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Knight Foundation Summer Institute

Liane D'ALlessandro, Haverford College


This lesson (modified from 202 Oozing, Bubbling, Dripping, and Bouncing Experiments, p. 34) is an excellent example of the "typical" scientific experiment in that it tests the effects of a certain variable on a process. Specifically, in this experiment, the variable is the yeast and the process being tested is the decomposition of a banana. Therefore, the control set up will be a banana without any yeast on it. It will be compared to the banana with yeast on it. The experiment also provides a good opportunity for the students to make observations over several days on what they see happening -- a crucial component to any scientific investigation. But because of the specifics of this experiment, it would also fit well into a biology unit dealing with the characteristics of different classes of organisms.

It may be interesting to have the students perform the experiment first and then discuss the "science" behind it later. That way, they would be more surprised at the results and perhaps be able to interpret the results on their own. Either way, the reason that the banana with yeast decomposes (ripens) much more quickly than the control banana is that yeast is an organism classified in the fungi group. Since this group of organisms does not contain chlorophyll, as green plants do, they cannot make their own food. Therefore, they rely on other sources for their food. More specifically, in this experiment, the yeast feeds on the banana causing it to decompose more quickly. This experiment provides exciting results within four days (maybe sooner depending on the temperature of the room that the experiment is performed in). After the students have proved to themselves that yeast feeds on other organisms, they can then understand how this phenomenon makes possible delicious pizza, bagels, and other breads (see "Extensions" section).


  1. to record and interpret observations in a scientific manner
  2. to identify the characteristics of the fungi group that causes yeast to feed on other organisms
  3. to understand what effects yeast has on food decomposition









for each student:

  • (2) slices from a banana (each approximately 2 cm. long) (2) plastic sandwich bags 1/2 tsp. of dry yeast plastic spoon and knife
  • tape and marker to label bags
  • pen and paper to record observations


Each student can do this on an individual basis:

  1. Cut two slices from a banana each approximately 2 cm. long.
  2. Place each slice in a separate plastic bag.
  3. Add approximately 1/2 tsp. of yeast on top of ONE of the bananas. (Make sure the yeast touches the banana.)
  4. Seal both bags and label each bag with the date and contents (e.g. " 10-25-97 banana without yeast " and " 10-25-97 banana with yeast").
  5. Record an observation for this first day. What do the contents in each bag look like?
  6. Each day for the next 3-4 days observe the two bags and record all observations.


  • Have children turn in their observations.
  • Discuss the results of the experiment as a class.
  • Make sure each child can explain what happened in the experiment using the vocabulary above.


Have the student modify this experiment. For example, they could test what effect yeast has on an apple or a piece of bread or anything else that interests them. They could also perform the experiment with a substance other than yeast to test if that substance is a "decomposer" like yeast The children could also observe their results from their own experiments and the original one by looking at the the smear of yeast on the banana (or whatever else they chose to use) under the microscope.

An interesting application of this knowledge about yeast is that it explains how bread is made. The reason that dough rises is that when yeast reacts with sugar, carbon dioxide gas is produced. This gas is what makes the dough rise. The children can observe the formation of carbon dioxide when yeast is added to sugar and water (in the beginning of a bread recipe) because bubbles appear: Later in the recipe when flour is added, the yeast reacts with the flour by breaking down the flour (complex carbohydrates) into simpler sugars. The yeast then reacts with these simpler sugars in the same way as mentioned above. Only this time, instead of seeing bubbles of carbon dioxide gas, students see the dough rise. As a fun, educational, and delicious activity, the class could make their own pizza. But wait . . . the fun and learning continues as pizza could also be used in a fractions lesson! The possibilities are endless! (As a related side note, this demo of making bread shows that the reaction of yeast and sugar produces carbon dioxide; therefore carbon dioxide should also have been present in the banana with yeast bag. Have the students record whether they noticed that the bag with the yeast "blew up" any in comparison to the bag without the yeast. The amount of carbon dioxide produced may not be abundant enough but it would be an interesting result to look for.)

A great web site dealing with yeast and bread is

Another application of this knowledge about decomposers involves gardens. Yeast is not the only decomposer in our environment. The fertilizer that is used in gardens is also one. It breaks down the dead material found in soil into a form that is usable by plants. Perhaps the class could do an experiment to test the effects that fertilizer has on the growth of plants.

Related to this idea of plants, the class could also make a compost pile. Essentially a compost pile is a large mixture of decomposing organic material that provides a garden with tons of nutrients. Incidentally, making compost piles also cuts down on the abundance of waste that this country produces.

Philadelphia Science Content Standards:


This experiment satisfies Benchmark 1 for grades 5-8: "design, modify, and conduct an investigation through testing, revising, and occasionally discarding ideas, all of which lead to a better understanding of how things work." It also satisfies Benchmark 3 for grades 5-8: "collect and summarize data from an experiment and interpret the results in terms of the data."


This experiment satisfies this standard because it allows the students to "develop an understanding of the characteristics and life cycles of organisms" -- in this case, the organism ~s the yeast.


This experiment would fit in great on a biology unit on the characteristics of various organisms. It could also be a lesson on chemical reactions (enzymes in yeast reacts with the sugar to form carbon dioxide.

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