DETECTION OF DETERIORATION 227 



ous substances, absorption of strongly odorous matter, 

 addition of foreign matter, bitter, stale, sour, microbic 

 changes) must not be sold and should be condenmed. 



b. Determination of the reaction. This may be done by 

 using red and blue litmus paper. Normal, quite fresh 

 milk has an amphoteric reaction. If the test shows ex- 

 cessive alkaline or acid reaction, it should be examined 

 more closelj". An alkaline reaction is frequently shown 

 in the milk of old milking cows; mixed milk, on the 

 contrary, is never alkaline. Sometimes an alkali is 

 added; if so, it can be proven by the test previously 

 mentioned, or, quite easily, })y titrating with tenth- 

 normal sodium hydrate solution, with which phenol- 

 phthalin is used as indicator. Normal milk shows an 

 acid reaction to phenolphthalin, so that from IS to 19 

 c.c. of tenth-normal sodium hydrate solution must be 

 added to 100 c.c. of milk to make the red color apf>ear ; 

 if the milk becomes red at once when phenolphthalin is 

 added, then it is abnormally alkaline and, as a rule, alkali 

 lias been added; if a smaller quantity of sodium hydrate 

 solution is necessary, the milk may either have been 

 treated with alkali or diluted with water. 



If the litmus paper indicates an acid reaction, the 

 degree of acidity of the milk may be determined hj 

 means of titrating with tenth-normal solution of so- 

 dium li}'drate, using phenolphthalin as indicator. If, to 

 neutralize 100 c.c, requires more than IS to 19 c.c. of 

 tenth-normal sodium hydrate solution, then the for- 

 mation of lactic acid has commenced, and its extent can 

 be measured exactly by determining the amount of 

 standard alkali re(|uired to neutralize it. 



For titrating, an ordinary burette is used with a scale 

 divided into spaces sliowmg 0.1 c.c. After the addition 

 of about 0.25 c.c. phenolphthalin solution to the milk, the 

 standard sodium solution is dropped in, little by little, 



