208 MILK AND THE PUBLIC HEALTH CHAP. 



after cooling, definite fractions of raw milk were added to 

 portions of it and well mixed. The following results were 

 obtained : 



The results obtained show that when pasteurisation is 

 carried out at the lower temperatures, such as 60 and 70 C., 

 this fact cannot be detected by any of the above tests. The 

 last table also shows that, even if the pasteurisation has been 

 carried out at high temperatures (e.g. 80 C.), the operation 

 can be effectively disguised, as far as these ferment-tests are 

 concerned, by the addition of 10 per cent of -raw milk. This 

 fact is of very great importance in considering the applicability 

 of using bacteriological standards to judge the cleanliness of 

 collection of rnilk supplies. 



THE DETECTION OF PRESERVATIVES 



The detection of preservatives in milk, although a strictly 

 chemical procedure, is one intimately related to the bacterio- 

 logical examination, since their presence may exert a profound 

 influence upon the bacterial content and totally mask any 

 bacteriological evidences of want of cleanliness in preparation 

 or of staleness of the milk. A bacteriological examination of 

 milk should therefore include an examination for preservatives. 

 The following preservatives may have to be looked for : 



Boric Acid and Borates. Evaporate 15 to 20 c.c. of milk 

 to dryness in a platinum or porcelain dish, first making 

 alkaline with dilute sodium hydrate solution, or preferably 

 baryta water. Ignite and then dissolve the ash in a little 

 hydrochloric acid. Filter and evaporate filtrate to dryness. 

 Moisten the residue with a very little dilute hydrochloric acid 

 solution and dissolve in a little water. Soak a piece of 

 turmeric paper in the solution, then remove and dry in the 

 hot-air oven. A red colour indicates the presence of boric 



