The chemical part of digestion is performed by a series of digestive juices, alternating between alkalies and acids. The active principles in these juices or fluids are ferments known as enzymes. All true digestive juices contain enzymes. These are substances which possess the power of instigating chemical reactions, without themselves being transformed or destroyed in course of the process. Strictly speaking, an enzyme is an organic compound formed by a living cell, while other substances which bring about chemical changes are called by the broader term, catalyzer. An enzyme is simply a special kind of catalytic agent, or a catalyst produced by a living organism.

Digestive enzymes bring about chemical changes in the food eaten. They are known as protein-splitting or proteolytic, fat-splitting or lipolytic, and starch-splitting or amylolytic, according to the type or food-stuff upon which they act. They are specific in their action, by which is meant, they are not capable of inciting several different reactions but each enzyme acts upon but one class of food. If a digestive juice affects two distinct types of food it is considered to contain two enzymes. Enzymes are destroyed by heat short of boiling and are prevented from acting by cold, although as a rule this does not prevent them from resuming their activity upon being warmed. The enzymes in the human body are most active at body temperature (about 98° F.) and begin to break down at a higher temperature. Fever prevents their action.

If they are compared with other chemicals a very striking peculiarity is disclosed. This is, enzymes are not used up in proportion to the work they do. If one is pouring hydrochloric acid upon iron to make hydrogen gas he is forced to continue pouring the acid if he is to continue evolving the gas. But if starch is being converted into sugar by pytalin the amount of sugar formed depends less upon the amount of saliva present than upon the time the enzyme acts upon the starch. A small amount of digestive juice, containing a much smaller amount of enzyme, may, under favorable conditions, act continuously with but the most gradual loss of power.