98 BACTERIA, YEASTS, AND MOLDS 



in the production of carbon dioxide and alcohol, is due to 

 the action of the yeast upon the sugar and not to the 

 beer extract. The extract is added in these cases chiefly 

 to produce a peculiar flavor in the product, which renders 

 it palatable. The commercial beer extracts simply give a 

 pungent taste and perhaps stimulate the growth of the 

 yeast ; but it is the fermentation of the sugar that causes 

 the sparkle due to the carbon dioxide, and any sugary 

 solution will ferment in a similar way if yeast is added. 

 The product is not palatable, however, unless something 

 is present to give it a flavor. The only reason why such 

 homemade beers are less intoxicating than commercial 

 beers is because the fermentation is allowed to continue 

 but a short time, long enough to produce an abundance 

 of carbon dioxide but only a little alcohol. 



Fermented Milk. A mild fermented beverage is occa- 

 sionally made from milk by means of a yeast. It is called 

 kumiss, and is regarded as useful for invalids, since it 

 is supposed to be more easy of digestion than raw milk. 

 Its preparation is as follows. Into a quantity of milk is 

 placed a little common sugar, — from four to eight table- 

 spoonfuls to a gallon of milk, — and yeast is added just as in 

 homemade beer, one fourth of a cake of compressed yeast 

 in a little water being sufficient for a gallon of milk. The 

 mixture is put in a warm place and fermentation sets in. 

 After twenty -four hours' fermentation the material is 

 bottled and placed on ice ; when cool it is ready for use. 

 The milk becomes slightly soured, giving a taste much 

 relished by some people. It is filled with carbonic diox- 

 ide and contains a small amount of alcohol, and is thus a 

 sort of beer made from milk. It is not much used in this 



