YEAST {Torula or Saccharomyces Cerevisice). 



Yeast is a substance which has been long known on account 

 of the power which it possesses of exciting the process termed 

 fermentation in substances which contain sugar. 



If strained through a coarse filter, it appears to the naked 

 eye as a brownish fluid in which no- solid particles can be 

 discerned. When some of this fluid is added to a solution of 

 sugar and kept warm, the mixture soon begins to disengage 

 bubbles of gas and become frothy ; its sweetness gradually 

 disappears ; it acquires a spirituous flavour and intoxicating 

 qualities; audit yields by distillation a light fluid — alcohol 

 (or spirits of wine) which readily burns. 



When dried slowly and at a low temperature, yeast is 

 reduced to a powdery mass, which retains its power of exciting 

 fermentation in a saccharine fluid for a considerable period. 

 If yeast is heated to the temperature of boiling water, be- 

 fore it is added to the saccharine fluid, no fermentation takes 

 place; and fermentation which has commenced is stopped 

 by boiling the saccharine liquid. 



A saccharine solution will not ferment spontaneously. 

 If it begins to ferment, yeast has undoubtedly got into it in 

 some way or other. 



If the yeast is not added directly to the saccharine fluid, 

 but is separated from it by a very fine filter, such as porous 

 earthenware, the saccharine fluid will not ferment, although 

 the filter allows the fluid part of the yeast to pass through 

 into the solution of sugar. 



M. 1 



