34 



No. 3, p. 65;. For method of testing strength of rennet extracts, 

 see Lecture 5, paragrapli 3. The enzyni contained in rennet extract 

 is essentially the same as pepsin. 



(6) Strength of rennet to coagulate milk. One part of good com- 

 mercial rennet extract, which is only a dilute form of rennet enzym, 

 readily coagulates five thousand parts of milk (reference No. 3, p. 6ft). 



(c) Chemical action of rennet. When rennet coagulates the cal- 

 cium casein of milk it forms a substance called calcium paracasein, 

 which is in general properties about like calcium casein. Cheese curd 

 is an impure form of calcium paracasein. This is quite different from 

 the solid substance formed by calcium casein when treated with acids. 



5. Conditions of rennet action. — The conditions of rennet action 

 should be carefully studied in an experimental way. 



(a) Acids affect action of rennet. Milk must be neutral or acid for 

 coagulating action of rennet. Rennet will not coagulate milk having 

 ,an alkaline reaction. Increased amounts of acid increase the rapidity 

 and completeness of coagulation (reference No. 3, p. 66, or No. 8, 

 p. 40). 



(b) Temperature affects time of coagulation by rennet. The quick- 

 ness of coagulation increases with increase of temperature (refer- 

 ence No. 3, p. 66, or No. 8, p. 41). The character of coagulated sub- 

 stance is soft and loose when coagulation takes place below 60° F. or 

 above 120° F. At 77° to 110° F. the curd is firm and soHd. Heating 

 rennet above 120° F. weakens its coagulating power. 



(c) Strength and amount of rennet extract. Strength of rennet 

 extract, as well as increased amounts of rennet, increases the rapidity 

 and completeness of coagulation (references No. 3, p. 66, and No. 8, 

 p. 42). Dilution of milk by water decreases rapidity of rennet action. 



(d) Action of rennet is affected by different chemical compounds 

 added to milk. Rennet coagulation is delayed by common salt, for- 

 malin, borax, and other substances (references No. 3, p. ()7, and 

 No. 8, p. 43). 



(e) Effect of heating milk upon rennet action. When milk is 

 heated above 150° F. for some time it coagulates less quickly than 

 unheated milk. Boiled milk coagulates slowly or not at all. Coagu- 

 lation may be restored by aildition of acid (reference No. 3, p. 67). 



(/) The relation of soluble lime (calcium) salts to rennet coagulation. 

 Rennet will not coagulate milk when soluble lime salts are not present. 

 Rennet coagulation occurs in two stages: (1) Rennet changes cal- 

 cium casein into calcium paracasein, but no coagulation takes })lace; 

 (2) coagulation occurs in the presence of soluble lime salts (reference 

 No. 1, p. 23). 



The action of acid in hastening rennet action is explained by the 

 increased formation of soluble lime salts. Boiling milk lessens the 

 amount of soluble lime salts, and so interferes with action of rennet. 



