Normal Digestion

Milk is not altered in the mouth, but on reaching the stomach the casein is precipitated by a curdling ferment, called rennin. The curds, or coagulae, vary in size and toughness according to the quality of the milk, its degree of dilution, and other circumstances.

The gastric hydrochloric acid also coagulates milk by neutralising the alkali which holds the casein in solution. Hence milk becomes really a solid food almost as soon as it enters the stomach. In coagulating, the curds entangle the fat globules, but they are soon dissolved by the ferment of the gastric juice (pepsin) and converted into peptones or intermediate products called albumoses. The fat globules are again liberated, their albuminous envelopes are dissolved, and they coalesce to larger droplets, in which condition they pass with the chyme into the intestine. The usual period for the digestion of milk in the stomach by normal gastric juice occupies about three hours.

The salts and water of the milk, and possibly to some extent the sugar, are absorbed in great part from the stomach wall. In proof of this is the fact that the diuretic action of milk is often obtainable within half the time required for the complete digestion of the albuminoids. When not absorbed by the stomach they pass into the intestine and are absorbed through the villi.

Those curds which are not fully dissolved by the gastric juice may enter the duodenum, where their digestion is completed by the pancreatic juice. The latter saponifies the fat which is absorbed by the lacteals of the villi. In the digestion experiments of C. H. Woods and L. H. Merrill (U. S. Department of Agriculture Bulletin No. 85, 1900) the digestibility of milk casein was always increased by the addition of bread to the milk.

Rennin

Rennet is the name given to an infusion of the middle stomach of the calf in brine which possesses special milk-curdling power which is due to a ferment called rennin.

This milk-curdling ferment is believed to be developed by the action of acids from an antecedent material called zymogen. Rennin is not obtainable from the human stomach in large quantity, but it undoubtedly exists in the gastric juice, for pure pepsin has scarcely any curdling action upon milk; moreover, milk may curdle in either neutral or alkaline media in which pepsin is inert. Rennin is most active in an acid medium, but it also works in the alkaline pancreatic juice. It possesses little proteolytic power. It is believed to exist more abundantly in the infant stomach than in that of the adult.

Rennet may be prepared as a powder; it is sold by grocers in alcoholic solution, and when added to milk which is gently warmed, a light coagulum is formed, causing the milk to partially solidify. The coagulum contracts and squeezes out fluid, which constitutes whey. The rennetised milk when flavoured with vanilla or powdered cinnamon or nutmeg makes an agreeable invalid food or dessert, which is highly nutritious, for it still contains all the original ingredients of milk.