384 MILK AND THE PUBLIC HEALTH CHAP. 



been added almost all exert a direct inhibitory action upon 

 bacteria, but one of them (sodium carbonate), sometimes found 

 in milk, is added not so much to check bacterial growth as to 

 neutralise the acidity resulting from bacterial growth. 



The preservatives most frequently added to milk are 

 formaldehyde and boric acid or one of its compounds. Pre- 

 servatives occasionally added are benzoic acid, salicylic acid, 

 acid potassium fluoride, sodium carbonate, and hydrogen 

 peroxide. 



As regards efficiency there is no doubt that by the use of 

 chemical preservatives the development of bacteria can be 

 retarded or even absolutely inhibited. It is merely a question 

 of dosage. As actually used in milk, preservatives are 

 employed not to inhibit all bacterial growth, but to retard it 

 so that the milk will keep good for a further period of 

 12 to 24 hours. The quantities of preservative necessary 

 to do this have been studied by several workers, chiefly in 



regard to boric compounds and formalin, 

 the following table : 



Eichmond 



gives 



Table of the Length of Time in Hours that Milk will keep at 

 various Temperatures when Preservatives are added. 



It is assumed that milk turns sour when an acidity 

 increase of 25 degrees has taken place (i.e. 100 c.c. of milk 

 require 25 c.c. more N/10 soda solution to neutralise to 



1 Report of Departmental Committee on the Use of Preservatives, etc., in Food, 

 1901, p. 409. 



