1. Digestion Of Starch

There is little doubt that primitive man accomplished much of the digestion of starch in the mouth. The saliva contains two enzymes, ptyalin, and mal-tase. By the action of ptyalin starch is changed to dextrins and these to maltose; by the action of maltase, maltose is converted into dextrose, the end-product of salivary digestion. The crude uncooked food required prolonged mastication before it could be swallowed, and during this mastication the enzymes had opportunity for action. Raw starch is so very slowly affected by enzymes, that much of the food value of uncooked cereals is lost, so that the practice of baking ground grain into bread developed very early. This fact in regard to starch digestion is often disregarded by advocates of a return to raw foods.

There is said to be some evidence that a pharyngeal reflex once existed which prevented the bolting of foods which are capable of salivary digestion, or which are not easily digestible without mastication. In recent years, Mr. Horace Fletcher has counseled a return to prolonged mastication, in order that salivary digestion may be carried to its fullest extent. It is claimed that the more perfect utilization of all food eaten necessitates the taking of a smaller amount. There is no doubt that thorough mastication renders the digestion of starch easier and more complete, but it will not greatly alter the total food requirement.

The starchy food mixed with saliva passes from the mouth to the stomach and lodges in the fundus, the portions last eaten always going to the center of the mass. Thus, while the gastric juice is at work upon the outer layer, attacking the protein of the diet, the inner portions remain alkaline for a considerable time, so that salivary digestion may continue undisturbed.

From time to time, the soluble products of salivary and gastric digestion pass into the small intestine. Here the conversion of starch and intermediate products into simple sugars is completed. The pancreatic juice contains an enzyme called amylopsin, which acts like ptyalin, i. e., changes starch to dextrins, and finally to maltose.

The intestinal juice contains an enzyme capable of changing maltose to dextrose, the final product in starch digestion.

Summary

The chemical changes in starch during the process of digestion are shown in the following table: