PROBLEM 93 115 



Problem 92 : Do yeasts grow wild ? 



Materials. — Molasses or nutrient solution, Petri dish, fer- 

 mentation tube. 



Method. — Place a Petri dish, or other flat dish, with nu- 

 trient solution in it for a day in any locality exposed to ordinary 

 drafts of air. Then pour its contents into a fermentation tube 

 and plug with absorbent cotton. 



Observations. — Note any change in the contents of the closed 

 end of the tube. 



Conclusion. — 1. Is there any yeast present? If so, where did 

 it come from? 



NOTE. — Spores (reproductive bodies) of yeast are found in the air, and yeasts 

 grow on grape, apple, pear, and other fruit skins. 



2. What causes wine to ferment, cider to become hard (ferment), 

 etc.? 



Problem 93 : To determine the conditions favorable to the 

 growth of yeast. 



Materials. — Fruit jars, yeast cake, molasses. 



Method. — Label three pint fruit jars A, B, and C. Add one 

 fourth of a compressed yeast cake to two cups of water containing 

 two tablespoonfuls of molasses or sugar. Stir the mixture well and 

 divide it into three equal parts and pour them into the jars. Place 

 covers on the jars. Put jar A in the ice box on the ice and jar B 

 over the kitchen stove or near a radiator; boil jar C by im- 

 mersing it in a dish of boiling water, and place it next to B. After 

 forty-eight hours, look to see if any bubbles have made their 

 appearance in any of the jars. 



Observations. — Which jars, if any, show bubbles on the surface? 

 After bubbles have begun to appear at the surface, the fluid in 

 jar B will be found to have a sour taste and will smell unpleasantly. 

 The gas which rises to the surface, if collected and tested, will be 

 found to be carbon dioxide. 



Conclusion. — 1. What conditions are favorable for the growth 

 of yeast ? 



2. How do you know that yeast has grown ? 



