34 PLANT PHYSIOLOGY 



contain more (like port, with 17% or more) must be "forti- 

 fied," tha^BL alcohol must be added to them; while whis- 

 key, brandy, \tc., are still more artificial, being distilled 

 liquors. 



The souring of milk, the so-called lactic-acid fermentation, 

 is due to jfa activity of a great number of aerobic and 

 anaerobic fflranisms, especially bacteria. These ferment the 

 sugars founoin solution in milk, first inverting the sugar if 

 necessary, then splitting it mainly into lactic acid, thus 



C 6 H w 0. - 2 C, H. 3 



but producing also, in proportions characteristic of the spe- 

 cies, acetic, formic, and other organic acids, and carbon- 

 dioxide. Since these organisms can be active in the presence 

 of only a comparatively small amount of free acid, only a 

 small part of the available sugar is, under natural condi- 

 tions, fermented by them. If, however, the free acid be 

 neutralized by the addition of calcic-carbonate to the milk, 

 fermentation will be resumed and continued until again an 

 excess of free acid accumulates. The addition of lime-water 

 (a dilute solution of calcic hydrate) to the milk given to 

 infants is advantageous because it neutralizes the traces of 

 free acid which may have been formed in the milk. 



Butter becomes rancid when the obligate aerobic and the 

 facultative or obligate anaerobic bacteria invariably present 

 in butter succeed in decomposing the dextrose and other 

 non-nitrogenous substances, possibly also some of the pro- 

 teids, into a number of simpler compounds of which butyric 

 acid is the most abundant and characteristic. The by- 

 products, gaseous and other, odorous or not, vary with the 

 organisms accomplishing the fermentation, while the sub- 

 stances acted upon, the course of the reactions, and the 

 activity of the fermentation vary also with the conditions. 



It must not be inferred that all so-called fermentations are 

 the result of splitting the compounds acted upon into sim- 

 pler ones. The acetic acid fermentation, for example, is an 

 oxidation carried on by aerobic bacteria (two species, ac- 

 cording to Hansen,* Bacterium aceti, B. Posteu rianu m), 



* Hansen. Loc. cit. 



