PRESERVATION OF MILK 87 



are more or less completely inhibited or destroyed after they 

 have gained access to the milk. 



Various chemical and physical agents are made use of to 

 destroy the bacteria in milk or to prevent their growth. The 

 chemicals used are those known as antiseptic substances, 

 formalin, borax, boracic acid, or proprietary compounds which 

 contain one of these antiseptic substances. Other substances, 

 such as sodium carbonate, by neutralizing the acid produced, 

 lengthen the time before the milk acquires an acid taste. 

 Others, such as hydrogen peroxide, act even more powerfully, 

 destroying the bacteria without altering the taste or appear- 

 ance of the milk, if they are not used in too large amounts. 



The use of practically all chemicals for the preservation of 

 milk is classed as an adulteration and is forbidden by law. 

 The same objection cannot be urged against the use of physi- 

 cal agents, such as heat and cold, employed separately or in 

 combination. The attempt has been made to freeze the milk 

 wholly or in part (Casse system), but this process has not met 

 with much success. Eefrigeration or icing, so as to lower the 

 temperature to 4.5 C. or below, is most successful. While such 

 temperatures prevent in considerable measure the growth of 

 the lactic organisms, there are some species of bacteria capable 

 of multiplying at even very low temperatures (0-1.5 C.). In 

 refrigerated but unfrozen milk these may develop abundantly, 

 although usually they do not alter the normal characteristics 

 of the milk (taste, smell, etc.) to any great extent. 



By the use of high temperatures it is possible to destroy 

 effectively the bacteria present. The temperature at which 

 any form is destroyed is known as its thermal death point. 

 This temperature varies greatly with the organism, depend- 

 ing upon its ability to produce endospores. The exact point 

 is also correlated with the length of time for which the 



