THE GRADING OF MILK 161 



THE RENNET TEST 



As milk drawn from diseased udders generally coagulates badly 

 with rennet, this test affords the same indications as the two tests 

 just described ; it has also a practical significance, for milk which 

 coagulates badly can hardly be made into good cheese, even if it 

 is satisfactory from the bacteriological point of view. The test 

 may be applied with advantage when difficulty is experienced in 

 making the curd sufficiently dry, so that the particular consign- 

 ments of milk which are at fault may be detected. In the labora- 

 tory the test is generally carried out by means of Schaffer's 

 apparatus, which consists simply of a shallow water bath with a 

 false bottom on which a number of beakers can be placed. One 

 cubic centimetre of one of Hansen's rennet tablets, No. 2, in 

 500 c.c. of water is added to 100 c.c. of milk which is kept at 35 C. ; 

 normal milk will coagulate in nine to nineteen minutes. MarschalUs 

 apparatus is better suited for use in the dairy ; this consists of a 

 graduated enamelled iron cup having a fine opening in the bottom. 

 The hole is closed by a finger, the cup is filled, and milk is allowed 

 to run out until the level comes to the top graduation mark in the 

 cup. One cubic centimetre of the rennet solution is rapidly and 

 thoroughly mixed with the milk, and the finger is removed from 

 the opening. The milk will cease to run out the moment coagula- 

 tion sets in, so that the capacity for coagulation will be inversely 

 proportional to the amount of milk which has run out. Con- 

 versely, this apparatus may be used for the estimation of the 

 coagulating capacity of rennet, which is a test of some importance 

 in the Swiss dairies which make their own rennet. The means 

 available for correcting a deficient capacity for coagulation are 

 discussed on p. 130. 



DETERMINATION OF ACIDITY 



The degree of acidity is generally understood to be the number 

 of cubic centimetres of standard sodium hydroxide solution 

 required to neutralise a given volume of milk, with phenol phthalein 

 as indicator. The Soxhlet-Henkel degrees, which are largely used 

 on the Continent, express the number of cubic centimetres of 

 quarter normal alkali per 100 c.c. of milk. In British and American 

 works, degrees of acidity are understood to represent the number 

 of cubic centimetres of normal sodium hydroxide per litre of milk ; 

 the titration is often carried out with decinormal alkali, using 

 smaller quantities of milk, the results being calculated to the 

 above standard ; as little as 10 c.c. of milk is sometimes used, but 

 more exact results can be got by using larger quantities. No 

 water should be added to the milk or cream before titration, as 



D.B. 11 



