PROPERTIES AND COMPOSITION OF MILK, BUTTER, &C. 389 



The main cause of butter becoming rancid is the chemi- 

 ' cal decomposition which the casein or curd it contains un- 

 dergoes by exposure to the air. This chemical change in 

 the cheesy matter may be prevented 



1st, By thoroughly washing and salting before the cheesy 

 matter has had time to become altered by exposure to the 

 air ; 



2d, By taking care that any water that may remain in 

 or around the butter be kept perfectly saturated with salt ; 



3d, By carefully excluding the air from the vessel in which 

 the butter is packed. 



About half a pound of the best Ashton salt is used to 10 

 pounds of butter. 



Milk contains a peculiar kind of sugar called milk-sugar, 

 which, being highly soluble in water, passes off in the whey 

 and goes to fatten pigs. In some countries it is extracted 

 and made an article of commerce. 



The main cause of milk becoming sour is the chemical 

 change which this sugar undergoes, without fermentation 

 *and therefore without loss, into an acid called lactic acid. 



This lactic acid is the cause of the curdling of the milk, 

 which may be hastened by hastening the change of the milk- 

 sugar into lactic acid by the addition of any other acid, such 

 as vinegar or rennet. 



Pure casein is nearly insoluble in pure water, either by 

 boiling or otherwise. By adding, however, a little soda to 

 the water, it dissolves and returns to its milky condition j 



