PRODUCTS OF METABOLISM 167 



compounds can be fermented only after being made soluble by an 

 enzyme, the amylase (see mechanism of metaboKsm). Amylase is 

 produced by most molds, by none of the fermenting yeasts, by a few 

 torulas, and perhaps mycodermas, and by a great many of the bacteria. 

 The sugar thus produced from starch is decomposed according to the 

 main types mentioned under sugars. The lactic bacteria and the coli 

 bacteria do not attack starch, but some acid-gas fermentations of 

 starchy foods do take place. Butyric fermentation of starch is com- 

 mon. Alcoholic fermentation can be accomplished only by some of 

 the Mucors, and Aspergilli. 



Cellulose is decomposed only by very few organisms; these must 

 be very active and very numerous, to judge from the enormous amounts 

 of cellulose produced and destroyed every year on earth. Molds and 

 higher fungi play probably the main r61e in its decomposition; the 

 products have not been determined, but we may well assume a complete 

 oxidation, since no intermediate products have ever been mentioned. 

 No yeast is known to decompose cellulose, and among the bacteria we 

 find but very few species. Some species have recently been isolated 

 which decoippose cellulose in the presence of air; the products have not 

 been determined; we can, however, assume a partial oxidation, eventu- 

 ally a complete oxidation. Besides the aerobic fermentation, we have 

 two types of anaerobic fermentation which are ordinarily described as 

 the hydrogen fermentation and the methane fermentation. In these 

 fermentations the gases mentioned, together with carbon dioxide, are 

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

 The marsh gas of the marshes originates in this way. 



Summing up aU 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 

 compUcation 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. 



CsHeOs 4- 60 = 3CO2 + 3HaO 



