SPONTANEOUS FERMENTATION 83 



have learned to make fermented drinks from the juices of 

 plants or fruits by simply collecting the sweet liquor and 

 allowing it to stand until it ferments. 



These wild yeasts explain another phenomenon occa- 

 sionally seen in the household. The housewife finds that 

 some of her preserved fruits or jellies at times undergo 

 an alcoholic fermentation. This is quite different from 

 molding or decay, and is found only in sugar-holding 

 materials. The preserve develops a peculiar, sharp, pun- 

 gent taste, easily recognized but difficult to describe. It 

 is particularly liable to occur in jellies, partly because 

 they contain much sugar, and partly because, even when 

 covered in jelly tumblers, they are still somewhat exposed 

 to the air and hence are liable to inoculation with wild 

 yeast. Sometimes this phenomenon is also found in con- 

 densed milk and in canned foods that have not been properly 

 protected. It is not uncommon to find a similar fermen- 

 tation occurring in certain types of sugar. Maple sugar 

 which is kept in the pantry for weeks, until it becomes moist, 

 may ferment and develop the peculiar sour taste characteris- 

 tic of this phenomenon. In all such cases the trouble is due 

 to the presence of the wild yeasts which are floating in the 

 air, and which settle and multiply upon any food suited to 

 their gfowth. These wild yeasts are so sure to be present in 

 the air that it is very difficult to protect a fermentable mate- 

 rial from their action unless the air is completely excluded. 



Such wild yeasts do not, of course, live permanently in 

 the air, since the air would itself furnish no food for them. 

 They live and grow in the soil, in decaying fruit on the 

 ground, on the surface of fruit on the trees, and in a 

 variety of other places. The air simply distributes them. 



