Test for Starch.—If we boil water with a piece of laundry starch in a test tube, then cool it and add to the mixture two or three drops of iodine solution,[3] we find that the mixture in the test tube turns purple or deep blue. It has been discovered by experiment that starch, and no other known substance, will be turned purple or dark blue by iodine. Therefore, iodine solution has come to be used as a test for the presence of starch.
Test for Starch.
Starch in the Bean.—If we mash up a little piece of a bean cotyledon which has been previously soaked in water, and test for starch with iodine solution, the characteristic blue-black color appears, showing the presence of the starch. If a little of the stained material is mounted in water on a glass slide under the compound microscope, you will find that the starch is in the form of little ovoid bodies called starch grains. The starch grains and other food products are made use of by the growing plant.
Test for protein.
Test for Oils.—If the substance believed to contain oil is rubbed on brown paper or is placed on paper and then heated in an oven, the presence of oil will be known by a translucent spot on the paper.
Protein in the Bean.—Another nutrient present in the bean cotyledon is protein. Several tests are used to detect the presence of this nutrient. The following is one of the best known:—
Place in a test tube the substance to be tested; for example, a bit of hard-boiled egg. Pour over it a little strong (60 per cent) nitric acid and heat gently. Note the color that appears—a lemon yellow. If the egg is washed in water and a little ammonium hydrate added, the color changes to a deep orange, showing that a protein is present.
If the protein is in a liquid state, its presence may be proved by heating, for when it coagulates or thickens, as does the white of an egg when boiled, protein in the form of an albumin is present.