462 PRACTICAL ORGANIC AND BIO-CHEMISTRY 



The factors concerned in the clotting of milk are illustrated by the 

 following experiments. The various mixtures of milk and rennet are 

 placed in a water-bath at 40 and the time taken to produce coagula- 

 tion is noted : Time of 



Coagulation* 



(1) 5 c.c. of milk + 2 c.c. of rennet extract + 



(2) 5 c.c. of milk + 2 c.c. of boiled rennet extract o 



(3) 5 c - c - f m ilk + i or 2 drops dilute acetic acid + 2 c.c. of rennet extract -f- 



(More acetic acid precipitates caseinogen) 



(4) 5 c.c. of milk + 2 c.c. dil. N^COg solution + 2 c.c. rennet extract . a 



(5) 5 c.c. of milk + i c.c. of potassium oxalate solution + 2 c.c. of rennet 



extract o 



(6) 5 c.c. of milk + i c.c. of potassium oxalate solution + 2 c.c. of rennet 



extract for 10 minutes. The rennet is now destroyed by boiling, 

 the solution cooled and i c.c. of CaCl 2 solution added ; clotting or 

 precipitation occurs -f- 



(7) 5 c.c. of cold boiled milk + 2 c.c. of rennet extract o 



(8) 5 c.c. of cold boiled milk + i c.c. of calcium chloride solution + 2 c.c. of 



rennet extract + 



(9) 5 c.c. of cold boiled milk + 2 or 3 drops of dilute acetic acid -t- 2 c.c. of 



rennet extract + 



(The acid produces soluble calcium salts and accelerates the clotting) 



The coagulation of milk takes place normally in digestion in the 

 stomach and is usually attributed to the presence of the enzyme rennin, 

 but Pavloff and other workers consider that the clotting of milk is 

 produced by the action of pepsin in neutral, or very faintly acid, solution. 

 The identity of the two enzymes, pepsin and rennin, is emphasised by. 

 the fact that the proteoclastic enzymes of plants, such as occur in pine- 

 apple juice, clot milk if their solutions be nearly neutralised. Trypsin 

 in very small amounts will also clot milk. 



If the change of calcium caseinogenate into calcium caseate is a stage 

 of pepsin digestion, casein may represent the stage of metaprotein ; 

 the calcium salt of it is insoluble and is therefore precipitated. 



Cheese. 



The action of rennin upon milk is used in the preparation of 

 junket and cheese. Extract of rennet is added to milk, or skimmed 

 milk, and the mixture kept in a warm place. It clots and forms junket. 

 On standing the clot contracts and a clear liquid, termed whey, exudes. 

 By cutting the clot into cubes the whey oozes out more rapidly, and as- 

 it oozes out the cubes are stirred, cut smaller and gradually piled one 

 upon another, so that they press out the remainder of the whey. The 

 cubes gradually form a mass which is again cut up and the process 

 continued till the mass is sufficiently dry. Salt is added and the mass 

 is pressed out so that it forms a solid. lump. It is allowed to stand 

 and ripen, a process which is due to enzyme action. These are hard: 

 cheeses. By the action of moulds and bacteria they become green. 



Soft cheeses are prepared in the same way, but the whey is not 

 removed by cutting the curd ; the clot is allowed to contract and 

 most of the whey may be pressed out, but the soft cheese always con- 

 tains some whey. It ripens more rapidly than hard cheese and the 

 ripening is due to the action of bacteria. 



The whey which has oozed out contains the lactose, phosphates 

 and soluble proteins and is used for preparing milk sugar. 



