DESTRUCTIVE METABOLISM 411 
access. As in the alcoholic fermentation, the accumulation of the 
products brings the action to a standstill. When 8 per cent of lactic 
acid has accumulated (or less in milk), the bacterium becomes inactive. 
Acetic fermentation. The acetic fermentation is due to bacteria, 
which oxidize ethyl and other alcohols to acids. The commonest form 
converts ethyl alcohol into acetic acid, CH 3 CH 2 OH + O 2 ^ CH 3 
COOH + H 2 O. In the quick process for the manufacture of vinegar, 
in which this fermentation is applied, dilute alcohol (6-10 per cent) is 
allowed to trickle over beech shavings in a deep vat, which have become 
covered with a slimy coating of the organisms. By the time the alcohol 
has reached the bottom it has been oxidized completely to acetic acid. 
Butyric fermentation. Butyric fermentation, by which butyric acid 
is produced from various sugars, especially lactose, and indirectly from 
polysaccharides, through the agency of bacteria, underlies the production 
of desirable flavors in butter and cheese. 
Putrefactions. The putrefaction of proteins is wrought by various 
bacteria, but little is known of the details. Among the numerous end 
products are the disagreeable gases hydrogen sulfid, mercaptans, skatol, 
etc. 
So a multitude of fermentations might be named, each concerned 
with a particular compound and due to a particular organism. By 
the single or successive action of such organisms, complex organic matter 
is gradually reduced to simple forms, like those from which it was con- 
structed, which then may enter again into the cycle and be built up, 
through the agency of green plants, into foods. 
Advantage. The precise role of fermentations in the life history of 
the organism that produces them is not certainly known. It is possible 
that they are, as respiration is supposed to be, a source of energy. The 
minuteness of the organisms would make possible the appropriation of 
this energy, even though, in contrast to that set free by respiration, it is 
released outside the body. From this point of view it would seem that 
fermentation might be considered as a substitute for respiration, though 
a rather ineffective one, and hence requiring an exaggerated decom- 
position of organic matter. On the other hand, it has been suggested 
that fermentation serves for the production of substances in which the 
producers can live, but by which other organisms are injured and so 
prevented from competing with them for food and room. This sugges- 
tion, however, seems forced and inadequate. Yet again, it may be 
