lOO BACTERIA, YEASTS, AND MOLDS 



any one to obtain good results with them. Their great 

 advantage is that they need not be absolutely fresh. The 

 cakes may be preserved for many weeks or even months, 

 and their powers will not be destroyed. They cannot 

 decay or mold, since they contain little water. It is always 

 well to remember, in using them, that the drying of the 

 yeast destroys some of the yeast plants and in time kills 

 them all. If such a yeast cake is examined week after 

 week, a decreasing number of living yeast plants will be 

 found, and finally they will all disappear. The fresher 

 the cakes are, the better ; and those that are very old are 

 useless. But in spite of this fact these dried yeast cakes 

 may be kept for many weeks, and for persons who have 

 not a ready access to a market they are much more con- 

 venient than the compressed cakes. 



Sometimes yeast is prepared in the form of a dry powder. 

 It is not a very common form, and the statements made 

 concerning dried yeast cakes will apply equally well to 

 yeast powder. 



Brewer's Yeast. Yeast has frequently been sold in a 

 liquid form by brewers to bakers, to be used in raising 

 bread. Yeast from such a source is different in variety 

 and action from that of the compressed or dried yeast 

 cake. It grows in the brewer's fermenting vats, either as 

 the " top " yeast or the " bottom " yeast. The former 

 grows as a scum on the top of the vat, while the latter 

 sinks to the bottom, the former alone being used for bread 

 raising. The first that appears on the surface of the vat 

 is commonly removed, since it is liable to be filled with 

 dirt and harmful bacteria. The flavor of bread raised 

 with brewery yeast is a little different from that raised by 



