290 PHYSIOLOGICAL CHEMISTRY. 



exact nature of this emulsion our knowledge is imperfect. It 

 has been held also that a membrane of casein encloses the fat 

 globules and that this prevents the ready extraction of fat 

 when ether or similar solvent has been added to milk. If the 

 milk is previously shaken with a little acid which is supposed 

 to break or destroy this membrane the ether added will now 

 dissolve it. But this membrane cannot be directly detected 

 with the microscope and experiments on the formation of fat 

 emulsions by the aid of casein and weak alkalies have shown 

 that the presence of a membrane is not necessary to account 

 for the round form or the failure to dissolve readily in ether. 

 The surface of the globule is not the same as the interior por- 

 tion, as it appears to take a stain by certain agents which does 

 not penetrate. But in the conflict of views advanced it is not 

 yet known what the surface actually is. 



Casein and Lactalbumin. These compounds have been 

 mentioned in the chapter on proteins and their place in the 

 general scheme of classification pointed out. In the free, pure 

 state the casein is a distinctly acid body which neutralizes al- 

 kali and forms salts with rather sharply defined properties. 

 Casein may be easily separated from milk in this way: 



Ex. Dilute 500 cc. of skimmed milk with about 2 liters of water in a 

 large jar; add enough dilute acetic acid to make not over o.i per cent 

 of the whole. This causes a precipitation of the casein in fine white 

 flakes which soon settle, leaving a nearly clear whey. After some hours 

 decant this whey and add a greater volume of distilled water and stir up 

 well. Allow this mixture to settle and pour off the water. Add a liter of 

 water and enough weak sodium or ammonium hydroxide to dissolve all 

 the casein and produce an opalescent solution. This in turn is re- 

 precipitated with dilute acetic acid after adding considerable water, and 

 these operations are repeated several times. In this way a casein nearly 

 free from calcium salts is obtained. It is washed well with water by 

 decantation, then poured on a Buchner funnel, drained, washed with alco- 

 hol, until the water is removed and finally several times with ether to 

 take out the fat. On drying a fine white powder is obtained with which 

 the important properties of casein may be shown. 



Ex. Weigh out 5 to 10 gms. of casein into a beaker or flask and add 

 distilled water. Note that it appears to be quite insoluble. (This might 

 be shown by filtering and testing the filtrate by evaporation.) Add a few 

 drops of phenol-phthalein reagent and run in standard sodium hydroxide 



