200 OUTLINES OF BACTERIOLOGY 



due to the activities of these living cells — their growth, reproduction, 

 nutrition, etc. When the nutritive matter is exhausted, or when from 

 any other causes growth and multiplication are no longer possible, the 

 fermentation ceases. There will obviously be a greater number of 

 yeast cells at the end of fermentation than there was at the beginning. 

 When a little of the yeast that is left over is placed in a fresh stock of 

 the nutritive medium, fermentation begins once more. 



§3. EXAMPLES OF FERMENTATION. 



Yeast Fermentation. The active growth and multiplication of 

 yeast has resulted in a transformation of the sugar into alcohol. It 

 will at once be seen that the connection between the life of the 

 yeast and the fermentation of the sugar is a very close one, and it 

 would appear at first that the connection is of the same nature as that 

 of the act of respiration to the living activities of the cells of our own 

 bodies. But such is not the case with regard to this particular 

 fermentation, for it is possible to kill the yeast cells without 

 retarding the activity of the fermentation, and, on the other hand, it 

 is possible to stop the fermentation without necessarily killing the yeast 

 cells. The conclusion is therefore arrived at that, in this instance, 

 though the fermentation originates from a living organism, its further 

 activity is not bound up with, and that it does not cease with, the 

 life of the organism which initiated it. What has happened is, that a 

 substance has been secreted by the yeast which, though present 

 only in small quantity itself, has the power of converting the sugar 

 into alcohol. This substance is called a ferment, and the process 

 set up by it fermentation. The ferment of yeast is called zymase. 

 The transformation of sugar into alcohol liberates energy, which is 

 utilised by the yeast cells to carry on the work of life. 



Malt Fermentation. Our next illustration is drawn from another 

 familiar fermentation, viz. that resulting in the conversion of starch 

 into sugar through the agency of the ferment contained in malt. 

 The process of malting essentially consists in strewing barley grains 

 on the floors of the malting-house, and providing them with as 

 much heat and moisture as is sufficient to cause germination. When 

 the germination has reached a certain point, the maltster puts an end 

 to the process by raising the temperature. During germination the 

 barley secretes a substance which has the power of converting the 

 starch contained in the barley, into sugar. This particular secretion 

 is known as diastase, and germinated barley grains containing diastase 



