The composition of a food shows the quantities of its constituents. Thus is disclosed the possible nutritive value of food. How these constituents act in the human digestive tract controls their use to the human body; this is their digestibility. Food must be or become soluble and ultimately somewhat liquefied for passage through the digestive tract and into use by body-tissues. Though milk is in liquid form and composed of soluble substances, it undergoes a number of changes before it comes into actual use to the body. How the body is able to effect these changes determines the digestion of a food. Though the general process of digestion is alike in all persons, all have not the same degree of vitality in all parts of the digestive tract, therefore cannot digest equally well all foods. Milk is one of the most digestible of foods (95% is digested), yet some persons do not digest it easily or quickly. (See p. 218).

Different digestive agencies make the different food-constituents of use to the body. Therefore what happens naturally or otherwise to change food-constituents must be observed, if food is to be made digestible. (See p. 113).

Milk-solids (13% in all) are its nutrients. These are held in solution in the 87% of water in milk. But in the stomach, milk becomes a solid food that must be broken up again. In this usually lies the digestive difficulty whenever it exists. Protein in milk is in two forms, casein and lact-albumin. The latter is only 1/7 of the protein; it coagulates with prolonged heating.

Casein (3 % of milk-solids) becomes a solid when milk sours, or acid or rennet is added, or it is heated. This is called the curd. The liquid left is the whey; it holds the sugar and mineral matter. When rennet is added to milk, casein coagulates and changes; this happens in the stomach.