CASEIN. 1029 



which abstracts the sulphuric acid, and the casein becoming 

 free is dissolved by the water. The solution of casein is eva- 

 porated to dryness, the dry matter reduced to powder and 

 digested in boiling ether, to dissolve out fat; the residue of 

 casein is afterwards dissolved again in water, and precipitated 

 by the addition of alcohol, to separate it from other matters. 



When dry, casein is yellowish like gum, insipid, and does not 

 dissolve again readily in water. Its solution is also yellowish 

 and somewhat viscid, and smells like boiled milk; left to 

 itself it putrifies and smells like old cheese. It comports 

 itself with reagents very like albumen ; it is precipitated by 

 acids, even by acetic acid which does not affect albumen. The 

 precipitates formed are soluble in an excess of their acid, and 

 also, it is said, in alcohol. Like albumen, casein exists both in a 

 liquid and solid form. Its coagulation is effected by rennet, the 

 inner coat of the calf's stomach, after it is well washed in hot 

 water. Skimmed milk placed in contact with a small portion of 

 this membrane, or mixed with an infusion of it, and heated 

 to 90 or 100, is thickened, and coagulates so completely that 

 not a trace of the casein remains dissolved in the whey. This 

 action of rennet is that of the pepsin it contains, but how the 

 latter operates is unknown. Berzelius observed that 1 part 

 of the membrane, washed and dried, placed in 1800 parts 

 of skimmed milk gradually heated up to 122, occasioned 

 complete coagulation. The membrane taken out afterwards, 

 washed and dried, was found to have lost 6 per cent of its 

 weight. The coagulum, mixed with the fat or butter, in sweet 

 milk, strongly compressed and dried, forms cheese. The fat 

 may be dissolved out of the latter by ether. 



Coagulated casein is insoluble or only very sparingly soluble 

 in water. It dissolves easily in dilute warm vinegar and 

 alkalies. Casein always leaves behind it, on incineration, a 

 portion of phosphate of lime, which milk contains in consider- 

 able quantity. Casein contains a little sulphur but no phos- 

 phorus, in chemical combination. It belongs to the class of 

 protein compounds, and may be considered a combination of 

 10 atoms of protein with I atom of sulphur. Its identity with 

 vegetable legumin (page 912) has already been more than once 

 adverted to. 



When milk is heated in an open vessel, it soon becomes 

 covered by a pellicle composed of insoluble matter. M. Scherer 



