CHKMICAI. RKACTKlX OF MILK 33 



milk was not reliable. The fat-globules themseK'es are said to be 

 almost transparent, )-et the color and opacity of milk is largel)- 

 due to their presence. I'his characteristic nra\' perhaps be 

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

 light instead of allowing it to pass through them. 



After the fat has been removed, the milk still continues to 

 be opaque. When the albuminoid matter has been remo^•ed 

 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 alkaline 

 and an acid reaction when tested with litmus paper. It turns 

 blue litmus paper red, and red litmus paper blue. This peculiar 

 beha\'ior ol milk is saifl to be due to the caseous matter in the 

 milk, which itself has an acid reaction, while 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, an acici reaction is obtainefl. 

 After standing, milk soon becomes distinctly acid, 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 acr^uired a false imj)ortance, as he 

 believes that the neutrality, as measured by the action of litmus 

 paper, is not chemical neutrality. 



Specific Gravity of Milk.— By si>ecific gra\'it}- of milk we 

 mean the weight of the milk as ccjmpared to that ol an ecjual 

 volume of water at the same temperature. If a certain \()lume 

 of water weighs looo pounds, an equal ^'olume ot milk at the 

 same temperature and under the same conditions will weigh 

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

 always done, the comparison between the two equal \-olumes of 

 water and milk will be i and i.o:;2. This latter figure represents 

 the average specific gravity of normal milk. 



It can be readily seen that the correct specific graxity can 

 be obtained only at one given temperature, for, as the tempera- 

 ture of the substance becomes higher, the density of it grows less, 

 and consequently the specific gravity will be less. The tempera- 

 ture at which the lactometers are standardized is 60° F. 



