METHODS OF OBTAINING YEAST 93 



to the old method of using leaven. In the seventeenth 

 century the use of yeast began again, and from that 

 time on it has been used more and more widely. As 

 methods of cultivating yeast developed it became pos- 

 sible to obtain a more reliable product, and as the relia- 

 bility of the product increased so did its usefulness in a pro- 

 portionate degree. At the present time yeast has very 

 largely taken the place of leaven in baking, because it has 

 proved easier to handle and more reliable in its results. 



Methods of obtaining Yeast 



The original source of all forms of cultivated yeast is 

 wild yeast, which, as we have seen, may easily be obtained 

 by exposing any sugary solution to the air. To obtain 

 such yeast in quantity sufficient for the purposes of house- 

 hold fermentation, various devices have been practiced. 

 Some of these, though little used at present, are instruct- 

 ive. A very interesting method of obtaining yeast called 

 "salt raising" was frequently practiced by housewives 

 before the introduction of compressed yeast. To a quan- 

 tity of milk was added a little salt, sufficient to delay 

 the growth of the common bacteria which otherwise 

 quickly sbur it. The milk was then placed in a warm 

 place for several hours. The yeasts present in the milk 

 were not injured by the salt, and therefore grew rapidly. 

 The milk soon began to froth from the carbon dioxide thus 

 developed. This material was then used to mix with the 

 dough for the raising. The method here described has 

 nearly gone out of use, and no study has been made of 

 the kinds of microorganisms actually concerned in the 



