5O PRACTICAL DAIRY BACTERIOLOGY 



readily undergo alcoholic fermentation, this is not true of milk 

 sugar. Fruit juices are quickly acted upon by yeasts, and con- 

 verted into Carbon dioxide and alcohol. Milk sugar, although 

 readily affected by bacteria, is not easily acted upon by yeasts, 

 and does not normally undergo alcoholic fermentation. The 

 reason for this has to do with the chemical nature of the 

 sugar molecules, and does not concern us here. It is very im- 

 portant to remember, however, that when solutions of fruit 

 sugar are left exposed to the air, they ferment, giving rise to 

 alcohol; but when milk sugar is exposed to similar conditions, 

 lactic acid is produced, but never alcohol. 



It is, however, possible to produce an alcoholic fermentation 

 in milk by adding thereto a little cane sugar and then inoculating 

 it with yeasts. 



This fact has been learned by manufacturers of condensed 

 milk to their sorrow. The common brands of condensed milk 

 are preserved by the addition of a large amount of cane sugar. 

 Since bacteria cannot thrive upon cane sugar, it serves as a 

 partial disinfectant for the milk. But although bacteria will 

 not grow in condensed milk, yeasts are capable of growing 

 there, and of fermenting the sugar. It sometimes happens 

 that yeasts get into the milk, and the process of condensing does 

 not kill them. When the milk is subsequently sealed in the cans 

 as condensed milk, the living yeasts are inclosed. They find 

 the sugar a medium favorable to growth, and an alcoholic fer- 

 mentation occurs. Immense losses have at times occurred as the 

 result of such growths. To prevent this loss, the process of 

 condensing and final sealing must be carried on in such a way 

 as to avoid yeast contamination, or the yeasts must be destroyed 

 by heat. 



The possibility of making milk undergo an alcoholic fermen- 

 tation by the addition of yeasts is made use of in the manu- 

 facture of kummys. 1 This beverage was originally prepared by 



Sehipin, Cent. f. Bact., II., vi., p. 775, 1900. 



