28 FERMENTATION, PUTREFACTION, AND DECAY 
mentation occurs in food after it is mixed with saliva in the 
mouth. 
Putrefaction and Decay.—If any proteid body—meat, eggs, 
or the like—be left for some time exposed to the air, it will give off 
unpleasant odors, for it is undergoing putrefaction and decay. 
These two processes, though frequently considered the same, are 
slightly different. Both are the result of the chemical decomposi- 
tion of organic compounds, and the terms are commonly applied 
only to the decomposition of material that contains proteids. 
Both result in chemical decompositions which are very complete, 
and more complex and indefinite than the other two types of fer- 
mentations. They are produced by microérganisms, chiefly bac- 
teria, which feed upon the putrefying mass, taking certain atoms 
out of the organic molecules. These molecules, thus losing some 
of their atoms, change their chemical nature. The remaining 
atoms necessarily rearrange themselves to form new compounds 
which are simpler in structure. The distinction between putre- 
faction and decay consists in the fact ihat decay is the term ap- 
plied to decomposition in the presence of oxygen, while putrefac- 
tion takes place in the absence of oxygen. The former is much 
more complete than the latter, resulting in the more complete 
destruction of the substance decomposed. 
Organized and Unorganized Ferments.—These three ex- 
amples of fermentation are very different from one another. One 
seems to break the sugar molecule into two simple portions, car- 
bonic acid and alcohol; the second simply adds a molecule of 
water to one of starch; while the third results in a complete de- 
composition of a highly complex proteid into a large number of by- 
products, both known and unknown. But in some important 
respects all three agree. Each differs from ordinary chemical 
processes in several respects and all agree in the following points: 
1. They are all closely associated with life processes; i.e., are 
brought about directly or indirectly by living agents. 
2. They are all closely dependent upon temperature, ceasing 
