96 Lectures on Bacteria. [^ ix. 



bottles well corked and frequently shaken. The bottled sour 

 milk, which is more or less highly effervescent, is fit to drink 

 in one or more days. It has the somewhat acid taste indicated 

 by its name, and contains an amount of carbonic acid varying 

 according to the temperature and the duration of the fermenta- 

 tion, but sometimes sufficient to burst the bottles or drive out 

 the corks, and, as has been already said, a certain amount of 

 alcohol, which in the cases examined in Germany was less than 

 I per cent, but according to other accounts may be 1-2 per cent. 



The changes in the milk which produce the drink here de- 

 scribed are brought about by the combined activity of at least 

 three ferment-organisms. The kefir-grains, as has been already 

 stated (page 13), consist chiefly of the gelatinous filamentous 

 Bacterium which has been named by Kern Dispora caucasica ; 

 intermixed with this organism and enclosed in the tough Zoo- 

 gloea are numerous groups of a Sprouting Fungus, a Saccharo- 

 myces, resembling the yeast-plant of beer; thirdly, there is 

 the ordinary Bacterium of lactic acid, which partly adheres to 

 the grains in company with some unimportant Fungi and other 

 impurities, and partly is introduced each time with the fresh 

 milk. 



We know at least enough of the ferment-effects of these 

 organisms or of their near allies to enable us to form a probable 

 idea of the course of the changes which have been described. 

 The acidification is caused by the conversion of a portion of the 

 milk-sugar into lactic acid by the Bacterium of that acid. The 

 alcoholic fermentation, that is, the formation of alcohol and of a 

 large part at least of the carbonic acid, is indebted for its 

 material to another portion of the milk-sugar, and for its exist- 

 ence to the fermenting power of the Sprouting Fungus. The 

 kefir-grain, like its constituent the Sprouting Fungus working by 

 itself, gives rise to alcoholic fermentation in a nutrient solution 

 of grape-sugar, though of a less active kind than that caused 

 by the Sprouting Fungus of beer-yeast. But alcoholic fermenta- 

 tion is produced in milk-sugar as such neither by Sprouting 



