134 MANUAL OF MILK PRODUCTS 



three drops of the indicator are put in milk, the color will not 

 change, because the milk is acid in reaction. The instant that 

 just enough alkali is added to the milk to neutralize all of the 

 acid, the solution will turn pink. 



It is a chemical fact that equal volumes of acids and alkalies 

 of the same chemical strength will exactly neutralize one an- 

 other. In 1 c.c. of a normal solution of lactic acid there are 

 .09 gram of lactic acid. According to the above rule 1 c.c. 

 of any normal alkali solution would just neutralize .09 gram of 

 lactic acid. 



In actual practice a solution weaker than a normal solution 

 is usually employed, because a normal solution is so strong 

 that any small variation in the amount used makes a big varia- 

 tion in results. A common solution used is 1/10 normal (ex- 

 pressed n/10) . One c.c. of an n/10 alkali solution would neutral- 

 ize .009 gram of lactic acid. An example will illustrate how the 

 percentage of acid in milk is calculated. Suppose it took 6 c.c. 

 of n/10 alkali solution to neutralize the acid in 20 grams of 

 milk. What is the per cent of acid ? One c.c. of n/10 alkali 

 will neutralize .009 gram of lactic acid. Six c.c. will neutralize 

 6 X .009 = .054 gram of acid. .054 -s- 20 = .0027. .0027 

 X 100 = .27 per cent acid in the milk. Formulated, the above 

 example is expressed as follows : 



09 X 6 x 100 = .27 per cent. 

 20 



If the milk for the acid test is measured in cubic centimeters, 

 it should be reduced to grams by multiplying by the specific 

 gravity of milk. The acid is obtained in terms of grams, and 

 we cannot divide grams by cubic centimeters and obtain per 

 cent. 



Farrington has devised some alkali tablets, each one of 

 which will neutralize .03492 gram of lactic acid. These 

 tablets are dissolved in water and an alkali solution made. 



