64 BACTERIA, YEASTS, AND MOLDS 



Spontaneous Fermentation. These wild yeasts are so 

 common in the air that they are sure to be present in 

 most localities, and they fully explain certain phenomena 

 of fermentation that seem at first sight somewhat puz- 

 zling. Almost any sugary solution will contain them. If 

 the juice of an apple is squeezed from the pulp, it forms 

 a sweet liquid which tastes at first almost exactly like 

 the apple from which it is taken. But if it is allowed to 

 stand in a warm place a fermentation begins in it which 

 rapidly changes its character, pro- 

 ducing in a few hours what we call 

 cider. A typical alcoholic fermen- 

 tation has started, just as truly due 

 to the growth of yeast as are similar 

 fermentations in a brewery. Since 

 the yeast has not been planted con- 

 FiG. 36. WUd yeast from sciously in the cider, the fermenta- 

 the juice of an apple, tion must be due to the wild yeasts 

 which causes the fermen- which find their way into the juice, 

 either before it has been squeezed 

 from the apple pulp or afterwards. The apple has been 

 growing in the air for many weeks, and the wild yeasts 

 have had plenty of chances to lodge on its skin. When the 

 juice is squeezed from the pulp it is sure to contain these 

 yeasts, and they promptly start a fermentation (Fig. 36). 



In a similar way other spontaneously fermented prod- 

 ucts are made from the juice of various plants or fruits ; 

 for any sweet juice from such natural sources will be sure 

 to become inoculated with wild yeast and will consequently 

 undergo fermentation. This fact has been learned by 

 almost all people from experience. Most savage tribes 



