122 NUTRITION AND METABOLISM. 



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 mentioned: indol, 

 skatol, hydrogen sulphide, mercaptan, the amins and ammonia, butyric 

 acid, and some of the higher alcohols. There remain to be mentioned cer- 

 tain 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 will be 

 discussed extensively in the following chapter and toxins are treated 

 more extensively in Pt. Ill, Div. VII, Chap. II. 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 

 bodies, but even this has not been proved. A pure toxin has never been 

 obtained and we have no criterion for its purity. The presence of a 

 toxin is recognized only by an animal test, and in this way the com- 

 parative concentration can be determined approximately. Such standard- 

 ization of toxin solutions is only comparative, however, and gives no clue 

 as to the actual amount of toxin present. Not all animals are sensitive 

 to all toxins. It is quite possible that all bacteria produce compounds 

 with chemical qualities similar to toxins, and only a few of them happen 

 to react upon men or animals. 



