194 MODERN METHODS OF TESTING MILK 



sufficient to make the milk neutral. One can usually 

 add 2 or 3 cc. of alkali at the start and then add it in 

 smaller portions, until the milk begins to show signs of 

 neutrality, after which the alkali is added a drop at a 

 time. 



(3) Precipitation of casein. This step of the pro- 

 cess needs to be considered in relation to the addition 

 of acid and temperature. 



(a) Addition of acid. Into the neutralized sample 

 of diluted milk, which should be at a temperature of 

 60 to 80 F., one now runs from a burette some of the 

 standardized acetic acid, adding the acid approximately 

 in 5 cc. portions and agitating vigorously for a few se- 

 conds after each addition. It is usually safe to add 

 about 25 cc. of acid before examining the milk to see 

 if the casein separates in the form of white flakes. 

 After adding 20 to 25 cc. and shaking, the mixture is 

 allowed to come to rest. If enough acid has been ad- 

 ded, the casein separates promptly in large, white 

 flakes, and, on standing a short time, the liquid above 

 the settled casein appears clear and not at all milky. 

 If the addition of 25 cc. of acid is insufficient to sepa- 

 rate the casein properly, add I cc. more of acid and 

 shake; continue the addition of acid, I cc. at a time, 

 until the casein is observed to separate promptly and 

 completely on standing at rest a short time. The num- 

 ber of cubic centimeters of acid used to effect precipi- 

 tation is noted and this result is recorded as A. 



(b) Influence of temperature. For convenience of 

 work and uniformity of results, the temperature of 

 the mixture at the time of the addition of acid may be 

 between 60 and 80 F. Under these conditions, many 



