32 PLANT PHYSIOLOGY 



products, as the quality of the fermentable mixture into 

 which they are introduced, which give the peculiar color, 

 texture, flavor, and odor of the different brands. Unless the 

 mixture to be fermented is first sterilized and then inocu- 

 lated with pure yeast exclusively, every bottle of wine, beer, 

 or other liquor, and every loaf of bread, represents the com- 

 bined action of yeasts and bacteria. It contains the pro- 

 ducts, alcoholic and other, so far as they are not driven 

 off or decomposed in the process of manufacture, of all 

 the organisms in the fermenting substance. Preliminary 

 sterilization, the inoculation with pure cultures of yeasts, 

 and fermenting under uniform conditions, are the se- 

 cret of the uniformly good quality of the well-known Ger- 

 man beers and wines. In America such precautions are 

 unusual. 



The substances acted upon bear a very definite relation to 

 the active organisms. Not all substances are fermentable, 

 not even all substances of the same proportional composi- 

 tion. Though there may be in one substance the same, and 

 the same number of, atoms, .these may be so differently ar- 

 ranged that the fermenting organism can act upon the one 

 but cannot upon the other. The yeasts are able to ferment 

 only a limited number of sugars, which are in the main 

 characteristic of the different species.* Some of the yeasts 

 secrete enzymsf which convert starch and cellulose into 

 sugar. Thus ordinary bread yeast, unable to ferment the 

 starch with which it may be brought into contact, secretes 

 an enzym which converts the starch into sugar. Further- 

 more, cane-sugar cannot be directly fermented, and when 

 present in dough, etc., it must first be acted upon by an 

 enzym, "inverted." The resulting sugar, differing only in 

 the arrangement, not in the number or kind, of atoms, is 

 directly fermentable. These chemical changes can be sug- 

 gested, though, on account of the by-products already 



* For tables of these see Miigge's Mikroorganismen, 3te Auflage, Bd. I., 

 p. 204, and the papers by EmU Fischer and collaborators in Ber. d. 

 Deutsch. Chem. GreseUschaft for the last few years. 



t Effront, J. Enzyms and their applications. English translation by 

 Prescott, S. C. 1902. 



