76 BACTERIA, YEASTS, AND MOLDS 



process, though it probably involved both yeasts and bac- 

 teria. It is interesting to-day simply because it was a 

 method of utilizing the wild microorganisms for the pur- 

 pose for which we now use cultivated yeasts. 



Other devices obtained by spontaneous fermentation 

 have frequently been practiced. Bakers sometimes make 

 a brew which is allowed to ferment spontaneously, and 

 use the product for bread raising. In making the Scotch 

 barms, a brew is prepared containing hops and flour, with 

 other ingredients, and this, at least in making " virgin 

 barm," is allowed to ferment spontaneously. 



In all such cases yeasts are obtained, but they are 

 always mixed with bacteria, which may materially inter- 

 fere with their successful working. The uncertainty of 

 results due to these impurities has led to cultivating yeasts 

 especially for household purposes. Cultivated yeasts are 

 simply wild yeasts from the air which have been freed 

 from impurities and planted in some pure food material, 

 where they grow in abundance, giving finally a mass 

 of pure yeast. Cultivated yeasts are now used almost 

 universally by all bread makers because of their greater 

 reliability. 



FERMENTING POWER OF DIFFERENT YEASTS 



The cultivated yeast used to-day in bread raising has 

 been gradually selected from a large variety of species. 

 The microscopist recognizes many different kinds of 

 yeasts, varying in their microscopic appearance, their 

 rapidity of growth, and their power of producing fermen- 

 tation, as well as in other important characteristics. Most 



