248 NUTRITION AND METABOLISM 



fluorescent pigments and perhaps a few other groups are distinguished, 

 but their chemical nature is rather vague as yet. The absorption of 

 distinct lines of the spectrum by solutions of these pigments is claimed 

 to be a very reliable means of distinguishing the pigments of different 

 species. 



AROMATIC SUBSTANCES constitute another group of metabolic prod- 

 ucts. The chemical analysis accomplishes more with these com- 

 pounds than with pigments, since they are frequently well-known 

 compounds. The main difficulty arising in their identification is in 

 the very minute quantities of the products available. Some substances 

 with strong, mostly very disagreeable odors have already been men- 

 tioned: indol, skatol, hydrogen sulphide, mercaptan, the amins and 

 ammonia, butyric acid, and some of the higher alcohols. There re- 

 main to be mentioned certain oils and esters giving rise largely to 

 pleasant aromas. The formation of aromatic oils has been established 

 although their nature is entirely unknown. The same is true with the 

 esters. The substance causing the fishy flavor in butter is volatile 

 with steam and is neither of an alkaline nor acid nature. The strong 

 odor of freshly plowed earth is caused by an Actinomyces; the odor 

 can be traced to a very volatile oil the nature of which has not been 

 determined. The aroma of fermented liquids wines, beers, and 

 many others is partly due to compounds constituting the fermenting 

 material, and partly to the fermenting agent. Some yeasts are 

 known to produce fruit-esters, as succinic-acid-ethylester and the 

 corresponding esters of malic and other acids. Besides, some glucosides 

 may be split and traces of hydrocyanic acid and benzoic acid may be 

 liberated. The change of flavor with the aging of wines is probably 

 more a chemical than a biochemical change. 



ENZYMES AND TOXINS. Among the most interesting and least 

 understood products of microbial action are the enzymes and the toxins. 

 These two groups are related in many respects The enzymes have 

 been discussed extensively in a preceding chapter and toxins will be 

 treated more extensively on pages 676, 740. Toxins and enzymes are 

 formed by the cells in such small quantities that they would never have 

 been discovered by ordinary chemical means were it not for the unusual 

 effects which they produce, the enzymes acting upon food substances, 

 and the toxins acting physiologically upon organisms. Toxins and 

 enzymes are chemically unknown. It is assumed that they are chemical 



