238 NUTRITION AND METABOLISM 



liberated, and butyric and acetic acids are formed at the same time. 

 The marsh gas of the marshes originates in this way. 



Summing up all the products formed from carbohydrates, we find 

 several acids, among them lactic and acetic acids most commonly, 

 and ethyl alcohol, rarely other alcohols, besides carbon dioxide, 

 hydrogen and water. The variety is not so great, but with these few 

 compounds, a number of different combinations are possible, and the 

 complication of the study of such fermentations lies mostly in the 

 simultaneous formation of several of the compounds. 



ACIDS AND ALCOHOLS. The organic acids and alcohols can be 

 decomposed further by bacteria and molds, also by some yeasts, to 

 simpler compounds. Ordinarily, this decomposition consists in the 

 complete oxidation. Thus, Oidium lactis will destroy the lactic acid 

 of sour milk and of soft cheeses by complete combustion. 



C 3 H 6 3 + 60 = 3 C0 2 + 3 H 2 



By the same process, the acidity of sauerkraut, ensilage, pickles is 

 reduced by mycoderma species. Another Mycoderma is known to 

 destroy acetic acid and thus spoil vinegar or fruits and vegetables kept 

 in vinegar; the yeast grows in a thin, dry white scum over the surface, 

 and oxidizes the acetic acid. 



CH 3 CO 2 H + 4 O = 2 CO 2 + 2 H 2 O 



The oxidation of alcohols is not always complete. Especially ethyl 

 alcohol is usually oxidized first to acetic acid; this is the common vinegar 

 fermentation. Many different kinds of vinegar bacteria are known, 

 some forming gelatinous masses of cell membranes called mother-of- 

 vinegar, while others remain as separate small cells. They all oxidize 

 alcohol first to acetic acid. 



CH 3 CH 2 OH + 2 O = CH 3 CO 2 H + H 2 O 



But most of them will oxidize later the acetic acid completely to carbon 

 dioxide, after the alcohol is all exhausted, unless the oxygen supply is 

 shut off. This behavior reminds one of the formation and destruction 

 of oxalic acid by Aspergillus, mentioned previously. It may be re- 

 marked here that the vinegar bacteria cannot attack the sugar directly 

 to any appreciable degree, and the manufacture of vinegar from sugar 

 requires two agents, the alcohol-forming yeast, and the alcohol-oxidizing 

 bacterium. 



