Digestion begins in the mouth, where by means of mastication the food is comminuted and mixed with saliva. This fluid contains a ferment which converts the insoluble starch of such foods as bread and puddings into the more soluble form of maltose or malt sugar. This transformation does not take place in one stage, for there are several steps in the process, and unless the food has been carefully cooked hardly any maltose is formed in the mouth at all. Ptyalin, however, which is the name of the ferment, is able to carry on its salutary work of conversion in the stomach for something like half an hour after the food has been swallowed, or at all events until the bland alkaline saliva has been neutralised by the acid gastric juice. The stages in the best circumstances in the mouth are starch, dextrin, maltose, and the process is not advanced much further in the stomach, although any cane sugar present may be decomposed (or inverted as it is called) by the acid of the gastric juice and ferments contained in the swallowed food into grape sugar (dextrose) and fruit sugar (laevulose).

Up to this point none of the uncooked starch has been changed in any way. It is therefore manifest that digestion is hastened and economy effected if all starchy foods are cooked before consumption. This cooking swells up the starch grain, which therefore ruptures its insoluble coating of cellulose and is quickly transformed into dextrin by the heat. For this reason even bread is the better for being subjected to a second baking process in the oven, and what is called zwieback (double-baked) or pulled bread is quite a favourite form in which to eat bread.

After being ejected from the stomach into the intestinal canal all carbohydrate, whether cooked or uncooked, so far as it can be reached through its envelope of cellulose, is attacked by the amylopsin - a ferment in the pancreatic fluid - and converted into one of the sugars, usually maltose. Finally this mixture of sugars, maltose, lactose, cane sugar, and laevulose, is, by means of another ferment called invertase, changed (in large degree at any rate) into glucose (grape sugar), the most convenient form for absorption.