APPENDIX 305 



33. Effect of Light. Prepare two tubes in the same way and set 

 one in a bright light and the other in a dark place. This may 

 be best done by wrapping the tube in velvet or heavy black paper 

 to keep out the light. Keep both tubes at the same temperature 

 and determine whether light has any effect upon the rapidity of 

 growth. 



34. Effect of Age on Yeast. Obtain an old sample of dried yeast 

 cake. Prepare two tubes of molasses and water and inoculate one 

 with a small quantity of the old yeast cake and two others with a 

 similar quantity of a fresh cake. Set aside in a warm place and 

 determine in which the fermentation starts sooner, and in which it is 

 the more vigorous. Examine with a microscope after fermentation 

 begins, to see if either contains other organisms besides yeast. 



35. Comparative Fermenting Power. Make a dilute mixture of 

 flour and water. Fill three fermentation tubes with the mixture, as 

 shown in Fig. 38. Inoculate one with compressed yeast, a second 

 with dried yeast cake, and a third with brewer's yeast, if it can be 

 obtained. Set all three aside in a warm place for one day, and 

 determine the relative fermenting power of the different yeasts by 

 comparing the quantities of gas that collect in the closed tubes. 



36. Action of Yeast on Bread. Mix up a little flour and water to 

 about the consistency of dough for bread making, and divide into 

 three lots. Into a and b place a little compressed yeast. This may 

 best be done by dissolving the yeast in water and stirring it into the 

 dough during the mixing, a and b are then to be placed in a warm 

 place for five or six hours, w^hile c, without the yeast, is to be baked 

 at once. After a and b have risen under the influence of the yeast, 

 bake b at once in the oven, while c is to be thoroughly kneaded and 

 then baked. Compare the results of a, b, and c, noticing the differ- 

 ence in the textures of the bread. 



37. Overraising. Mix another lot of dough with yeast in the 

 same way and allow it to rise in a warm place for twelve hours or 

 more. Test with litmus paper to see if it is acid. Bake and taste 

 to see if it has become sour. 



38. Bread raised by Wild Yeast. Put a small amount of salt in a 

 little milk and then allow it to stand in a warm place until a froth 

 appears. Mix it with flour to make a dough and set aside to rise. 



