18 DAIRY LABORATORY GUIDE 



EXERCISE 6 



LACTIC ACID IN DAIRY PRODUCTS 



Object: To determine the percentage of lactic acid in 

 dairy products according to various methods. 



MANNS' TEST 



Apparatus: Samples of whole milk, skimmilk, butter- 

 milk, cream, butter, whey, and starter; Manns' testing outfit. 



Preparing the Alkali Solution. The best way to do, 

 where the facilities are not at hand for standardizing, is to 

 secure at the drug store a quantity of standard solution of 

 sodium hydroxide; then dilute it to N/10 as needed. This 

 can be done by putting 100 c. c. of the standard solution into 

 a liter flask and filling to the 1000 c. c. mark with distilled 

 water or with soft water. 



Preparing the Indicator. Obtain a small quantity of the 

 compound phenolphthalein at the drug store; then dissolve 

 1 gram of the dry powder in 30 c. c. of 90% alcohol. It is 

 necessary to use only a few drops at a time. 



Steps: 1. Put 50 c. c. of the sample to be tested into 

 the white cup; then add a few drops of the phenolphthalein 

 indicator. 



2. Add the N/10 alkali solution from the burette drop 

 by drop until a permanent pink color appears. 



3. One c. c. of N/10 alkali solution contains .004 grams 

 of sodium hydroxide, which is sufficient to neutralize .009 

 grams of pure lactic acid. Read from the burette the num- 

 ber of c. c. of alkali used and substitute in the formula. 



Formula: 



c.c. alkali used X .009 



X 100 = percentage of acid. 



c. c. of sample 



