26 METHODS OF QUANTITATIVE ANALYSIS. 



acid has been formed (0.604 P^r cent., according to 

 Timpe), because the ferment is destroyed when the de- 

 composition reaches a certain degree of concentration 

 wherein the conditions for its growth are removed. 



In order to ascertain the age of milk from the strength 

 of the acid reaction, the initial reaction must be known. 

 If the souring has advanced so far that it may be tasted 

 the casein will soon begin to coagulate. The milk is then 

 doubtless old and spoiled. Before this stage is reached, 

 however, one may determine the age of milk with some 

 degree of accuracy, in the following manner : old milk 

 coagulates by treating with carbon dioxide and sub- 

 sequently heating to boiling. Older milk, without treat- 

 ment vtfith carbon dioxide coagulates by boiling, while 

 with still older milk coagulation takes place without 

 boiling by simply forcing in carbon dioxide. Quite fresh 

 milk, especially cow milk at the beginning of her lactation 

 period, will sometimes curdle by boiling, but the result- 

 ing curd in this case consists not of casein, but of another 

 proteid, viz., globulin. If this point is to be proved, the 

 sample in question is treated with di-sodium phosphate 

 solution till the acidity is almost neutralized, and then 

 boiled. Under these conditions albumin and globulin 

 alone will be precipitated out, if the milk is fresh. On the 

 other hand, if the sample should curdle by boihng with- 

 out adding di-sodium phosphate it is evident that the 

 milk is old and that fermentation has begun, forming lac- 

 tic acid. The curd in this case is casein. When alkaline 

 or neutral milk is boiled the curd produced is always 

 albumin.* 



*Sembritzki found that by continually removing the scum 

 that forms on boiling milk, he could obtain 1.023 per cent of the 

 milk in this way. Since albumin is not present in milk in such 

 large quantity, it follows that some other body — probably case- 

 in — must enter into the composition of this scum or curd. See 

 "Poggendorf's Annalen der Physik und Chemie," 37, page 460.) 

 Translators. 



