58 MODERN METHODS OF TESTING MILK 



the action of the acid is much the same as if it were 

 too strong, producing dark-colored specks or a dark- 

 ened appearance in and below the fat-column. Ex- 

 perience shows that when the milk and acid are at a 

 temperature between 60° and 70° l\, there is no dan- 

 ger of too slight or too strong action of acid. More 

 acid can be used at lower temperatures and less at 

 higher temperatures with satisfactory results, but this 

 involves experimenting ; the best way w^ill be to use 

 the regular amount of acid and regulate the tempera- 

 ture of the milk and acid. 



MIXING MILK AND ACID IN TEST-BOTTLE 



When the measured amount of acid has been placed 

 in the test-bottle, die acid and milk should be thor- 

 oughly mixed. This is best done by giving the bot- 

 tle a rotary motion, with gentle shaking, until the 

 whole mass becomes liquid and free from solid parti- 

 cles of casein. Much motion up and down should be 

 avoided, since milk might be thrown up into the neck 

 of the bottle beyond reach of the acid, in which case 

 coagulated casein would contaminate the fat-column 

 and impair the results. 



When the acid and milk first mix, the casein is 

 coagulated in a somewhat solid mass, which gradu- 

 ally redissolves as the mixing becomes complete. The 

 mixing, once begun, should continue until the casein 

 appears to be redissolved. If the operation of mixing 

 milk and acid is incomplete or is interrupted, black 

 particles may appear in the fat-column at the end of 

 the test. 



It is a wise precaution to allow the bottle to stand 



