1 82 MILK 



The Acidity of Milk. 



Fresh milk always shows a distinct acidity with regard to 

 phenolphthalein, owing to the presence in it of calcium hydrogen 

 phosphate and dissolved carbon dioxide. 



On keeping milk, the acidity is found to increase owing to the 

 production of lactic acid by fermentation of the milk sugar. 



The quantitative determination of the acidity of milk is of 

 importance as it affords valuable indication as to how far the 

 lactic acid fermentation has proceeded, and in the case of cream 

 for butter-making, the process of " ripening " can be followed. 



The acidity of milk is best determined by direct titration 



with — caustic soda, using phenolphthalein as indicator, 

 lo 



Although it is somewhat difficult to detect the first appearance 



of the pink coloration owing to the opaqueness of the milk, the 



latter should not be diluted to remedy this, as the addition of water 



brings into solution certain alkaline phosphates which have a 



marked effect in decreasing the acidity. 



179. • Determination of Acidity. 



Fifty c.c. of fresh milk are placed in a conical flask, 



a few drops of phenolphthalein added, and directly 



N 

 titrated with — caustic-soda solution. In order to 



lO 



facilitate the detection of the pink colour, the titration 



should be carried out while holding the flask over a 



piece of white (or cream) paper. As the end-point of 



the titration is approached, the flask should be inclined 



after each addition of alkali, as it is easier to detect the 



coloration in a thick layer of liquid. 



N 

 The number of c.c. of — alkali required multiplied 



by 2 will give the amount required for lOO c.c. of milk. 

 This number is called the degree of acidity. Fresh milk 

 generally has about i8° to 19° of acidity, and this value 

 increases to a marked extent on keeping. To show 

 this, a sample of the same milk should be kept in a 



