52 
LABORATORY MANUAL 
EXERCISE NO. 38 
Object: To become acquainted with Van Slyke’s Volumetric method of casein 
determination. 
Procedure: The milk constituents which determine the yield of cheese 
are fat and casein. Since the casein in different milks does not 
increase in as great a ratio as the fat content, a problem arises 
as to the proper method of payment at cheese factories where 
milk having a wide range in fat content is sold. Several sug¬ 
gested methods are: Fat +2; Fat +.6; Fat +calculated casein. 
A. Test each of the lots of milk provided in duplicate as follow: 
(1) Measure out 17.5 cc. of milk into a 200 cc. flask, and add 80 cc. 
of distilled water and 1 cc. phenolphtalein solution. Titrate 
carefully with a standard NaOH solution, shaking vigorously 
after each addition until just a permanent pink tinting of the 
milk is obtained. 
(2) Add standard acetic acid solution in 5 cc. quantities until 25 cc. 
have been added. Shake thoroughly after each addition. If 
the casein does not settle readily, add more acetic acid in quan¬ 
tities of 1 cc. and shake. Record the cc. of acetic acid use as 
“A.” 
(3) Fill the flask with distilled water to the 200 cc. mark. Shake 
10-15 seconds and filter through a dry filter into a dry cup. 
The filtrate should be clear. 
(4) Take 100 cc. of the filtrate and titrate carefully with a standard 
NaOH solution until a permanent pink color which will remain 
a half minute or so, is obtained. Record the cc. of the standard 
NaOH solution as “B.” 
(5) Calculate the per cent casein in the milk by the following 
formula: 
A 
-B = % casein. 
2 
B. Make a fat test of the milk provided and compare the ratio of 
the casein to the fat. 
Observations: 
Sample 
% Casein 
% Fat 
Ratio 1 : ? 
1 
2 
Conclusions: 
Questions: (1) How does the casein content of buttermilk and skimmilk 
compare with that of whey? 
(2) What effect has freezing on the physical and chemical prop¬ 
erties of milk? 
(3) How does casein compare in food value with fat? With sugar? 
(4) W hat part of milk is albumen, and of what commercial value 
is it? 
References: F. & W. If 18, 23, 307; Van S. pp. 189-199. 
