Neither ptyalin nor amylopsin can act in an acid media. This indicates that starch and acids do not form a suitable food combination. Tea, probably, by its astringent property, arrests the activity of both the salivary glands and the pancreas. As the secretions from these organs are the only agencies in the system by which starch is digested, tea and starchy food evidently do not form a hygienic combination. It follows, then, that the proper time for tea drinking is not until the food has left the stomach then it may be taken as a stimulant. Neither ptyalin nor amylopsin is developed in children until the teeth begin to appear: hence it were wise to give no starchy food to children until after the first year, except that which has been converted into dextrin or maltose outside the body.

The starch in a piece of bread, properly toasted, and in the crust of a loaf, properly baked, are easily digested, because the starch has been changed to dextrin and, being dry and hard, they cannot be swallowed until masticated for some time, thus insuring the digestive action of the pytalin and the change into maltose. By the law of opposites this explains why fresh bread, which of itself is no more indigestible than stale bread, is considered unwholesome and indigestible. If mastication be but continued long enough, fresh and stale bread are equally digestible.