Starch turns a characteristic blue color in the presence of iodine. This is an unfailing starch test, but must be used in the cold.
1. Grate a piece of potato into a small amount of water, and strain out the pulp. The starch settles from the water in a few minutes. Pour off the water, and add a drop of diluted iodine to the remaining starch. If a microscope is available, dilute this mixture and with a dropper tube place a drop upon a slide. The potato starch granules are comparatively large and easy to see through the microscope.
2. Drop a teaspoonful of dry starch into boiling water.
3. Mix a teaspoonful of starch with a small quantity of cold water, and stir this into boiling water.
4. Mix a teaspoonful of starch with ¼ cup of cold water, and bring the water to the boiling point, stirring the mixture as it heats.
Why are 3 and 4 similar in result, and different from 2?
Fig. 39.—Changes of starch cells in cooking: a, cells of a raw potato with starch grains in natural condition; b, cells of a partially cooked potato; c, cells of a thoroughly boiled potato. Farmers’ Bulletin No. 295, U. S. Dept. Agriculture.
Desserts from the starches.—Cornstarch, in particular, is often disliked, because it is undercooked, and too large a proportion is used. It may be made very palatable, and is too valuable in cooking and too inexpensive to be discarded.