78 BACTERIA, YEASTS, AND MOLDS 



may be inferred from the amount of gas produced. It will 

 be seen that the tube on the left has in the inclosed arm 

 a much larger amount of gas than is found in either of 

 the other samples. This tube was inoculated with a dis- 

 tillery yeast (compressed yeast), and the experiment shows 

 that this type of yeast has a greater fermentative power 

 upon flour than either of the other two forms. It sug- 

 gests also that this yeast will be the most satisfactory 

 for the ordinary domestic purpose of bread raising. 



There are also other factors concerned in the choice of 

 a proper species of yeast. Some kinds of yeast give a 

 sour or otherwise unpleasant taste to bread, and others 

 give to the bread an undesirable color. From the many 

 varieties of yeast which might be used for this purpose 

 certain ones have been chosen by the brewers as par- 

 ticularly well adapted for their type of fermentation ; 

 others are commonly used in distilleries. But this does 

 not necessarily make them the best for bread raising. A 

 long experience in baking has resulted in the selection 

 of the type best adapted for bread raising, and this is 

 a species that grows quickly in dilute sugar solutions 

 and hence raises the bread in a few hours. At the same 

 time it gives rise to a pleasant, agreeable taste, and pro- 

 duces no color. Consideration of all these phenomena 

 has been clearly essential in selecting a yeast which is 

 best adapted for bread making. 



All of the yeasts used by brewers and distilleries 

 to-day belong to the same species, and this species is also 

 the best for bread raising. But although all one species 

 there are several quite distinct varieties having different 

 fermenting powers. 



