32 BUTTER-MAKING. 



reliable. The fat-globules themselves are said to be almost 

 transparent, yet the color and opacity of milk is largely due 

 to their presence. This characteristic may perhaps be explained 

 by assuming that the fat-globules in milk deflect the light 

 instead of allowing it to pass through them. 



The opacity of milk, after the fat has been removed, is 

 due to the presence of nitrogeneous matter. After the fat 

 has been removed from the milk, the milk still continues to 

 be opaque. When the albuminoid matter has been removed 

 and filtered off the filtrate becomes clear and transparent. 



Chemical Reaction of Milk. — Milk when fresh shows an 

 amphoteric reaction, which means that it exhibits both an 

 alkahne and an acid reaction when tested with htmus paper. 

 It turns blue litmus paper red, and red litmus paper blue. This 

 peculiar behavior of milk is said to be due to the caseous matter 

 in the milk, which itself has an acid reaction, but the remainder 

 of the serum has a slight alkaline reaction. By testing the 

 reaction of fresh milk with a tenth normal alkali solution, 

 and using phenolphthalein as an indicator, it will be found 

 to give an acid reaction. After standing, milk soon becomes dis- 

 tinctly acid, which is due to a change of the milk-sugar into 

 acids, chiefly lactic acid, through the action of micro-organisms. 

 Richmond maintains that the amphoteric reaction of milk 

 has acquired a false importance, as he believes that the neu- 

 trality, as measured by the action of litmus paper, is not chemi- 

 cal neutrality. 



Specific Gravity of Milk. — By specific gravity of milk we 

 mean the weight of the milk as compared to that of an equal 

 volume of water at the same temperature. If a certain volume 

 of water weighs 1000 pounds, an ecjual volume of mitk at the 

 same temperature and under the same conditions, will weigh 

 about 1032 pounds. Reducing the figure to a basis of 1, as is 

 always done, the comparison between the two equal volumes 

 of water and milk will be 1 and 1.032. This latter figure 

 represents the average specific gravity of normal milk. 



It can be readily seen that the correct specific gravity can 



