90 MILK 



It is worthy of note that no carbohydrate radical was found. 

 Glycocoll is also absent, while cystin is present in small quantity. 

 On the other hand, tyrosin and mono-amino-acids are well repre- 

 sented. It is thought that these conditions render casein easy of 

 digestion. 



Casein is soluble in dilute solutions of salts, and according to 

 Van Slyke and Hart more so in hot solutions than in cold ones. 

 The solutions are opalescent and turn milky when boiled. The 

 casein is precipitated from these salt solutions with ammonium or 

 magnesium sulphate. 



^Casein is precipitated by saturation with NaCl,- MgS04, 

 CuS04, and mercury salts. 



Van Slyke and Hart found a salt-soluble substance in Cheddar 

 cheese when it was extracted with a dilute solution of sodium 

 chlorid. This body was present in large amount only when lactic 

 acid was used. It occurs in fresh cheese, but the quantity dimin- 

 ishes as the cheese ages. When paracasein is treated with dilute 

 lactic acid a substance is obtained which resembles this salt- 

 soluble substance extracted from cheese both physically and 

 chemically. Paracasein, which is the main component of green 

 cheese, combines with acids in two different proportions: one is 

 a monobasic salt, the other a dibasic salt. Lactic, hydrochloric, 

 and sulphuric acids will form similar compounds with paracasein. 

 Casein also combines with acids in a similar manner. The un- 

 saturated salts formed by casein and paracasein with acids are 

 soluble in dilute solutions of sodium chlorid and hot 50 per cent, 

 alcohol, but they are insoluble in water. The unsaturated salts 

 are practically insoluble in salt solution, hot alcohol, or water. 

 In making Cheddar cheese the curd is tested with a hot iron, and 

 when strings are formed from the hot iron the curd is ready for 

 further treatment. These strings are due to unsaturated para- 

 casein lactate. The ripening process of Cheddar cheese com- 

 mences with paracasein lactate. The water-soluble nitrogen in 

 cheese increases as the unsaturated paracasein salt decreases in 

 quantity. Therefore, ripening commences with a peptic diges- 

 tion of unsaturated paracasein lactate. 



RENNET COAGULATION 



Rennet extract contains an enzym which coagulates casein 

 and changes it into a non-coagulable substance. Rennet extract 

 is prepared from the fourth stomach of a suckling calf, but similar 

 enzyms are found elsewhere in nature, as, for example, in the 

 stomach, the pancreas, in some plants, and as products of bac- 

 terial metabolism. Coagulating enzyms differ in some of their 



