22 PEOTBIDS. 



trary 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 form- 

 ation 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.^ Further, 

 the clot is entirely different from casein : jt is mucli less soluble 

 ^i acids and alkalis than the latter,* always leaves as ordinarily 

 prepared a large and constant residue of. ash (calcium phosphate) 

 on ignition, and even it it ~5e~Treed 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 neu- 

 tral salts, have resulted in tbe 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 

 casern! Casein freed from this salt and dissolved in dilute alkali 

 wlfl not yield a clot ; dialysed milk similarly yields no clot, but 

 if the dialysate be concentrated and added to the milk it now 

 clots on the addition of fenrTtTT vviien pure casein is dissolved in 

 iTme^water and neutralisecTwith phosphoric acid it ndw 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 oQlie 

 altered product. 6 JN either Is the calcium salt alone essentiaTTfor 

 be replaced, but with less efficient results, Jjv the similar 



salts of magnesium, barium, and strontium. 7 



The question as to the identity or the reverse of casein and 

 alkali-albumin as obtained by the action of alkalis on other pro- 

 teids has given rise to much controversy. Some authors have 



1 Soxhlet, Jn.j. pr. Chem. Bd. vi. (1872), S. 1. 



2 Hammarsten, Maly's Ber 11. (1872), S. 118, iv. (1874), S. 135. Heintz, Jn. f 

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



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

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



5 Koster; foe. cit. S. 14. 



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

 casein see Ringer, Jl. of Physiol. Vol. xi. (1891), p. 464, xii. (1891), p. 164. 



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



