STERILISED MILK. 395 



Sterilised Milk. 



Milk is a product which affords all the necessary nourishment 

 for the growth of micro-organisms ; these not only develop 

 products which cause alteration of the milk e.g., lactic acid and 

 proteolytic enzymes but also are in some cases injurious to 

 health. 



They are destroyed by heat. Hence milk is frequently 

 " sterilised " by heat, the object being to bring about the de- 

 struction of the micro-organisms. 



Many processes are used. Pasteur originally recommended 

 heating to 70 C. for a short time, a process which was sufficient 

 to destroy all adult forms of pathogenic organisms and, practi- 

 cally, all others. The spores, however, were left untouched and 

 retained their vitality ; on cooling to the mean air temperature 

 these developed into the adult forms and resumed their activity. 

 To destroy the spores, a process of continued " pasteurisation " 

 has been used. This consists of alternately heating to 70 C. 

 for, say, twenty minutes ; cooling to a lower temperature and 

 keeping at this temperature for a Sufficient length of time to 

 allow the spores to develop ; again heating to 70 ; and repeating 

 this process many times. By this process, which is very tedious, 

 the taste and composition of the milk undergo but little alter- 

 ation. 



It has been found that most spores can be killed by continued 

 exposure to 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 



