488 DIGESTION 



Clotting of Milk in the Stomach 



Besides its power of digesting protein, the gastric juice is also endowed 

 with the property of clotting milk. This action is commonly attributed 

 to the presence of another enzyme besides pepsin, namely, rennin; but 

 in recent years considerable controversy has raged around the question 

 as to whether pepsin and rennin are not the same thing. One strong 

 argument in favor of this view is that all digestive juices that are capable 

 of digesting protein can also clot milk. In any case, when gastric juice 

 acts on milk, it splits the casein* of the milk into two portions, one of 

 which, called paracasein, immediately combines with calcium to form an 

 insoluble colloidal compound, which is precipitated and, by entangling 

 the fat of the milk, forms the clot; the other protein remains in solution 

 and is known as whey albumose. From studies on molecular weight it 

 is believed that the paracasein is produced from casein by the splitting 

 of the molecule of the latter into two, from which it would appear that 

 the action of this enzyme is nothing more than the first stage in the 

 hydrolysis of the casein molecule. ' The whey albumose, according to this 

 view, is a by-product. 



There are many investigators, however, who believe that rennin and 

 pepsin are not identical, since an infusion of the stomach of a calf has a 

 powerful clotting action on milk but a very weak digestive one on egg 

 white, whereas a similar infusion from the stomach of a pig shows exactly 

 the reverse properties. This question is one of so controversial a na- 

 ture that it would be out of place to go into it further here. It 

 should be pointed out, however, that, when the gastric contents are acid 

 in reaction, milk will become clotted by the action of the acid itself 

 quite independently of any pepsin or rennin the juice may contain. 

 This acid clotting of milk is probably of a different chemical nature 

 from that produced by the enzymes. 



On other foodstuffs than proteins the action of the gastric juice is 

 relatively unimportant, although polysaccharides may be considerably 

 broken down in the cardiac end of the stomach on account of the action 

 of swallowed saliva (see page 454), and disaccharides, as we have seen, 

 may become split by the hydrolyzing effect of the hydrogen ion. Fat 

 digestion also takes place in the stomach when the fat is taken in an 

 emulsified condition, as in milk and egg yolk, but not when in masses, 

 as in meat or butter. This action is due to the presence of a fat-splitting 

 enzyme, or lipase, in the gastric juice. 



,*In the above nomenclature casein is the same as caseinogen, and paracasein the same as casein, 

 of the English physiologists. 



