304 BACTERIA, YEASTS, AND MOLDS 



are stained red. It should be found that most of them are unstained, 

 although a few are stained deep red. 



27. Staining Boiled Yeast. Put some yeast in a test tube with 

 some water. Heat to boiling for a few seconds and then remove 

 some of the yeast with a pipette and stain it as above described. 

 After washing, study to see if the yeast which has been killed by 

 boiling stains better than the living yeast. 



28. Effect of Boiling. Prepare two test tubes of molasses and 

 water and inoculate both with a drop of yeast. Plug with cotton. 

 Place one test tube in water and boil for ten minutes, and then leave 

 both test tubes side by side in a warm place for two days, and deter- 

 mine whether the boiling has been sufficient to kill the yeast. 



29. Wild Yeast. Prepare several test tubes of molasses and 

 water as described and, without plugging with cotton, leave exposed 

 in various places for two or three days. Determine by the appear- 

 ance of bubbles whether fermentation occurs. If any change takes 

 place in the liquids, examine with a microscope to determine whether 

 yeasts have found entrance from the air or whether some other 

 microorganisms are growing in the solution. Commonly bacteria 

 will be found more abundantly than yeasts. 



30. Fermentation of Cider. Grind up a few apples and strain the 

 juice from the same by squeezing through cheese cloth. Collect the 

 juice in test tubes and allow it to stand for a few days. A fermenta- 

 tion soon appears and the juice turns into cider. Examine the sediment 

 with a microscope and detect the presence of yeast. Close up the tube 

 with a cotton plug and leave it for a number of weeks, determining 

 whether it subsequently becomes acid by the developmentof acetic acid. 



31. Fermentation of Grape Juice. Proceed as above, using grapes 

 instead of apples. The juice will become wine if fermentation occurs 

 properly. 



32. Effect of Temperature. Fill three test tubes with molasses and 

 water as above described and inoculate each with three drops of 

 yeast in water. Place one tube in a refrigerator, a second in a 

 moderately warm temperature, about 70°, and a third in a warmer 

 place, near a stove or radiator (temperature about 90°). Compare 

 the three at the end of three, six, and twenty-four hours, and note 

 the effect of temperature upon growth. 



