178 NUTRITION AND METABOLISM 



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 

 rnaterial, 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 t9xins are treated 

 more extensively on pages 575, 676. 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 

 standardization of toxin solutions is only comparative, however, and 

 gives no clue as to the actual amount of toxin present. Not all ani- 

 mals are sensitive to all toxins. It is quite possible that all bacteria 

 produce compounds with chemical quaUties similar to toxins, and only 

 a few of them happen to react upon men or animals. 



Toxins are not always the product of microbial action. Vegetable 

 toxins or phytotoxins are known, among which the ricin of the castor- 

 oil bean is perhaps the most studied representative. The best-known 

 zootoxin is the rattlesnake poison. These non7microbial compounds 

 have the' same quality as the microbial ^oxins — they are extremely 

 poisohous. Toxins are the cause of disease in diphtheria, tetanus and 

 botuUsm. If a culture of these organisms is filtered through a porcelain 

 filter which removes all bacterial cells, the filtrate injected into an 

 animal will cause the disease with all its accompanying symptoms 

 though there are no riiicroorganisms introduced into the animal body. 



