196 ELEMENTAEY AGRICULTURAL CHEMISTRY 



But it is impracticable to keep milk free from access of 

 micro-organisms, especially such widely distributed ones as the 

 lactic bacteria. 



The methods to be adopted in preserving milk are, there- 

 fore, based upon either the destruction of the micro-organisms 

 which have entered it or the prevention of their growth. 



The second of these methods, the prevention of the growth 

 of the micro-organisms, cannot be done perfectly, but if the 

 temperature of the milk can be kept low the growth is slow 

 and the milk will keep for some days. Rapid cooling, after 

 milking is of great importance, as multiplication of the 

 micro-organisms goes on very quickly in warm new milk. 



In order to destroy the organisms which have gained access 

 to milk recourse may be made to either of two methods : 



1. Sterilisation by heat. 



2. The use of antiseptics. 



To effect complete sterilisation — i.e., the destruction of all 

 bacteria and their spores — by heat requires a high temperature 

 (about 115° C), which can only be applied to milk under 

 pressure, and unfortunately this produces undesirable chemical 

 changes in the milk. Some of the sugar is turned brown, the 

 albumen and a portion of the calcium citrate are precipitated, 

 a peculiar burnt or cooked flavour is imparted and the casein 

 becomes less readily coagulable by rennet. The fat rises more 

 slowly and a very small quantity of richer cream is obtained. 



In order to avoid these disadvantages a modified process 

 known as Pasteurisation is often substituted for sterilisation. 

 The milk is heated to only G0° or 80° C, whereby its flavour 

 is hardly affected and the active bacteria are killed, though 

 the spores are not. It fortunately happens that the micro- 

 organisms to which the souring of milk is due- viz., the 

 lactic bacteria — do not readily form spores, so that Pasteurised 

 milk will usually keep sweet and good for some days. Occa- 

 sionally, however, spore-forming bacteria are present in milk, 

 and it may happen in such cases that the milk very soon 



