CHAPTER V 

 MILK SUGAR AND LACTIC ACID 



49. Physical Properties of Milk Sugar. — Milk sugar, 

 or lactose, is present in milk to the extent of about 

 5 per cent. When milk sours, the sugar is converted 

 during the process of fermentation into lactic acid, 

 and hence tlie milk sugar takes an important part 

 indirectly in all dairy operations. Milk sugar is 

 not as soluble in water as cane sugar. It lias about 

 the same chemical composition as ('iine sugar, but 

 differs from it in all of its physical and chemical 

 properties. In butter and cheese making the main 

 function of milk sugar is the production of lactic 

 acid. 



50. Fermentation of Milk Sugar. — Under favor- 

 able conditions milk sugar undergoes lactic acid fer- 

 mentation. This change is brought about by the 

 action of minute organisms known as the lactic acid 

 ferments. The conditions necessary for lactic acid 

 fermentation are : favorable temperature, 70° to 

 90° F., and the presence of the ferment body. The 

 lactic acid ferments gain access to the milk through 

 the dust particles of the air, from unclean dairy 

 utensils, and from other sources. The spores or 

 seeds of the lactic acid organisms readily develop in 



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