This section is from the book "The American Woman's Cook Book", by Ruth Berolzheimer. Also available from Amazon: The Domestic Arts Edition of the American Woman's Cook Book.
Some of the foods which are used most frequently are rich in starch; for instance:
Breakfast Foods - Wheat, oat, corn-meal, rice, barley.
Starch-rich Foods Must be Cooked Thoroughly if they are to have fine flavor and be easily digested. This is because starch occurs in foods in the form of tiny, hard, dry grains which are not soluble in cold water and which are difficult for the digestive juices to act upon. When starch is cooked, it is easy to digest and much improved in flavor, because cooking changes the form of the starch.
When Starch is Cooked in Liquid, the heat causes the starch grain to absorb liquid, swell and soften. When flour or corn-starch or any other finely divided meal is cooked in a liquid, it thickens the liquid.
When Starch is Cooked by Dry Heat, that is, with very little moisture, the heat, unless it is great enough to burn the starch, breaks down the starch grain and changes the starch to a substance called dextrin. Dextrin does not thicken liquid, but, like starch cooked in water, it has a better flavor and is easier to digest than raw starch.
The baking of a loaf of bread illustrates both these changes. The starch in the dough in the inside of the loaf absorbs the water used in making the dough and swells and softens. The water in the dough on the outside of the loaf evaporates and the starch in the outer layers of dough is partly changed to dextrin. As a result, the crust has more flavor and is sweeter than the crumb, and has a different texture.
In baking a potato, the water for cooking the starch is supplied by the potato itself.