IMPURITIES IN COMMERCIAL YEAST 91 



are obtained by a moderately active growth of the yeast, 

 which will produce a sufficient amount of lightness in the 

 dough in the course of eight or ten hours. 



Impurities in Commercial Yeast 



One factor largely determining the value of commercial 

 yeast is its purity. It rarely or never happens that a yeast 

 cake, or yeast culture of any form, is composed purely of 

 yeast. There is almost certain to be mixed with it a quantity 

 of bacteria. Frequently there is also present a variety of 

 mold spores, and especially in summer the yeast cake is 

 likely to contain other species of yeast besides the one 

 desired for bread raising. These impurities maybe abun- 

 dant or scanty in any cake of yeast. If they are present 

 at all, they may produce trouble, but this will depend upon 

 circumstances. The yeast plants themselves are present 

 in such overwhelming proportions that, under ordinary cir- 

 cumstances, the impurities get little opportunity to develop. 

 If the raising is conducted at a proper temperature, the 

 impurities will rarely do much injury. In the common 

 use of yeast, therefore, in spite of the fact that various 

 bacteria and mold spores may be mixed with the dough 

 when the yeast is added, the bread rises in a normal way, 

 and the impurities produce no trouble. 



These foreign bacteria in the yeast cake are quite sure 

 to increase with its age. While the yeast plants do not 

 multiply in the compressed yeast cake, the bacteria are 

 almost sure to do so, especially if the cake is kept in a 

 moist condition for some days before using. An old yeast 

 cake is therefore quite sure to contain more of these 



