208 MILK 



and the mixture is stirred with a thermometer. The time elaps- 

 ing between addition of the rennet extract and coagulation of the 

 milk is noted accurately. By floating a few dark particles on the 

 surface of the milk and setting the milk in motion with the ther- 

 mometer, the exact moment when coagulation takes place can be 

 sharply noted by the arrested motion of the particles. For Cheddar 

 cheese making coagulation should be complete in thirty to sixty 

 seconds. 



The Marschall Test. One c.c. of rennet extract is measured 

 into an ounce bottle, previously half filled with water. The 

 pipet is rinsed by drawing up the water two or three times. The 

 milk is placed in a cup of pint capacity which has graduated spaces 

 from at the top to 7 at the bottom. In the bottom is fastened a 

 metal tube of small bore. The milk will flow through the tube, 

 and as soon as the surface has reached the mark the rennet will 

 quickly mix with it. As long as the milk is liquid it will continue 

 to flow through the tube. When coagulated the amount of milk 

 that has escaped will be indicated by the graduated scale. When 

 two and one-half spaces are uncovered the milk will be in suitable 

 condition for cheese making. 



Hastings and Evans think that the rennet test is superior to the 

 acid test for determining the ripeness for Cheddar cheese making. 



The age of milk can also be determined in a rough manner by 

 a rennet test, because milk coagulates more rapidly in propor- 

 tion to the increase in the amount of acid present. 



Commercial pepsin is used to some extent in place of rennet 

 extract. This can be used for the tests described if 5 grams of so- 

 called 1 : 3000 pepsin are dissolved in 120 c.c. of water. This solu- 

 tion can be used like rennet extract. 



THE ALCOHOL TEST 



The alcohol test for acidity is used to some extent abroad, but 

 has not found much favor in this country. Normal fresh milk 

 should not coagulate when mixed with an equal amount of 68 

 per cent, alcohol. If curdling takes place with this amount of 

 alcohol, the acidity is assumed to be about 0.25 per cent, expressed 

 as lactic acid. Ayers and Johnson have found that the test is not 

 as reliable as has been believed. Milk may curdle when mixed 

 with alcohol even when the acidity is low, as other conditions may 

 produce coagulation of the casein. A positive test with an equal 

 volume of 68 per cent, alcohol means something abnormal in the 

 milk, and the cow producing such milk should be examined. The 

 test may also be positive when rennet-producing bacteria are 

 present. The coagulation may, therefore, be the result of the 



