240 MILK : ITS ORIGIN AND COMPOSITION 



(ft) Add i c.c. of commercial "rennet" to 10 of water: then 

 pour i c.c. of the mixture into 10 of milk : casein is precipitated. 



Note the difference in the texture of the curds obtained in 

 the two cases. 



Ex. 114. Heat 20 c.c. of fresh milk in a wide test-tube to 35 

 or 40 C. and add to it 5 c.c. of a weak solution of commercial 

 rennet (i in TO of water). 



Note the formation of curd or precipitation of casein in a minute 

 or less. 



Ex. 115. Try the previous experiment, using boiled rennet 

 extract. 



Ex. 116. Take 50 c,c. of fresh milk in a flask warmed to 40 C 

 and add a few drops of acetic acid. 

 Filter off the curd and dissolve : 



(a) One part of it in a weak solution of caustic soda ; 



(b) another part in lime water. 



Add a dilute solution of rennet to both and keep them at 40 C. : 

 in which of them is a curd reproduced ? 



Ex. 117. To some fresh milk in a test-tube add one or two 

 drops of a saturated solution of ammonium oxalate : then add a 

 few drops of a solution of rennet : keep at 40 C, for an hour and 

 note absence of coagulation. Now add a few drops of a 2 per 

 cent, solution of calcium chloride : coagulation occurs rapidly. 



Ex. 118. (a) To 20 c.c. of milk add 10 c.c. of water : heat to 

 60 to 70 C. and add a few drops of 20 per cent, acetic acid. 



Caseinogen is precipitated. 



Filter twice. 



(b) Test a portion of the filtrate for lact-albumin by Millon's 

 reagent (Ex. 45). 



(c) Boil the rest and note the coagulation of the lact-albumin. 



(d) Filter and test the filtrate for sugar. Add a few drops of 

 copper sulphate solution and strong caustic potash until a blue 

 clear solution is obtained and then boil : a yellow precipitate 

 indicates the presence of sugar, 



