342 MILK AND MILK PRODUCTS. XIV. 



since usually it is desirable to hinder as much as possible the 

 growth of bacteria. The micro-organisms which find their 

 way into milk are of various types ; in most cases, perhaps, the 

 majority are such as convert milk sugar into lactic acid, while 

 organisms of almost all kinds may be found, many objection- 

 able and some highly dangerous. The first evident effect of 

 the growth of the organisms is usually the souring and con- 

 sequent curdling of the milk. 



It is obvious, therefore, that if the milk is to be kept for any 

 length of time some means of preventing the growth of these 

 organisms must be taken. Two chief methods suggest them- 

 selves : (1) To prevent the entrance of the germs, or (2) to 

 iise some means which will either kill them or prevent their 

 growth. 



The first method is almost impracticable on the large scale, 

 but the second is often adopted. This is either by " sterilisa- 

 tion" or "Pasteurisation." 



In the former process the milk is heated to a sufficiently 

 high temperature (about 115 C.) to destroy all germs. This is 

 usually effected in steam under pressure. The milk is then 

 kept excluded from the air, or air which has filtered through a 

 thick layer of cotton wool may be admitted. Milk so pre- 

 pared will not sour and may be kept indefinitely. Unfortu- 

 nately certain undesirable changes are produced in the milk 

 by this treatment. The taste and smell are altered, a portion 

 of the calcium citrate and the albumin are precipitated, and 

 the casein (probably by the precipitation of the calcium com- 

 pounds) becomes much less coagulable by rennet. Moreover, 

 the milk becomes brown, and the enzyme originally present in 

 the milk and which has the power of giving a blue coloura- 

 tion with hydrogen peroxide and para-phenylene diamine, 

 CH 4 (NH 2 ) 2 , is destroyed. The fat rises much less readily, and 

 the cream layer, though very thin, is richer in fat than ordi- 

 nary cream, containing often over 40% instead of 20 to 30% 

 as in the case of fresh cream. 



In Pasteurising, a lower temperature about 60 or 80 is 

 employed, and the milk is subjected to this two or three times, 

 with intermediate cooling. The taste and properties of the 

 milk are not so much altered by this treatment, but the 



