202 OUTLINES OF BACTERIOLOGY 



On the other hand, the term fermentation is restricted by some to 

 those processes only which are brought about as the result of the 

 secretion of a definite ferment by living organisms. This, however, 

 would mean that such processes as the change of alcohol into acetic 

 acid through the agency of the acetic-acid bacteria, the souring of milk 

 through the agency of the lactic-acid bacteria, and other similar processes, 

 which since the beginning of observations on this subject have always 

 been regarded as being the most typical of fermentative processes, 

 would be thrust out altogether. Further, the nature of the process in 

 these cases is exactly the same as in those from which ferments have 

 been extracted, so that there seems no reason for their separation. 



Properties of Ferments. Some ferments are excreted into the sur- 

 rounding medium, there to carry on their work ; others, though not 

 excreted, can easily be extracted from the living organism by glycerine 

 or water. A third class of ferments can pass through the cell-wall 

 only after the death of the cell, whilst a fourth kind remains inside the 

 cell even after the death of the latter, and can be reached only by 

 breaking up the cell-walls. No ferment has been extracted in a pure 

 condition. An analysis of the purest form yet obtained shows that 

 carbon, hydrogen, oxygen, and nitrogen enter into their composition, 

 but more than this cannot be established, for the process of further 

 extraction with the object of obtaining a purer ferment has resulted 

 only in a weakening of its power. 



A remarkable characteristic of ferments is their selective power. 

 The same results achieved by them can often be produced by purely 

 chemical means. Thus, dilute acids as well as diastase can change 

 starch into sugar ; but whilst acids can act on almost all substances, a 

 ferment is strictly confined to one, or at most a very small number of 

 closely allied substances. 



Most ferments cease their activity at 70 C. Hence, in all fermenta- 

 tive industries great care is taken that this temperature is not exceeded 

 in cultivating the cells which secrete the ferment. The presence of 

 ferments is made known solely by their action, for they do not respond 

 to any specific chemical test. In general they are partly precipitated 

 by alcohol, and are also carried down when a precipitate such as calcium 

 phosphate is produced in solutions containing the ferment. It has 

 been asserted that they give certain colour reactions, e.g. with orcin and 

 sulphuric acid, but it is more than suspected that these colour reactions 

 are due to the impurities that are always present in ferment solutions. 



As a class, ferments possess a slight degree of diffusibility, some 

 being more diffusible than others. With regard to the amount of 



