SO-CALLED ' BIOLOGICAL PROPERTIES OF MILK 65 



sources of contamination since milking. The bacteria which are 

 present are capable of producing different varieties of ferments 

 in considerable quantities. Evidently, if investigations are being 

 made into the ferments which are present in milk as such, it is 

 essential that the presence of ferments due to subsequent bacterial 

 action should be eliminated. This consideration did not, however, 

 receive due attention among the earlier observers, and in a large 

 number of instances the milk used for investigation was collected 

 with no special precautions, and the bacterial content, which was 

 probably considerable, was not taken into account. In some 

 cases the observers appear to have been conscious of a defect 

 in their investigations, and endeavoured to kill the bacteria by , 

 introducing antiseptics before they proceeded with their investiga- 

 tions upon the presence of ferments. This method, although it 

 may have been successful in killing the bacteria, did not remove 

 the ferments which had been manufactured by them before they 

 were killed. 



A great many investigations have been carried out with a view 

 to discovering a method of detecting whether milk had been boiled 

 prior to being sold. In 1898, when the campaign against tuber- 

 culosis in cattle was commenced in Denmark, regulations were 

 issued for compulsory pasteurisation, in order to limit the spread 

 of tubercular disease by means of milk. It was necessary to pro- 

 vide a test which would ensure the fulfilment of the prescribed 

 pasteurisation, and such a test was based upon the presence in 

 milk of a ferment which gave a specific colour reaction. This 

 ferment, now known as peroxidase, gives a peculiar grey-blue 

 colour when treated with paraphenylene-diamine and hydrogen 

 peroxide. If the milk has been boiled, this reaction is modified 

 or destroyed. Storch, who first worked out this reaction, believed 

 that it was reliable for differentiating between milk which had 

 been heated to 80 C, and above, and milk which had not been 

 heated to so high a temperature. This reaction, about which 

 more will be said later, is very generally known as Storch's reaction 

 or test. 



Efforts have also been made to utilise a colour reaction produced 

 by a ferment for estimating the number of bacteria present. A 

 considerable number of bacteria have the property of ' reducing ' 

 methylene blue. By their action, they slightly alter the chemical 

 composition of this substance, and the colour completely disappears, 

 the milk to which this substance has been added becoming white. 



Efforts have been made to utilise the ferment action of milk 

 for yet another purpose, namely, to discover the presence of milk 

 from a diseased cow when the milk from such a cow has been added 

 to a general milk supply. A different ferment, catalase, has been 

 investigated in connection with this matter, and is one which acts 

 upon hydrogen peroxide with the evolution of oxygen in the form 

 of gas. This gas can be collected over the surface of the milk, and 



