PRESERVATIVES 297 



Incubate all at 35 C. for 48 hours. 



Count the colonies on the several plates, and note the effect 

 of the heating on the numbers of bacteria in the milk. 

 Observe which colonies are acid on each plate. 



(iii) Preservatives. The use of saltpetre, salt, and 

 other substances for the purpose of preserving all kinds 

 of food from putrefaction and other fermentative changes 

 is widespread. Many attempts have been made also to 

 check the souring of milk by the addition of certain 

 chemical compounds which in small doses are known to 

 inhibit bacterial growth. Such powerfully poisonous 

 bactericides as mercuric chloride or carbolic acid cannot 

 of course, be used for this purpose, but it is unfortunately 

 too often the case that antiseptics with deleterious 

 properties are added to milk by farmers and dairymen. 

 The substances most commonly employed are boric acid, 

 borax, salicylic acid, and formalin ; in most countries 

 their use is illegal, and infractions of the law should be 

 punished. 



All materials which effectually check the development 

 of bacteria are certain to impair the digestive functions, 

 particularly of infants and young children, whose diet 

 consists largely of milk. There is little doubt that the 

 continued use of small amounts of these compounds 

 would result in damage to the health of adults also if 

 milk were extensively used as a food by such persons. 



The addition of small quantities by the farmer has 

 been repeated by the dealer and dairyman, and in some 

 instances by the householder as well, so that before the 

 milk was consumed it contained pernicious doses of these 

 harmful substances. 



Apart from the directly injurious effects of preserva- 

 tives upon the health of the community, their use 



