SAPROPHYTES 389 



of mold in food, however, will give it a changed flavor and this 

 we may not like. We are not likely to be harmed in any way, 

 though, by eating food in which the mold can be tasted. When 

 mold grows in food, the food loses some of its value for us, for 

 the mold has consumed some of the food which we might 

 otherwise have had, and our consumption of the mold does not 

 wholly make up for this. Ultimately, the molds, and the 

 bacteria which are likely to accompany them, will completely 

 consume the food and render it worthless for our use. On the 

 other hand, the growth of some molds in certain of our foods 

 greatly enhances their value by giving them a delicate flavor 

 which we very much desire. If you have ever eaten Roque- 

 fort cheese, you have had experience with one of these useful 

 molds. 



YEASTS 



441. The Prevalence of Yeasts. Yeasts have been used 

 by man for raising bread and for making fermented liquors 

 since before the time of historical records. Yet, notwith- 

 standing this, the actual relation which yeasts bear to these 

 processes was never clearly demonstrated until after the middle 

 of the nineteenth century We do not even yet need to put 

 yeast into fresh fruit juice in order to make a fermented liquor 

 out of it, for yeasts, like mold spores, are widely distributed and 

 are always sure to be present wherever there is suitable mate- 

 rial for them to feed on. Similarly, it is possible to make raised 

 or leavened bread without putting prepared yeast into the 

 mixture. Probably you have eaten what is called "salt ris- 

 ing" bread. This bread is made by putting a little salt into 

 some milk and allowing it to stand in a warm place for a time, 

 and then using this milk for mixing the bread. The salt keeps 

 other organisms from growing in the milk, and yeasts fall into 

 it and multiply until they are numerous enough to raise the 

 bread when it is made. 



In essentially these ways, yeasts have been used for many 

 centuries, but it was not until after the perfection of the micro- 



