CASEIN. 



THE PREPARATION OF CASEIN. 







When fresh, whole milk is left at rest, or is set in very 

 rapid rotary motion by means of special appliances (centri- 

 fugal machines), the cream, or portion richest in fat, collects 

 on the surface, and if this be skimmed off, skim milk is left. 

 This latter forms the raw material for the preparation of the 

 second chief product of milk, namely, the casein, which is 

 present to the extent of 2 to 4'5 per cent., the average being 

 3-2 per cent. Casein is not dissolved in milk, but is sus- 

 pended in a state of distension, as can be proved by a simple 

 experiment. If milk be filtered through a porous earthen- 

 ware plate the filtrate obtained consists merely of a solution 

 of milk sugar and saline constituents, the fat and casein, 

 neither of which is in solution, being left behind on the 

 surface of the filter. ] 



The entire behaviour of milk is determined by this peculiar 

 condition of the casein present. This is the cause of certain 

 physical properties of the milk, notably the viscosity, which in 

 turn influences the rate at which the cream rises. The con- 

 dition of the casein is not altered by warming or boiling the 

 milk ; but, on the other hand, it is precipitated by dilute 

 acids or rennet. The precipitated casein is insoluble in water 

 or dilute acids, and forms when dried a crumbling, horny mass ; 

 but it is dissolved by alkalies and concentrated acids. This be- 

 haviour of casein forces one to the conclusion that it does not 

 exist in a pure state in milk ; and, in fact, experiments have 

 shown that it occurs therein as a compound of lime. This 

 circumstance explains its behaviour towards dilute acids, i.e., 

 its precipitation thereby, the lime being extracted by the reagent 

 and the casein left in the solid form. Only a very small 

 quantity of acid is needed to effect this precipitation, 1 per cent, 

 of lactic acid (from the milk sugar) being sufficient to curdle 

 the milk. In addition to casein, small quantities of albumen 



