This section is from the book "Elements Of The Theory And Practice Of Cookery", by Mary E. Williams. Also available from Amazon: Elements Of The Theory And Practice Of Cookery; A Textbook Of Domestic Science For Use In Schools.
Under the microscope starch is seen to consist of irregularly shaped granules formed of layers folded around a central point. Starches from different plants differ from one another; granules of potato starch are larger than those of any other kind and something like oyster shells in shape and marking. Rice starch granules are angular and very small. When cooked, the granules lose their distinctive appearance.
A. Pour about two tablespoonfuls of boiling water upon one tea-spoonful of dry cornstarch, stirring as you pour. What happens? Break open one of the lumps. What do you find inside? Would pouring boiling water upon starch be a good way to cook it? Why not?
When boiling water is poured upon dry starch, lumps form, because the starch first touched by the hot water swells suddenly, forming a sticky envelope around the rest, thus keeping it from swelling.
The grains of sugar, by separating the starch granules, give the granules room to swell and thicken the liquid smoothly.
C. Repeat Experiment A, mixing one-half tablespoonful of cold water with the starch. Note result and explain.
Starch cooked with water forms a paste.1
Starch is used to thicken liquid in making sauces and gravies. In what three ways may lumping be avoided? Which of these ways is used in making white sauce?
Starch, heated dry, changes to dextrin, which is soluble in cold water. In browned flour part of the starch has undergone this change, lessening the thickening property of the flour, and at the same time part of the dextrin has been further changed to caramel, which causes the brown color. The brown crust under the skin of baked potatoes is largely caramel and dextrin. A temperature of 320° F. is required to dextrinize starch. Explain why dextrin is not formed in boiled potatoes.
A. Heat about one tablespoonful of starch in a test-tube (or on a small tin pan kept for use in experiments). When brown, take out part of it and test it for starch, (1) by heating with water, (2) by adding iodine.
B. Continue to heat the rest of the starch in a test-tube, until black. What is the black substance? What do you observe on the sides of the test-tube?
1 The starch takes up the water in such a way that we cannot drive off the water and leave the starch as it was before.
Starch is composed of carbon, oxygen, and hydrogen. When heated, the two last pass off as water, leaving the carbon.
Food after being eaten undergoes many changes before it can be absorbed by the body. This process of change we call digestion. One important step in digestion is to make all insoluble foodstuffs soluble foodstuffs. Saliva begins to digest starch in the mouth. Some of it is here changed first to dextrin and then to maltose, a kind of sugar. The saliva continues to act on it for a time in the stomach. Starch digestion is completed in the small intestine.
A substance in saliva, called amylase,1 causes this change (p. 131). The test for maltose is Fehling's solution,2 which forms with it a reddish or orange-colored substance.
Make a thin starch paste (about one-half teaspoonful of starch to three or four tablespoonfuls of water). Cool it to the temperature of the hand. Divide this between two test-tubes; in a third collect some saliva. Pour part of the saliva into one of the tubes of starch paste. Add a few drops of Fehling's solution and boil. Note the color. For comparison treat first starch solution and then saliva in the same way. Do either of these change color? Does either saliva or starch alone contain maltose? Explain the presence of this sugar in the mixture of starch and saliva.