84 BACTERIA, YEASTS, AND MOLDS 



liquid is to be strained and mixed with the mashed pota- 

 toes. It is well to boil again for a few moments to 

 destroy any microscopic organisms (bacteria or molds) 

 that may have found entrance into the brew during its 

 preparation. The material is then to be cooled and may 

 be allowed to ferment spontaneously. But, since the 

 results are then unreliable, there is usually added to it, 

 after it is cooled to the temperature of 70° to 80°, a small 

 quantity of pure yeast from some reliable source. The 

 whole is to be stirred occasionally and allowed to stand 

 until the yeast has developed for a few days. The yeast 

 will then be present in large quantity in the brew, and 

 can be used for any desirable purpose. 



In making the second of the above brews, the process 

 is nearly the same. The hops are steamed or heated with 

 the water, as in the first case, and then mixed with the 

 malt according to the proportions given in the formula. 

 The subsequent treatment is identical in both cases. 



Some such method of preparing yeast was in former 

 years almost universally used for baking. Brewing yeast 

 is inexpensive, simply requiring a little care ; but with the 

 introduction of the convenient compressed yeast at a small 

 price this method of making yeast has practically disap- 

 peared from the household. It is still retained, however, 

 in some places where large quantities of yeast are used. 



The hops are added to these brews, not as food for the 

 yeast, but for two other purposes : (i) They give a slight 

 nutty flavor which is subsequently imparted to the bread, 

 somewhat improving its taste. (2) The extract of hops is 

 a partial antiseptic, in a measure preventing the growth of 

 bacteria, though not injuriously affecting yeast. Without 



