CHAPTER XVII. 

 FERMENTATIONS IN MILK. 



i. Lactic Fermentation. When milk is first drawn 

 from the cow it possesses a sweetish taste, although when 

 tested by chemical means it is found to be slightly acid. 

 If it is allowed to stand in a vessel for a day or two 

 during warm summer weather, it becomes much more 

 sour or acid and at the same time curdles. The curdling 

 is due to the precipitation of the casein of the milk 

 by the action of an acid which chemical examination 

 has shown to be produced from the milk sugar. 

 The acid is known as lactic acid (or oxypropionic acid, 

 C 2 H 4 (OH)COOH), its production from the sugar being 

 generally indicated by the following equation : 



C 12 H 22 O n + H 2 = 4 (C 3 H 6 3 ) 



milk sugar lactic acid 



or lactose 



It is probable that the milk sugar is first hydrolysed 

 by an enzyme, lactase, the hexose sugars produced being 

 subsequently acted on with the formation of lactic acid, 

 thus : 



C 12 H 22 On + H 2 O = C 6 H 12 O 6 + C 6 H 12 O 6 = 4C 3 H 6 O 3 



lactose glucose galactose lactic acid 



The change is brought about by the activity of 

 bacteria which gain access to the milk after it is drawn 



from the udder of the animal. Theoretically the loss of 

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