598 TESTING MILK AND CREAM FOR ACID 



sodium hydroxide) and an alcoholic solution of phenolphthalein. 

 Both of these solutions may be purchased from creamery supply 

 houses or chemical supply houses. 



Making the Test. With the 50 c.c. pipette transfer 50 c.c. of 

 the cream into the white cup, rinse pipette with water and add rins- 

 ings to sample in cup. Add one-half c.c. of phenolphthalein solu- 

 tion. Fill the 50 c.c. burette with the tenth normal solution of 

 sodium hydroxide. Run this solution slowly from the burette 

 into the white cup, stirring the cream constantly. Each drop of 

 sodium hydroxide will give the cream a pink color which at first 

 immediately disappears. As more of sodium hydrate solution is 

 added the pink color disappears more gradually until, when the neu- 

 tral point has been reached the pink color of the cream remains con- 

 stant for several minutes. When this point is reached, that is, when 

 upon stirring a faint pink color remains in the cream, enough sodium 

 hydroxide has been added and the test is completed. Now read off 

 on the graduations of the burette the number of c.c. of decinormal 

 alkali solution that was used and multiply this by the factor .018. 



OOQ 

 ( ' X 100 = .018). This gives the per cent acid in the cream. 



N 

 Example. 35 c.c.yjy NaOH are required to neutralize 50 c.c. 



of cream. What is the per cent acid? 

 7C v OOQ 



C ' X 100, or 35 X -018= .63% acid. 

 *)U 



The detection of the pink color is facilitated by adding a pipette 

 full of water to the cream in the cup before titrating. 



A Practical Factory Test. For practical creamery operation 

 the Mann's acid test is modified to the extent of using a 17.6 c.c. 

 pipette for milk or an 18 c.c. pipette for cream instead of a 50 c.c. 

 pipette. Measure 17.6 c.c. or 18 c.c. of the milk or cream to be 

 tested into the white cup. Add 5 drops of phenolphthalein indi- 

 cator and then add tenth normal alkaline solution from the burette 

 until a permanent faintly pink color appears, stirring the milk or 

 cream in the cup with the rod while adding the alkaline solution. 

 Do not add alkali until the sample is deep pink. If not sure that 

 the neutral point has been reached and that enough alkaline solution 

 has been added, pour one drop of phenolphthalein indicator into cup. 

 If the milk or cream turn pink the test is completed, if no pink color 



