94 BACTERIA AND FERMENTATION 



Pasteur occupied six years (1857-1863) in the further elucidation 

 of his discovery of the potency of these hitherto unrecognised agents, 

 and the establishment of the fact that " organic liquids do not alter 

 until a living germ is introduced into them, and living germs exist 

 everywhere." It must not be supposed that to Pasteur is due the 

 whole credit of the knowledge acquired respecting the cause of 

 fermentation. He did not first discover these living organisms ; he 

 did not first study them and describe them ; he was not even the 

 first to suggest that they were the cause of the processes of fermenta- 

 tion or disease. But nevertheless it was Pasteur who " first plac'ed 

 the subject upon a firra foundation by proving with rigid experiment 

 some of the suggestions made by others." 



Kinds of Fepmentation 



Although fermentation is nearly always due to a living agent, as 

 proved by Pasteur, the process is conveniently divided into two 

 kinds.* (1) When the action is direct, and the chemical changes 

 involved in the process occur only in the presence of the cell, the 

 latter is spoken of as an organised ferment ; (2) when the action is 

 indirect, and the changes are the result of the presence of a soluble 

 material secreted by the cell, acting apart from the cell, this soluble 

 substance is termed an unorganised soluble ferment, or enzyme. The 

 organised ferments are bacteria or vegetable cells allied to the 

 bacteria ; the unorganised ferments, or enzymes, are ferments found 

 in the secretions of specialised cells of the higher plants and animals. 

 It will be sufficient to illustrate the enzymes by a few of the more 

 familiar examples, such as the digestive agents in human assimilation. 

 This function is performed, in some eases, by the enzyme combining 

 with the substance on which it is acting and then by decomposition 

 yielding the new " digested " substance and regenerating the enzyme ; 

 in other cases, the enzyme, by its molecular movement, sets up 

 molecular movement in the substance it is digesting, and thus changes 

 its condition. These digestive enzymes are as follow : in the saKva, 

 ptyalin, which changes starch into sugar ; in the gastric juice of the 

 stomach, pepsin, which digests the proteids of the food and changes 

 them into more soluble forms ; the pancreatic ferments, amylopsin, 

 trypsin, and steapsin, capable of attacking all classes of food stuffs ; 

 and the intestinal ferments, which have not yet been separated in 

 pure condition. In addition to these, there are ferments in 

 bitter almonds, mustard, etc. Concerning these unorganised ferments 

 • we have little further to say. Perhaps the commonest of them all 

 is diastase, which occurs in malt, and to which some reference will be 

 made later. Its function is to convert the starch, which occurs in 

 * E. A, Schafer, F.R.S., Text-hook of Physiology, vol. i., p. 312. 



