i 9 4 MILK AND DAIRY PRODUCTS PART 



is the most usual and is quite sufficient for ordinary practical 

 purposes. Devarda l has devised a more exact method which 

 can be used if more accurate determinations are to be made. 

 Devarda's method is based upon the comparison of the sample 

 of rennet with a " standard rennet " prepared according to 

 a definite formula and is carried out as follows. 



A commercial rennet powder of medium strength is used 

 as the standard rennet. It must be homogeneous and dry, 

 and should only contain a small quantity of mineral salts, and 

 ought also to give a clear solution with water. After being kept 

 in the dark for one or two years it ought not to have diminished 

 in strength even to a slight degree. To get a standard rennet 

 of constant strength Devarda prepared a mixture of the 

 commercial rennet powder with fine sugar. This preparation 

 coagulated milk in the proportion of 1 : 100,000 in 6 minutes. 

 The rennet solution must be prepared immediately before 

 each test. 



To determine the strength of the standard rennet, it is 

 absolutely essential to have a perfectly normal milk got from 

 a herd of cows fed on a standard ration. An average sample 

 of 1 litre of the milk is drawn and cooled at once to 15 C. 

 Before using this milk for testing the rennet, it must first of 

 all be thoroughly shaken to free it from carbon dioxide. Milk 

 that is a few hours old must not be used for this purpose. The 

 average number 12 is taken as the first value for the strength 

 of the standard rennet when the above test is made with 

 normal milk under the conditions given above. When the 

 strength of the standard has been determined once for all, it 

 can be used as a measure of the strength of an unknown rennet 

 preparation. The procedure when determining the strength is 

 as follows : 



Two hundred c.c. of fresh normal cows' milk are poured into a 

 300 c.c. glass flask and warmed to 35 C. To this 2 c.c. of rennet 

 solution are added, the time being carefully noted, preferably 

 on a seconds watch with stop action, and the contents of the 

 flask gently shaken. A thermometer is then sunk into the 

 milk, and the flask placed in a water-bath having a temperature 

 of 36 C. Whilst the flask is slowly moved to and fro, the 

 appearance of the milk as it flows from the sides of the flask is 

 1 Landwirtschaftliche Versuchsstationen, 1896, Vol. XLVII, p. 401. 



