16 Testing. Milk and Its Products. 



The quantity of casein in normal cows' milk will vary 

 from 2 to 4 per cent., and of albumen, from .5 to .8 per 

 cent. The total content of casein and albumen ranges 

 between 2.5 and 4.6 per cent., the average being about 

 3.4 per cent. Milk with a low fat content will contain 

 more casein and albumen than fat, while the reverse is 

 generally true in case of milk containing more than 3.5 

 per cent of fat. 



19. Milk sugar or lactose belongs to the group of organic 

 compounds known as carbohydrates. It is a commercial 

 product manufactured from whey and is obtained in this 

 process as pale white crystals, of less sweet taste and 

 less soluble in water than ordinary sugar (cane sugar, 

 sucrose). About 70 per cent, of the solids in the whey, and 

 33 per cent, of the milk solids, are composed of milk sugar. 



When milk is left standing for some time, viz., from 

 one to several days, according to the temperature of the 

 surrounding medium, it will turn sour and soon become 

 thick and loppered. This change in the composition 

 and the appearance of the milk is brought about through 

 the action of acid-forming bacteria on the milk sugar; 

 these are present in ordinary milk in immense numbers, 

 and under favorable conditions of temperature multiply 

 rapidly, feeding on the milk sugar as they grow, and 

 decomposing it into lactic acid. When this change alone 

 occurs, there is not necessarily a loss in the nutritive 

 value of the milk, since milk sugar breaks up directly into 

 lactic acid ; this is shown by the following chemical formula: 



C 12 H 22 O, j. H 2 O (lactose) = 4 C 3 H 6 O 3 (lactic acid). 1 



lOne molecule of milk sugar is composed of 12 atoms of carbon (C), 

 22 atoms of hydrogen (H), 11 atoms of oxygen (O), and one molecule of 

 water (H 2 O). In the same way, the lactic-acid molecule consists of 3 atoms 

 of carbon, 6 atoms of hydrogen, and 3 atoms of oxygen. 



