144 MILK [vin 



alkaline reaction is not capable of being coagulated 

 by rennet. A very slight acid reaction, on the other 

 hand, assists its action, which is largely influenced 

 by temperature, the most favourable temperature 

 being 30 C. (86 Fahr.). A temperature of 60 C. 

 (140 Fahr.) destroys its action. Milk which has 

 been boiled generally loses its power of being precipi- 

 tated by rennet, but such milk regains its suscepti- 

 bility at once if calcium chloride or other soluble 

 lime salt be added, or if the lime salts precipitated 

 by boiling be again dissolved by the addition of a 

 dilute acid. 



It has been sometimes noticed that even fresh 

 milk is not coagulated by rennet. Investigation 

 would probably show that such milk, owing to dis- 

 turbance in the milk-gland, exhibits a slightly alka- 

 line reaction, and does not contain soluble lime salts. 



It has already been stated that casein in milk 

 exists in a semi- dissolved or colloidal form. This 

 semi-soluble condition of casein is due to the fact 

 that it is combined in some way with lime. Accord- 

 ing to this view, the casein present in milk may be 

 regarded as of the nature of a salt, in which the 

 casein proper takes the part of the acid, and the lime 

 that of the base. The coagulation which results on 

 the addition of an acid may thus be explained by 

 the decomposition of this body, the lime being 

 neutralised by the acid and the casein set free in the 





