Which material—flour or corn-starch—is the cheaper to use for thickening?
How many persons does the quantity of Blanc Mange above serve?
LESSON XXV
TOAST: DIGESTION OF STARCH
EXPERIMENT 26: CHANGE OF STARCH INTO DEXTRIN.—Place a teaspoonful of flour in a frying pan and heat slowly until it becomes very dark brown and uniform in color. Put a little of the browned flour into a test tube, add water, then shake. Add a few drops of iodine. What indicates the presence of starch? Is starch present?
The starch has been changed to dextrin. Dextrin gives a purple (reddish blue) color when treated with iodine.
EXPERIMENT 27: THE SOLUBILITY OF DEXTRIN.—Pour the remainder of the browned flour from Experiment 26 into a test tube. Add water and shake. Pour through filter paper [Footnote 26: Liquids pass through filter paper, but solids do not. Hence if a mixture of solid and liquid is poured upon filter paper, the liquid passes through, but the solid remains on the paper.] into another test tube (see Figure 30). Notice the color of the liquid that has been filtered. Add a few drops of iodine to the filtered liquid. Is dextrin present? Is dextrin soluble in water? [Illustration: Figure 30.—Method of Folding Filter Paper.]
From these experiments, we find that dry heat has changed insoluble starch into a soluble substance called dextrin. Dextrin is found in small amounts in the crust of bread and in toast.
DIGESTION OF STARCH.—It was found in a previous lesson (Lesson XVI) that sugar is entirely soluble in water, and since digestion and solution are closely related, the digestion of some sugar is simple. Starch was found to be insoluble in cold water and only semi-soluble in hot water. In the process of digestion it would seem that some change must take place in the starch to make it soluble. Such a change does take place; starch is changed into a soluble carbohydrate or a sugar before it is digested.
Substances called enzymes which are in the saliva of the mouth [Footnote 27: Ptyalin and amylopsin are the ferments found in the mouth and intestines, respectively.] and in the digestive juices of the intestines [Footnote 28: Ptyalin and amylopsin are the ferments found in the mouth and intestines, respectively.] cause this change. To show that this change takes place to some extent in the mouth, try the following experiments: