No. 6. DEPARTMENT OF AGRICULTURE. 609 



(d.) Different chemical compounds added to milk affect the actioo 

 of rennet. For example, many acid salts like free acids, hasten 

 rennet action. Alkaline and alkaline salts prevent it. Retinet co- 

 agulation is delayed by common salt, borax, formalin and some 

 other substances used in milk preservatives. 



(e.) Heating milk above 150 degrees F. for some time makes it 

 coagulate less quickly with rennet than unheated milk. Milk heated 

 to boiling for some time coagulates either very slowly and incom- 

 pletely or not at all with rennet. The coagulating power of such 

 milks may be restored by adding small amounts of some acid or by 

 adding calcium chloride. 



(f.) Milk from different cows behaves very differently to rennet. 

 For example, in some tests made at the same time and under the 

 same condition with samples of milk obtained from fifteen different 

 cows, the time of coagulation by rennet varied in the individual 

 samples all the way from 50 seconds to 23 minutes. Just why 

 such differences exist no one knows. 



50. Methods of Testing Rennet Extracts. 



Since diff'erent brands of rennet extract vary somewhat in their 

 ability ro coagulate milk, it is important to have a meansi of testing 

 their strength, so that we may know definitely their value. The 

 strength of a rennet extract may be ascertained by finding out how 

 long it takes for a certain amount of extract to coagulate a fixed 

 amount of milk at some definite temperature. The two or three 

 rennet tests in use are based upon this general principle. The 

 two forms in common use are known under the names of (1) the 

 Monrad test and (2) the Marschall test. 



(1.) The Monrad Rennet Test. — In this test the amount of milk 

 used is 160 cubic centimeters (about five and one-half ounces fluid 

 measure), and the temperature is 82 degrees to 86 degrees F., and 

 the amount of rennet is one-half of a cubic centimeter, which is 

 diluted to 5 c. c. with water. The apparatus furnished for this 

 test by dairy-supply houses makes these measurements simple. 

 Having the rennet previously diluted, one makes the test as follows: 

 The given amount of milk is heated to 82 degrees to 86 degrees F., 

 5 c. c. of the dilute rennet solution are added and stirred in quickly 

 with the thermometer. A fixed temperature must be used always; 

 it can be any one point from 82 degrees F. to 86 degrees F., but the 

 point must always be the same one. The time when the rennet is 

 added is noted by the second hand of the watch and then again 

 the time when the milk has coagulated, and then we know how many 

 seconds it has taken for the milk to coagulate. The time when the 

 milk coagulates can be seen more sharply by scattering a few par- 

 89—6—1902 



