MILK. 699 



15 or 16 per cent., of water, besides traces of casein, salts, coloring- 

 matter, etc. For curing butter, common salt is often used ; the 

 quantity added should not exceed 5 per cent. 



The composition of buttermilk has been given above ; when 

 freshly obtained from sweet cream it is a pleasant drink and a whole- 

 some food. 



Milk-sugar. Lactose. The general properties of milk-sugar 

 have been mentioned on page 534. By hydrolysis it yields two 

 simple sugars, dextrose and galactose ; when boiled with nitric acid, 

 saccharic and mucic acids are formed, the latter being a characteristic 

 product of the oxidation of galactose. Solutions of milk-sugar are 

 dextrorotatory. Lactose occurs occasionally in the urine of preg- 

 nant women, and also in the urine after ingestion of large quantities 

 of milk-sugar. 



Experiment 93. (Preparation of milk-sugar.) Use the aqueous filtrate obtained 

 in Experiment 89. Free the solution from the remaining proteins by boiling 

 and filtering; evaporate to about 75 c.c., when calcium phosphate will be 

 deposited. Filter, evaporate to a syrup, and set aside, when crystals of lactose 

 will be formed. The crystals may be purified by treating their solution with 

 bone-black aud recrystallizing. 



Tests for milk-sugar. 



1. To solution of milk-sugar apply Fehling's, Trornmer's, Moore's, 

 Boetger's, and Nylander's tests, for which see Index. 



2. Add ammonio-silver nitrate and a drop of sodium hydroxide. 

 A mirror of metallic silver forms on heating the mixture. 



3. Indigo-test. To a dilute solution add enough indigo-carmine to 

 produce a blue color, and render alkaline with sodium carbonate. 

 On heating, the blue solution becomes successively red, yellow, and 

 colorless, or nearly so, in consequence of the deoxidizing power of 

 the lactose. Pour the cooled solution repeatedly from one test-tube 

 into another; the colors are reproduced in reverse order in con- 

 sequence of the absorption of oxygen. 



4. As grape-sugar responds to the above tests, the fermentation- 

 test may be used for distinguishing between the two sugars. Fill 

 two fermentation tubes with glucose and lactose solution, respectively ; 

 add yeast, let stand in a warm place, and notice that a gas rises from 

 the glucose, but not from the lactose solution. 



