180 



THE ESTIMATION OF PROTEINS. 



mono- and di-basic phosphates, and partly to the dissolved 

 carbonic acid. 



Van Slyke and Baker have shown that there is little free 

 lactic acid in sour milk until the salts of milk have been trans- 

 formed into mono-basic-diacid phosphates, lactates, and free 

 casein ; after these changes have taken place only a small amount 

 of lactic acid (about ^) is absorbed by the casein, the rest being 

 in solution. The sour smell and taste are not due to lactic acid, 

 but to a volatile compound. *- 



Soxhlet and Henkel have proposed the use of a normal 



Fig. 25. M'Creath Acidimeter. 



soda solution, and use a special apparatus (Fig. 26) ; they express 

 the acidity as degrees ; 1 Soxhlet-Henkel degree = the number 



of c.c. of -j- alkali used per 100 c.c. of milk ; it is to be regretted 



that this " degree " has been introduced, as it is 2-5 times larger 

 than the ordinary degree, and has caused confusion. 



Instead of using phenol-phthalein, delicate neutral litmus 



:per may be employed ; milk is practically neutral to this, 

 e acidity can be titrated with fair accuracy, though the end 



