimination of waste material.