APPLICATION OF ANALYSIS 57 



The Cause of Sour Milk. Practically the only 

 cause of milk turning sour is the formation of lactic 

 acid by the action of micro-organisms. Milk curdles on 

 boiling when the acidity reaches about 33, and spon- 

 taneously in the cold when the acidity is about 80 ; 

 milk curdled by boiling generally contains somewhat 

 hard lumps of curd, and the acidity of the whey may 



* 



& :', 



>* f '** 



fc ak 



A: w 



FIG. 29. Blood in Milk 



be 25 and upwards ; at lower temperatures the curd is 

 softer, and if the acidity is appreciably below 80, this 

 indicates that the milk has been kept warm, but not 

 warm enough to inhibit microbial action. 



Freshness of Milk. The freshness of milk may 

 be determined by an acidity estimation ; if the acidity 

 is more than 2' higher than the aldehyde figure, it may 

 be assumed thatr a development of acidity due to the 

 action of micro-organisms has taken place. 



Many micro-organisms secrete a reductase, whioh 

 decolourises methylene blue ; make a solution by di- 

 luting i c.c. of saturated alcoholic solution of methylene 

 blue to 100 c.c. with distilled water, add i c.c. of this to 

 25 c.c. of milk contained in a stoppered tube of little 



