410 PHYSIOLOGY 



they have been simplified by cleavage into hexoses. Why this limitation 

 exists, and vi^hy within this there are others even more specific, is not 

 known. The organisms concerned are chiefly those known as yeasts 

 (see Saccharomycetes, p. 70), but certain molds and bacteria also give 

 rise to ethyl alcohol, though the latter more commonly produce higher 

 alcohols (propyl alcohol, butyl alcohol, etc.). In this connection it is 

 to be remembered that even the higher plants produce ethyl alcohol in 

 the course of anaerobic respiration. 



The sugar is split up in large measure into CO2 and ethyl alcohol, 

 but there are other products, such as glycerin, succinic acid, etc., in 

 smaller quantity. Fermentation proceeds very slowly when the yeasts 

 are abundantly supplied with O2; then, however, they grow and mul- 

 tiply rapidly, and apparentiy use the sugar chiefly as food. But when 

 the supply of O2 is small, so that their vegetative processes are hindered, 

 fermentative action is increased. Though alcohol is produced at all 

 times, its quantity is in a sort of inverse ratio to the favorableness of the 

 conditions for life. When 12 per cent have accumulated in the liquid, 

 the action is retarded, and by 14 per cent it is stopped. 



Fermentation by yeasts was long believed to be due to the direct action 

 of their protoplasm on the sugar; now it has been proved that an extract, 

 made by grinding the yeast with sand and filtering the juice under high 

 pressure through porcelain, can produce the same effect. The active 

 substance, known as zymase, is soon destroyed, unless protected from 

 digestion by accompanying enzymes. Similar substances have been iso- 

 lated in higher plants, which are believed to act upon carbohydrates in 

 anaerobic respiration,' giving rise to alcohol and CO2 in the same propor- 

 tions as in fermentation. 



The economic uses of alcoholic fermentation are many. It plays a prominent 

 role in the lightening of bread, in which, however, other organisms share with yeast 

 the production of the gases that raise the dough; it is the source of commercial 

 ethyl alcohol, which is distilled from fermented liquids, in which hexose sugars are 

 first produced from corn and potato starch; it gives rise to the alcohol in a host of 

 fermented liquids used as beverages : wine, beer, koumiss, pulque, sak^ etc. 



Lactic fermentation. • — The lactic fermentation, giving rise to lactic 

 acid, is best known in the souring of milk, and may be produced whenever 

 lactose is present in a solution to which the lactic acid bacterium has 



^ The source of these carbohydrates is uncertain. They may be either the unassimi- 

 lated carbohydrates of the food; or, equally well, a carbohydrate nucleus from the 

 decomposition of the protoplasm. 



