STERILISED MILK. 193 



exposure at higher temperatures. The temperature of boiling 

 water is one much used, as it can be easily attained, but higher 

 temperatures are sometimes resorted to by heating the milk 

 under pressure ; the higher the temperature, the shorter the 

 time necessary to kill all microbial life. Another method 

 adopted is to alternate successive short periods of heating to 

 high temperatures with intervals during which the milk is kept 

 at the ordinary temperature. Numerous modifications of these 

 methods have formed the subjects of patents. 



Analytical Characters. — As, practically, no milk sterilised 

 by successive heating to a temperature not exceeding 70^ C. is 

 sold commercially, it will be sufficient to describe the methods 

 for characterising milk which has been heated above the coagu- 

 lating point of albumin. 



The most marked characteristic distinguishing sterilised milk 

 from new milk is the state in which the albumin exists. As 

 previously stated, it is probable that albumin exists in milk in 

 combination with a base ; on heating milk, no coagulation of 

 albumin takes place, but on acidifying, or saturating with mag- 

 nesium sulphate, the albumin separates with the casein. The 

 albumin appears to be changed from a soluble to a colloidal 

 form. (Not more than O'l per cent, of albumin is found in steri- 

 lised milk in the soluble form.) The casein separates on acidi- 

 fying in a more finely divided state. 



If the milk has been heated to 100° C. or a higher temperature 

 for any length of time, the rotatory power of the milk-sugar 

 undergoes a serious reduction, the cupric reducing power not 

 changing to any appreciable extent. The milk also assumes a 

 slight brownish colour, due probably to the formation of a 

 " caramelised " body of low rotatory power. 



The cream rises with extreme slowness ; in three hours, prac- 

 tically no cream is observed on the surface of the milk ; and 

 after six hours, the layer is only about one-tenth of that given 

 by new milk. If sterilised milk be allowed to stand for twenty- 

 four hours or more the bulk of the cream will rise to the surface, 

 but the quantity will be less than that yielded by new milk : 

 the cream will, however, contain a distinctly larger percentage 

 of fat, about 40 per cent., as against less than 30 per cent, in the 

 cream yielded by new milk. 



The diminished yield of cream is a property shared also by 

 milk which has been pasteurised by heating to about 70°, but 

 the rate of rise of cream in pasteurised milk is fairly rapid ; 

 practically the same amounts are found in three hours as in six 

 hours. The total quantity of cream from pasteurised milk is 

 about half that of fresh milk. 



The following figures, which, together with those in Table 



