FERMENTATION OF CARBOHYDRATES 187 



added and the sugar fermented to alcohol and carbon 

 dioxide. 



Alcoholic Fermentation of Sugar Solutions. The juices 

 of many fruits contain sufficient quantities of sugar, usually 

 dextrose, levulose, sometimes cane sugar, so that they fur- 

 nish a natural medium for the growth of yeasts. In fact, 

 yeasts in nature apparently grow for the most part upon 

 the surfaces of bruised and diseased fruits. They are 

 probably transported from one fruit to another by the 

 agency of flies and various sucking insects. It is apparent, 

 therefore, that when juice is pressed from fruits it usually 

 contains a considerable inoculation of yeasts. The juices of 

 fruits likewise usually contain considerable quantities of 

 acids, particularly malic, citric, and tartaric. The hydrogen 

 ion concentration in consequence is so high that most species 

 of bacteria are inhibited from growth. This acidity is some- 

 times intensified by the wine manufacturer by the so-called 

 process of sulphuring. In this process fumes of S0 2 are 

 introduced, or compounds readily yielding S0 2 are added. 

 The yeasts most commonly present on the surfaces of fruits, 

 and which develop in their juices, belong to the group 

 frequently termed Saccharomyces ellipsoideus because of 

 the ellipsoid form characteristic of the cells. 



Wines are prepared from the juice of the grape, cider 

 from the juice of the apple, and perry from the juice of the 

 pear. These may in turn be utilized for'the manufacture of 

 vinegar. It should be noted that -afeoholifc fermentation is 

 essentially an anaerobic process. If any of these juices are 

 left exposed to the air, the acetic bacteria will ordinarily 

 begin to grow, transforming the product into vinegar. The 

 juice of the century plant or agave in Mexico is fermented 

 to produce pulque, and in certain tropical countries the 

 juices of certain palms are fermented for the production of 

 palm urine. 



As previously noted, most of the common yeasts do not 



