=Pure starches.=——These occur as food materials in several forms. Cornstarch is the starchy portion only removed from the grain of corn. Wheat starch is more largely used for laundry purposes than for food. Rice flour may be treated as starch in cooking. Arrowroot is a fine starch from the roots of a family of plants growing in the West Indies and other warm climates. It is used principally in cooking for invalids. Cassava, manioca, tapioca, and sago are starchy materials in granular form. The first three are made from the pith of the cassava plant, the sago from the sago palm. The pure starches are all easily digested and inexpensive. Corn starch is the most abundant and cheapest pure starch in this country. =The starch granule.=——To understand the behavior of materials like cornstarch, rice flour, and arrowroot in cooking, we need to know more of the starch grain. Cornstarch is composed of myriads of tiny granules somewhat like those pictured in Fig. 39, but smaller. The starch granules of different plants differ in size and even in shape, but they all have a covering lying in folds, the pure starch being within. The granule unfolds or bursts when exposed to heat. When these granules are floating in water, and, being heated, open at the same moment, the starch paste is smooth; otherwise, the starch lumps. _A. Starch experiments._ 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? [Illustration: 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. =5. Chocolate cornstarch.=