THE INDIVIDUAL GROUPS OF PROTEINS 461 



Properties. 



Caseinogen forms a white granular powder, which is insoluble in 

 water ; it is insoluble in dilute acids in the cold, but on warming it 

 dissolves slowly, undergoing decomposition into derivatives. It dis- 

 solves in dilute alkalies forming salts ; these are best made by rubbing 

 the caseinogen in a mortar with small quantities of the alkali, or alkali 

 carbonate in the presence of a drop of phenolphthalein, more alkali 

 being added only when the pink colour disappears. I gm. of casein- 

 ogen dissolves in 8 c.c. of *iN alkali. On rubbing with carbonates 

 carbon dioxide is evolved. 



Caseinogen forms two series of salts ; those with calcium oxide con- 

 tain respectively I '5 and 2-5 per cent of calcium oxide. The neutral 

 salt containing I '5 per cent, of CaO is neutral to litmus ; the basic 

 salt containing 2*5 per cent, of CaO is neutral to phenolphthalein. A 

 solution of caseinogen in lime-water when made neutral to phenol- 

 phthalein is milky in appearance, but when neutralised to litmus the 

 milky appearance is much more distinct. 



A neutral solution of caseinogen in caustic alkali gives most of the 

 general reactions for proteins. 



The presence of phosphorus in caseinogen is shown by incinerat- 

 ing or oxidising it and testing for phosphates (p. 30). 



The phosphorus in caseinogen and phosphdproteins is separated by 

 the action of dilute alkali. On warming caseinogen with caustic alkali 

 of about I per cent, strength for five minutes in a boiling water-bath, 

 acidifying and filtering, the filtrate will show the presence of inorganic 

 phosphates when tested with nitric acid and ammonium molybdate. 



The Action of Rennin upon Caseinogen. 



The most characteristic property of caseinogen is its conversion 

 into casein, or paracasein, by the action of the enzyme, rennin or chymo- 

 sin. Rennin is prepared by extracting the fourth stomach (rennet) of 

 the calf with salt solution, or glycerin, and can be obtained commer- 

 cially either in solution or in the form of powder or tablets. The con- 

 version is most noticeable with the soluble calcium salt of caseinogen 

 in milk which is converted into the insoluble calcium salt of casein ; in 

 the presence of calcium salts the milk clots. 



The conversion takes place more rapidly in the presence of a trace 

 of acid or acid salts, but is prevented by alkali. The presence of cal- 

 cium salts is essential to the formation of a clot. Boiled milk does 

 not clot with rennin since soluble calcium salts (bicarbonates, acid 

 phosphates) are made insoluble (carbonates, phosphates), but it clots if 

 soluble calcium salts or a few drops of acid be added. The change 

 seems to take place in three stages : (i) the conversion of calcium case- 

 inogenate into calcium caseate ; (2) the formation of soluble calcium salts 

 from insoluble salts ; (3) the change in viscosity and clotting of the 

 calcium caseate. 



