294 PHYSIOLOGICAL CHEMISTRY. 



THE TEST FOR SUGAR. Measure out about 10 cc. of milk, and dilute it 

 with water to make 200 cc. Add to this 5 cc. of a copper sulphate solu- 

 tion, such as is used in making the Fehling solution (69.3 gm. per liter), 

 and then enough potassium or sodium hydroxide solution to produce a 

 voluminous precipitate containing copper with all the proteins and fat. 

 For this purpose about 3.5 cc. of a i per cent sodium hydroxide solution 

 will be required. Allow the precipitate to subside, pour or filter off some 

 of the supernatant liquid, and boil it with Fehling solution. The character- 

 istic red precipitate forms, showing presence of sugar. 



PROTEIN TEST. The presence of proteins in milk can readily be shown 

 as follows : Mix equal volumes of milk and Millon's reagent in a test-tube, 

 and boil. The bulky red precipitate which forms proves the presence of 

 the body in question. 



Action of Rennet on Milk. The mucous membrane of the stomachs of 

 most animals, and especially that of the young calf, contains an enzyme 

 known as the "milk curdling ferment," the "rennet ferment," or rennin, 

 the nature of which has already been explained in the chapter on the fer- 

 ments. 



A crude extract of the mucous membrane of the stomach from the calf 

 is commonly called rennet and has long been in use for the curdling of 

 milk in the production of cheese. This curdling consists essentially in the 

 coagulation or precipitation of the casein, which, it will be recalled, is 

 not readily thrown down by the usual methods. 



An active rennet can be readily obtained by digesting the stomach of the 

 calf with glycerol or brine. If a brine extract is precipitated by alcohol 

 in excess a white powder separates, which, when collected and dried, has 

 very active properties. Several powders of this description are now in the 

 market. Let the student try the following experiment with such a product : 



Ex. Warm some fresh milk to a temperature of 38 to 40 C. in a test- 

 tube or small beaker, then add about half a gram of commercial " rennin," 

 and after stirring it well keep for 15 minutes at a temperature not above 

 40. Then as the milk cools it assumes the consistence of a firm jelly. It 

 is essential in this experiment that the temperature be kept within the 

 proper limits, as the enzyme is not active at low temperature and it is, 

 like others, destroyed by high temperature. The casein or cheese which is 

 obtained in this way is not the same as that precipitated by acids as it 

 contains much calcium in combination. This form of casein is usually 

 called para-casein. 



Repeat the experiment by adding about 5 drops of a concentrated sodium 

 carbonate solution to the milk and then the rennet. Coagulation now 

 fails or is partial. 



The Action of Pancreatic Extract on Milk. The behavior of milk with 

 extract of pancreas is somewhat complicated because of the complex 

 nature of the milk; the sugar, the fat, and the protein bodies all suffer 

 some change under the influence of the several pancreatic enzymes. 



The most interesting of these changes, however, is that produced in the 

 proteins, and is commonly called peptonization. 



