18 QUALITATIVE EXAMINATION OF MILK. 



III. MILK SUGAR. 



To identify the characteristic carbohydrate of milk — 

 milk sugar or lactose — it first must be isolated in a pure 

 state. loo c. c. to 200 c. c. of milk are treated with acetic 

 acid in the same manner as described, or with rennet to 

 precipitate the casein, and filtered through a linen cloth. 

 The filtrate is then heated for ten minutes to coagulate 

 completely the remaining albuminoids and is again fil- 

 tered and evaporated to crystallization. After a few days 

 colorless glistening crystals (rhombic prisms) will have 

 separated out. The product may be purified by decant- 

 ing the mother liquor, dissolving in warm water and re- 

 crystallizing. The following reactions are characteristic" 

 of milk sugar : 



1. It reduces an alkaline copper solution (and Fehl- 

 ing's solution), red cuprous oxide, Cu^O, being precipi- 

 tated. Its reducing power is, however, dififerent from 

 that of grape sugar (see page 52). To apply this test, one 

 adds a solution of sodium hydrate to an aqueous solution 

 of the sugar and then adds drop by drop a solution of 

 copper sulphate as long as the resulting blue precipitate 

 dissolves by stirring. By heating this deep blue solution, 

 red cuprous oxide is precipitated. 



2. Lactose also reduces alkaline solutions of bis- 

 muth, silver and mercury. 



3. By boiling a solution of milk sugar for about five 

 minutes with a slight excess of lead subacetate and add- 

 ding ammonia to the boihng solution until a permanent 

 precipitate is obtained, a cherry red solution results. 

 After a short time a similar colored precipitate settles 

 out (Rubner). 



4. If a solution of milk sugar be heated for about 

 an hour upon the water bath with equal parts of phenyl- 

 hydrazine and acetic acid (50 per cent.), lactosazone is 



