NATURAL AND ARTIFICIAL CcRDLING OF MILK. 567 



tartar, and salt cf sorrel have been occasionally used, and in Switzerland 

 — especially in the manufacture of the schabzieger cheese — it is cus- 

 tomary to add merely a little sour milk for the purpose of producing the 

 curd. 



3°. Chemical action of the acid. — But how does the acid act in causing 

 the milk to curdle, and why is it necessary to allow a little time to 

 elapse and to apply ^Blso a gentle heat before the curd will completely 

 separate ? 



In regard to casein or the cheesy matter of milk, we have seen (p. 

 561)— 



a. That though nearly insoluble in pure water, it dissolves readily in 

 water containing in solution a small quantity of potash or soda, either in 

 the caustic or carbonated state. In other words the casein, which is an 

 acid substance, unites chemically with the potash or the soda, and forms 

 a compound ivhich is soluble in water. 



h. That when an acid is added to this solution, it takes the potash or 

 soda from the casein and combines with it, leaving the curd again in its 

 original insoluble state, and causing it, therefore, to separate from the 

 water. 



Now in milk, as it comes from the cow, the casein is in chemical 

 combination with a small quantity of soda, by which it is rendered so- 

 luble in the water of which the milk chiefly consists. When the milk 

 stands for a time in the air, the sugar of milk, as we have seen, is trans- 

 formed into lactic acid — this acid takes the soda from the casein, and 

 forms lactate of soda, and the cheesy matter, in consequence, being itself 

 insoluble in water, separates in the form of curd. The application of a 

 gentle heat acts in two ways. It aids the acid in more completely taking 

 the soda from the casein, and causes the latter at the same time to 

 shrink in, to become less bulky, and thus to separate readily from the 

 whey. 



If we add an acid artificially to milk, the effect is exactly the same. 

 Either muriatic acid, or tartaric acid, or vinegar, or sour milk, will, in the 

 same way, take the soda from the casein, and render it insoluble. And 

 that this is the true action is readily proved by adding a little soda to 

 curdled milk, when the curd will be re-dissolved, and the milk will be- 

 come sweet. Add acid to it now, or let it sour naturally a second time, 

 and the curd will again be separated. 



The action of rennet is in some degree different, though no less simple 

 and beautiful. Let us first, however, consider what rennet is, and how 

 \{. is prepared. 



§ 18. Of the preparation of rennet. 



Rennet is prepared from the salted stomach or intestines of the suck- 

 ling calf, the unweaned lamb, the young kid, or the young pig.* In 

 general, however, the stomach of the calf is preferred, and there are 

 various ways of curing and preserving it. 



1°. Preparing the stomach. — The stomach of the newly killed animal 

 contains a quantity of curd derived from the milk on which it has been 

 fed. In most districts (Switzerland, Gloucester, Cheshire) it «6 usual to 



• Dried pig's bladder is often employed instead of the dried kid'a stomach for curdling the 

 goat's milk on Mont Dor. 



