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.
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.
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 1/4 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?
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.