LESSON XVI 

 MILK 



1. Caseinogen in milk exists in the form of a salt (calcium caseinogenate). 

 Add acetic acid to milk, and this salt is decomposed, and free caseinogen 

 (with entangled fat) is precipitated. Collect the precipitate so produced from 

 a pint of milk on a filter, and wash thoroughly with distilled water : grind it 

 up with calcium carbonate in a mortar, and add about a pint of distilled 

 water ; allow the mixture to stand for about an hour. The fat rises to the 

 top : the excess of calcium carbonate falls to the bottom. The intermediate 

 fluid contains the caseinogen in solution ; it is usually very opalescent. 

 Take some of this solution and divide it into three parts, A, B, and C. 



To A add rennet. 



To B add a few drops of 2-per-cent. solution of calcium chloride. 

 To C add both rennet and calcium chloride. 



Put all three in the water-bath at 40 C. A clot of casein forms in C, but 

 not in A if all calcium salts have been successfully washed away, nor in B. 



2. The formation of casein from caseinogen is a double process ; the first 

 action is that of the ferment, which converts the caseinogen into what may 

 be called soluble casein ; the second action is that of the calcium salt, which 

 precipitates the casein in an insoluble form, or curd. This may be shown by 

 taking some of the caseinogen solution and adding rennet. Warm to 40 C. ; 

 no visible change occurs, but nevertheless soluble casein and not caseinogen 

 is now present. Then boil this mixture to destroy the rennet, cool, and add 

 calcium chloride. A formation of insoluble curd now occurs. 



3. Caseinogen may be precipitated as a salt from milk by the addition of 

 alcohol. This reagent also precipitates the other milk proteins. 



4. The method of salting out described in Lesson VI., Exercise 10 (p. 41), 

 may also be used. Add to some milk an equal volume of saturated solution 

 of ammonium sulphate.. Caseinogen as a salt is thus precipitated, and 

 entangles the fat; with it. Filter off the precipitate and examine the filtrate 

 as follows : Saturate it with sodium chloride ; a small amount of precipitate 

 comes down. This is the so-called lacto-globulin. This contains only a 

 trace of true globulin; it is mostly caseinogen previously left in solution 

 together with calcium sulphate. Filter it off; acidify the filtrate with a few 

 drops of 2-per-cent. acetic acid and heat it in a water-bath gradually. About 

 77 C. the remaining protein (lactalbumin) is coagulated. 



