50 PRACTICAL AGRICULTURAL CHEMISTRY ("138-139 



oxidise the lactose, producing lactic acid. This acid will 

 curdle the milk, as described in paragraph 133. 



The milk may be prevented from ' going sour ' in various 

 ways : 



(a) By heat or cold, thus destroying the bacteria or 

 suspending their activity. 



(6) By adding an alkali, e.g. carbonate of soda, so 

 neutralising the acid as it is produced. 



(c) By adding a 'germicide,' such as borax, boracic 

 acid, or formalin, to poison the bacteria. 



As, however, it is illegal to add any foreign substances 

 to milk, heat and cold are the only two methods officially 

 recognised. 



138. Preservation of Milk.— Half fill six test-tubes 

 with milk, and treat as follows : Allow the first portion 

 to remain in the test-tube stand without any further treat- 

 ment. Boil the second portion and place it also in the 

 test-tube stand. To the third add a few drops of forma- 

 lin (40 per cent, formic aldehyde) ; to the fourth add a 

 few drops of strong sodium carbonate solution ; and to 

 the fifth a few drops of borax solution. The sixth must 

 be placed in an ice-chest so that it may remain at a low 

 temperature. 



The milk should be examined daily, and the order in 

 which the different samples turn sour noted. The record 

 so obtained will give an indication of the efficiency of the 

 various methods employed. 



139. Tests for Preservatives in Milk.— The pre- 

 servatives usually found in milk are boracic acid, borax, 

 and formalin. The following tests should be applied to 

 samples of fresh milk; if no results are obtained, they 

 should be repeated with samples treated as described in 

 paragraph 138. 



Boracic Acid or Borax. — Make the milk alkaline with 

 lime water, evaporate to dryness and ignite until the re- 



