98 BACTERIA, YEASTS, AND MOLDS 



while fresh or the proper results will be lacking. If the 

 yeast cake is kept for a day. or two only, the plants begin 

 to die, and after three or four days only a small number 

 of them may be left alive. Such yeast when a few days 

 old will not produce as quick a raising of the bread as 

 the fresh cake. More than this, a result is frequently 

 experienced in old cakes that is worse than the loss of 

 activity. The commercial compressed yeast is never a 

 pure yeast, but contains a variety of other microscopic 

 plants, among which are bacteria as well as other yeasts. 

 These other organisms are liable to grow in the cake if 

 kept for a few days. The yeast may even decay, which 

 indicates an excessive growth of bacteria ; but if it does 

 not decay it is quite certain that in an old cake other 

 kinds of yeast or bacteria are relatively more abundant 

 than they are in the fresh cake. When such an old 

 yeast cake is used it may give rise to undesirable fermen- 

 tations in the bread, resulting in unpleasant flavors. If 

 it is necessary to keep a compressed yeast cake some 

 days before using it, it is best preserved by placing it 

 in cold water and keeping it on the ice ; but it should never 

 be allowed to freeze. 



Where the compressed yeast cake can be obtained fresh, 

 however, it is the most convenient form in use. It is so 

 cheap that the expense need not be considered in the 

 household, where only a small amount is needed. But 

 where large quantities of bread are to be made, com- 

 pressed yeast is rather expensive, and it may be cheaper 

 then to brew one's own yeast. Consequently bakers 

 long adhered to their own methods of making yeast, to 

 be referred to presently, instead of depending upon the 



