DEFINITION OF FEEMENT AND OF FERMENTATION 203 



ferment which is necessary to effect any particular change, it is found 

 that a very minute quantity of some ferments is quite sufficient to 

 effect a considerable fermentation ; in fact, the weight of the fermented 

 substance may be several hundred thousand times the weight of the 

 ferment. Thus, a very small amount of rennet will coagulate a very 

 large quantity of milk in the course of a few minutes. At the same 

 time, the amount of fermentation is not altogether independent of the 

 amount of ferment originally added, the fermentation being greater 

 when a larger amount of the ferment is added. 



In spite of the fact that ferments can act quite apart from the living 

 cell, they do not seem to be altogether devoid of the properties we 

 associate with living cells. They are pre-eminently susceptible to 

 external influences; in neutral solutions they work either not at all, 

 or in an extremely weak manner. Some absolutely require that a 

 small quantity of acid should be present; others, a small quantity 

 either of acid or alkali. When the solution is made more acid, up to 

 a certain percentage, which is very small, the fermentative activity 

 increases ; but beyond this percentage decrease follows till a point is 

 reached at w r hich the fermentation stops altogether, and subsequent 

 neutralisation will not cause the process to begin again. Further, the 

 presence of certain metallic salts or of antiseptics has an injurious effect 

 on the activity of ferments, though in this respect they possess more 

 resistance than living cells. The influence of temperature on ferments 

 is somewhat similar to its influence on living cells. For all living 

 organisms there are minimum, optimum, and maximum temperatures 

 of growth. Ferments behave in the same way. The minimum usually 

 lies near C. and the optimum between 40 C. and 60 C., whilst the 

 maximum is never as high as 100 C. and very seldom above 70 C. 

 If exposed to a high temperature, ferments do not regain their 

 activity when the temperature is again lowered; but, on the other 

 hand, their activity is only arrested, not destroyed, by exposure to 

 low temperatures. 



Ferments are sometimes described as catalytic agents, i.e. agents 

 which induce reactions to take place without being finally altered 

 themselves by the reactions. Many such are known to chemists. 

 Thus, finely divided platinum acts as a catalyst in inducing the com- 

 bination of sulphur dioxide and oxygen to form sulphuric acid : also, 

 dilute acids act as catalysts in inverting cane-sugar. It is possible that 

 when the action of inorganic catalytic agents is fully explained much 

 light will also be thrown on the action of ferments. In the meantime, 

 it will greatly facilitate an understanding of the subject if the student 



