1208 CASEIN. 



Action of rennin on casein. Contrary to the older views that 

 the formation of the clot is rather of the nature of a precipita- 

 tion than a true ferment action, we now know that by the action 

 of rennin the clotting of casein is due to a specific action of 

 the enzyme which results in the formation of a substance (which 

 may be conveniently called tyrein) differing essentially from 

 casein. The specific action of the enzyme is further shewn by 

 the fact that simultaneously with the formation of the clot, a 

 by-product is formed having the properties of a soluble albumin. 

 Further the clot is entirely different from casein: it is much 

 less soluble in acids and alkalis than the latter, always leaves 

 as ordinarily prepared a large and constant residue of ash 

 (calcium phosphate) on ignition, and even if it be freed from 

 the calcium salt by special methods and dissolved in dilute 

 alkalis, is not capable of being made to yield a clot by the 

 renewed action of rennin. 



The calcium salt plays an all-important part in the clotting 

 of casein. Casein freed from this salt and dissolved in dilute 

 alkali will not yield a clot; dialyzed milk similarly yields no 

 clot, but if the dialysate be concentrated and added to the milk 

 it now clots on the addition of rennin. When pure casein is 

 dissolved in lime-water and neutralized with phosphoric acid 

 it now clots with rennin. The action of the salt in the whole 

 process appears to be that it determines not so much the action 

 of the enzyme on the casein, but rather the subsequent separa- 

 tion from solution of the altered product. Neither is the calcium 

 salt alone essential, for it may be replaced, but with less efficient 

 results, by the similar salts of magnesium, barium and strontium. 



After the removal of casein from milk by precipitation, the 

 filtrate contains a small amount of coagulable proteid sometimes 

 spoken of as 'lactalbumin,' closely resembling serum-albumin 

 in its general properties, but differing slightly as to its specific 

 rotatory power and the temperature at which it coagulates when 

 heated. Also a minute quantity of a globulin closely similar 

 to that of blood serum. 



When milk is kept for some time at a temperature above 

 50° and below its boiling point, a firm skin is formed over its 

 surface composed largely of casein. Its formation is not to be 

 regarded as being specially characteristic of milk, for pure 

 casein dissolved in dilute alkalis exhibits the same phenome- 

 non, as also do alkali-albumin, chondrin, gelatin and the filtrate 

 from 1 p.c. starch when it is concentrated on a water-bath. Its 

 formation is probably due to the rate of evaporation from the 

 surface of the milk being more rapid than the fluid diffusion 

 into the upper layer; and in accordance with this it is found 

 that its appearance is considerably facilitated by blowing a 

 rapid stream of air or any indifferent gas over the surface of 

 the warmed milk. 



