Chemical Principles

Since raw starch is digestible only to a slight degree, it is essential to perfect digestion that starchy foods be cooked.

In the presence of moisture, starch is made soluble at the temperature of boiling water, i. e., 212° F. By long boiling, this soluble starch can be converted into dextrin, a still more easily digested form than soluble starch. This principle is applied in the making of gruels, especially for invalids and infants.

By dry heat, at a temperature considerably above 212° F. (300° F.- 4000 F.) starch can be converted quite readily to dextrin. This principle is applied in making toast. Some of the starch is further changed to caramel, giving a characteristic flavor to the crust of bread, toast, etc.

Mechanical Principles

When foods composed almost entirely of pure starch, such as flour, cornstarch, etc., are to be cooked with a liquid, care must be taken to prevent the formation of lumps, for these are not only unsightly, but consist of an outer layer of soluble starch surrounding a center of unchanged material and preventing its being made digestible.

There are three methods of avoiding lumps: -

1. By mixing the starch gradually with a small portion of cold water, so that a smooth fluid mass is formed, before adding the boiling liquid. This method is useful in making gruels, etc.

2. By mixing with fat before adding liquid. The melting fat separates the starch granules. At least as much fat as starch must be used. The liquid is preferably added cold, and all at once. If added hot, it must be stirred in very gradually. This method is employed in making gravies, sauces, etc.

3. By mixing with sugar. The melting sugar serves to separate the starch grains. This is practicable only when the sugar is greater in amount than the starch, and the liquid must be added gradually. This method is applied in the case of desserts, such as cornstarch pudding.

When foods consisting of a mixture of starch and cellulose, as cereal breakfast foods, are to be cooked, it is necessary to soften and break up the cellulose to enable the digestive juices to reach the starch. This necessitates long cooking at a temperature near the boiling point. The longer such a cereal product is cooked, the easier of digestion it becomes. Hence it is an excellent rule to cook such foods for infants and others of feeble digestive powers, twice as long as is commonly recommended.

Reheating of starchy foods, therefore, tends to increase their digestibility. Zwieback (or twice-baked bread) is more digestible, for example, than fresh bread.

Test For Starch

Starch is colored blue by iodin. Hence it is easy to detect its presence in any food. To make the test, a small portion of the material should be placed in a test tube, boiled with a little water, cooled, and two or three drops of dilute tincture of iodin added. If starch is present, the blue color will immediately appear. Dextrin gives a port-wine color with iodin, so that if any of this substance be present, the color is modified, becoming more or less purplish, or purple-red.