608 ANNUAL REPORT OF THE Off. Uoc. 



(2.) Strength of Keunet in Coagulatiug Milk. — How jjowerful the 

 action of louDet is iu cogulating milk-caseiu can be seen in cheese- 

 making , where we use only one part of rennet extract to five thou- 

 sand parts of milk, and rennet extract itself is only a dilute form of 

 rennin, the real coagulating substance. One part of absolutely pure 

 renniu can coagulate three million parts of milk. Apparently, ren- 

 net extract does not exhaust itself by its own action, but can be 

 used repeatedly. For example, if we could recover from whey and 

 curd the rennet used in coagulating milk, it would coagulate an 

 equal quantity of milk again. 



(3.) Chemical Action of Kennet. — When rennet acts upon milk- 

 casein, it changes the composition of the casein, forming a new 

 compound, and this new compound which appears as the coagulated 

 substance is called paracasein. Cheese curd is, then, an impure form 

 of paracasein. This paracasei« formed by rennet is quite different 

 from the solid substance formed by milk casein when treated with 

 acids. 



(•4.) Conditions of Kennet Action. — The conditions of rennet action 

 are quite well understood and we will consider some of them. The 

 rapidity and completeness of coagulation of milk casein by rennet 

 are dependent upon the following conditions: 



(a.) Acids Affect Action of Rennet. — Milk must be neutral or acid 

 for action of rennet. If milk is alkaline, rennet will not coagulate 

 it. Increased amounts of acids have a very marked effect in in- 

 creasing the rapidity and completeness of rennet action in coagulat- 

 ing milk casein. Moreover, very small amounts of acid have a very 

 pronounced influence. For example, a sample of fresh milk that co- 

 agulated in 110 seconds was coagulated in 30 seconds when one part 

 of lactic acid was added to 5,000 parts of milk. 



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

 ness with which rennet coagi]lates milk increases with increase 

 of temperature. For example, a sample of milk that coagulated 

 at 75 degrees F. in 270 seconds, was coagulated in 65 seconds at 

 95 degrees F. The character of the coagulated substance also varies 

 with the temperature at which coagulation takes place. Below 60 

 degrees F. and above 122 degrees F., the curd formed is soft and 

 loose, while at 77 degrees F. to 113 degrees F., the curd is much more 

 firm and solid. The activity or strength of rennet is weakened by 

 heat at 120 degrees F., while heated for sometime above 140 degrees 

 F., rennet becomes permanently weaker or entirely inactive. 



(c.) Strength of rennet extract increases rapidity and complete- 

 ness of rennet action, likewise increased amounts of rennet extract. 

 Dilution of milk by water has the effect of diluting rennet extract 

 and so a given amount of rennet extract acts less rapidly and com- 

 pletely in diluted milk than in the same quantity of milk undiluted. 



