Older students can test their ideas about bread yeast by actually baking bread. To do this, they will need to bake one loaf by following the recipe exactly, but change the amount of yeast in each subsequent loaf. For example, the basic recipe for bread calls for flour, water, salt, sugar and yeast. The student will bake one loaf without yeast, another using half the required amount and a third using twice the required amount of yeast. The student will run these trials to determine the relationship between yeast and bread. Does doubling the yeast double the size of the loaf?
For students who don't have the time or patience to bake multiple loaves of bread, making bread in a bottle is another option. The student will simply combine the standard bread ingredients in clean soda bottles and place a balloon over the mouth of each bottle to gauge the amount of carbon dioxide created by the yeast. The student will need to run multiple trials, increasing or removing one ingredient each time, to determine how that ingredient affects the role of the yeast. For example, the student may discover that too much salt inhibits the action of the yeast.
Many bread recipes call for the baker to leave the dough on the counter to rise overnight because yeast multiplies rapidly at room temperature. What effect does temperature have on yeast? Find out by mixing up a small batch of runny bread dough and pouring it into different measuring cylinders. Place one cylinder in a beaker of ice water, another in hot water and a third in room temperature water. Use thermometers to make sure the water temperature stays constant. Measure the starting volume and record changes every two minutes for half an hour.
For this experiment, the student will make and test a prediction about the ideal nutrients for yeast to grow. To do this, get three clean 1-liter soda bottles, a 2-liter bottle of soda that's gone flat and a gallon of drinking water. In the first bottle, add yeast to pure soda. In the second bottle, add yeast to pure water, and in the third bottle add yeast to a solution of half water and half soda. Fit balloons over the mouths of the bottles and record the diameters of the different balloons after 24 hours. From this experiment, extrapolate what ingredients must be necessary to make a loaf of bread rise.