178 BACTERIOLOGICAL AND ENZYME CHEMISTRY 



enzymes which give a blue colour with guaiacum tincture 

 without hydrogen peroxide are true oxidases. 



THE CLOTTING ENZYMES 



Rennet. This enzyme, which is also sometimes referred 

 to as lab or chymosin, has the property of curdling or clotting 

 milk. It is generally prepared from the stomach of the calf ; 

 an impure product can be obtained by macerating the stomach 

 with water or with a 5 to 10 per cent, solution of sodium 

 chloride in presence of a little acid. 



It is obtained in a purer state by digestion with sodium 

 chloride solution 0'5 per cent, strength at 30 C. for twenty- 

 four hours. On filtering the solution and adding acid up to 

 0*1 per cent, a precipitate of mucous matter is obtained which 

 can be filtered off; acid is then further added to the filtrate 

 up to 0'5 per cent., and the solution saturated with sodium 

 chloride. On standing and stirring for two or three days and 

 gradually raising the temperature to 30 or 35 C. a flocculent 

 scum of rennet separates which is soluble in water. 



An enzyme capable of clotting milk occurs in many animal 

 and plant extracts, e.g., in germinating castor oil seeds; 

 certain bacteria also secrete a clotting enzyme. 



The chemical action of rennet upon milk is of considerable 

 interest. On addition of rennet to milk a curd separates out, 

 but the whey still contains an albumin, which differs from 

 lact-albumin in that it is not precipitated by boiling. The 

 curd also is different from the precipitate produced by acids, 

 as this can be redissolved on neutralisation, while the curd 

 produced by rennet is insoluble. It has been found that 

 curd contains calcium phosphate, which is consequently present 

 in cheese. If calcium phosphate is dialysed out of milk, 

 curdling is no longer obtained by addition of rennet. It 

 would appear that the greater part of the albumin of milk 



