CHEMICAL BASIS OF THE ANIMAL BODY. 21 



criticised these views and concludes that casein is a unitary substance and not a 

 mixture or compound. 



Action of rennin on casein. This has been fully studied by Ham- 

 marsten whose results may be summarised as follows. Contrary to the 

 older views that the formation of the clot is rather of the nature of a 

 precipitation 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 (tyrein) differing 

 essentially from casein. It had been considered that the separation of 

 the clot was due to the formation of lactic acid from milk-sugar 1 , but 

 this is not so 2 ; pure casein free from every trace of lactic acid clots 

 with rennin. 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 3 . Further 

 the clot is entirely different from casein : it is much less soluble in acids 

 and alkalis than the latter 4 , 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 5 and dissolved in 

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

 action of rennin. 



It may be remarked here that no efforts to obtain a * curd ' from milk by purely 

 chemical means, such as the addition of acids or neutral salts, have resulted in the 

 production of a substance which by further treatment can be made to yield a 

 typical ripening ' cheese.' The latter can only be made by the use 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 ; dialysed milk similarly yields no clot, but if the dialy- 

 sate be concentrated and added to the milk it now clots on the 

 addition of rennin. When pure casein is dissolved in lime-water 

 and neutralised 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 ferment on the casein, but rather the 

 subsequent separation from solution of the altered product 6 . 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 7 . 



1 Soxhlet, Jn.f. pr. Ghent. Bd. vi. (1872), S. 1. 



2 Hammarsten, Maly's Ber. n. (1872), S. 118, iv. (1874), S. 135. Heintz, Jn.f. 

 prakt. Chem. N. F. Bd. vi. (1872), S. 374. 



3 Hammarsten. See also Koster (Swedish) in Maly's Ber. Bd. xi. (1881), S. 14. 



4 Al. Schmidt, Beitr. z. Kennt. d. Milch, Dorpat, 1874. 



5 Koster, loc. cit. S. 14. 



6 For further observations on the influence of salts on the clotting of milk and 

 casein see Kinger, Jl. of Physiol. Vol. xi. (1891), p. 464, xn. (1891), p. 164. 



7 Lundberg (Swedish). See Maly's Ber. Bd. vi. (1876), S. 11. 



