133-137] DAIRY PRODUCE 49 



DAIRY PRODUCE 



Milk 



133. Action of Acid on Milk.— Half fill a test-tube 

 with milk. Add about six drops of dilute sulphuric acid 

 and shake. Allow the tube with its contents to stand for 

 five minutes. At the end of that time the milk will be seen 

 to have curdled. The acid has coagulated the soluble 

 albuminoids, rendering them insoluble, and thus forming 

 clots which separate out from the milk. 



134. Fat in Milk.— Filter the curdled milk and save 

 the clear filtrate (136). Spread out the filter-paper which 

 contains the 'curd' on a glass plate and place in the 

 steam-oven to dry. When it has got quite dry the fat will 

 melt out of the curd, and form grease stains on the filter- 

 paper. 



135. Albuminoids in Milk.— The residue left on the 

 filter-paper, after the fat has been melted out as described 

 in the last experiment, is principally albumen and casein. 

 Both these substances contain nitrogen, which may be 

 tested for by scraping this residue off the filter, mixing with 

 soda-lime, and heating in an ignition-tube as described in 

 paragraph 85 ; ammonia will be given off. 



136. Sugar in Milk. — The clear liquid which has 

 been saved (134) contains sugar. This sugar is different 

 from either cane sugar or glucose, and goes by the name 

 of lactose, or milk sugar. It has an action, however, on 

 Fehling's Solution (266) similar to that of glucose. Add 

 to the clear liquid in the test-tube (134) a little potassium 

 hydrate solution until the liquid is no longer acid, then add 

 Fehling's Solution and boil; a yellow precipitate will be 

 formed, indicating the presence of sugar. 



137. Natural Acidification of Milk.— If milk is 

 allowed to stand in the air, certain bacteria will gradually 



