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Yeast Reproduction in Sugar Substitutes

Difficulty
Time Required Average (6-10 days)
Prerequisites None
Material Availability Readily available
Cost Low ($20 - $50)
Safety No issues

Abstract

There is nothing quite like the smell of fresh-baked bread to make your mouth water! As any baker can tell you, you cannot bake bread without yeast. Yeast actually eat sugar so that they can reproduce and make more yeast, and make bread dough rise. But can they use sugar substitutes to do this? In this science project you will get to investigate how well yeast grow with sugar substitutes as a food source. Pass the butter, please!

Objective

Investigate whether yeast will reproduce using various sugar substitutes.

Credits

Scott L. Karney-Grobe

Edited by Andrew Olson, Ph.D., Science Buddies

  • Elmer's is a registered trademark of Elmer's Products Inc.

Cite This Page

MLA Style

Science Buddies Staff. "Yeast Reproduction in Sugar Substitutes" Science Buddies. Science Buddies, 5 Apr. 2014. Web. 28 July 2014 <http://www.sciencebuddies.org/science-fair-projects/project_ideas/MicroBio_p005.shtml?from=Blog>

APA Style

Science Buddies Staff. (2014, April 5). Yeast Reproduction in Sugar Substitutes. Retrieved July 28, 2014 from http://www.sciencebuddies.org/science-fair-projects/project_ideas/MicroBio_p005.shtml?from=Blog

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Last edit date: 2014-04-05

Introduction

Did you ever wonder how bread gets its "spongy" structure? If you have ever baked homemade bread yourself, you know that you need yeast to make the bread dough rise. Yeasts are single-celled fungi. Like the cells in your body, they can derive energy from sugar. They can also break down larger carbohydrates (like starches present in flour) into simple sugars, which are then digested further.

Yeast can extract more energy from sugar when oxygen is present in their environment. In the absence of oxygen, yeast switch to a process called fermentation. With fermentation, yeast can still get energy from sugar, but less energy is derived from each sugar molecule. In addition to deriving less energy with fermentation, the end products of sugar metabolism in yeast are also different. When oxygen is present, the sugar molecules are broken down into carbon dioxide and water (plus energy that the yeast uses to grow and reproduce). In the absence of oxygen, the sugar molecules are not broken down completely. The end products are alcohol (with two carbon atoms), carbon dioxide (one carbon atom), and water. Less energy is extracted from each sugar molecule: the energy that could be extracted from the alcohol molecule if oxygen were present.

As you know, carbon dioxide is a gas (at least at room temperature and atmospheric pressure, for you gas law aficionados). In bread dough, carbon dioxide produced by yeast forms bubbles that make the dough rise, and give bread its spongy texture.

OK, so yeast can derive energy from simple sugars and complex starches. What about sugar substitutes? Can the yeast use sugar substitutes to grow and reproduce? In this science project, you will find out by preparing different yeast solutions, and "feeding" some with sugar, others with sugar substitutes, and still others with only warm water. To measure the reproduction of the yeast under the different conditions, you will collect the carbon dioxide gas from each solution.

Terms and Concepts

  • Yeast
  • Fungus (plural: fungi)
  • Sugar
  • Carbohydrates
  • Fermentation
  • Yeast metabolism
  • Carbon dioxide
Questions
  • What are some different types of sugar substitutes?
  • How is the simple sugar glucose similar to, and different from, sugar substitutes?
  • Do some additional research on yeast metabolism. Based on your research, and knowledge of the sugar substitutes you want to test, what do you predict will happen in your experiment? Which (if any) sugar substitutes will the yeast be able to use? Do you think yeast grown with sugar substitutes will produce more, less or the same amount of carbon dioxide as yeast grown with regular sugar?

Bibliography

You may wish to choose different sugar substitutes than the examples we list in the Materials section. This Wikipedia Category webpage has links to many possible choices:

These resources contain general information about sugar substitutes:

Materials and Equipment

These items can be purchased from Carolina Biological Supply Company, a Science Buddies Approved Supplier:
  • Graduated cylinder, at least 100 mL. Alternatively a 500 mL bottle can be substituted, if necessary.
  • Plastic tubing, approximately three times as long as the height of the plastic tub or bucket used. For example, for a 10 inch tall bucket, you will need 30 inches of tubing.
  • Thermometer to measure water temperature
  • Dry yeast. Alternatively this can be found in the baking aisle of the grocery store. Tip: Buying a whole jar is probably more economical than individual packets.
You will also need to gather these items:
  • Plastic tub or bucket
  • Water
  • Optional: Plastic wrap
  • Packing tape
  • Empty 500 mL plastic bottles (at least 6)
  • Cap. The cap must fit all of the bottles.
  • Drill or nail and a hammer
  • Elmer's® super fast epoxy cement or a silicone sealant that works with plastics
  • Permanent marker
  • Measuring tablespoon
  • Measuring teaspoon
  • Sugar (3 Tbsp.)
  • Sugar substitutes (3 Tbsp. of each type), for example:
    • Saccharin
    • Sucralose
    • Aspartame. The commercial name is NutraSweet.
    • Acesulfame potassium, also known as Ace-K
  • Measuring cups
  • Warm water, typically 110°F–115°F, but consult the recommendations on your yeast package
  • Clock or timer
  • Lab notebook

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Experimental Procedure

Working with Biological Agents

For health and safety reasons, science fairs regulate what kinds of biological materials can be used in science fair projects. You should check with your science fair's Scientific Review Committee before starting this experiment to make sure your science fair project complies with all local rules. Many science fairs follow Intel® International Science and Engineering Fair (ISEF) regulations. For more information, visit these Science Buddies pages: Projects Involving Potentially Hazardous Biological Agents and Scientific Review Committee. You can also visit the webpage ISEF Rules & Guidelines directly.

  1. Do your background research on yeast metabolism and sugar substitutes and develop a hypothesis based on what you find out.
  2. You will collect CO2 from the yeast by displacing water trapped in an inverted graduated cylinder, as shown in Figure 1, below. Here is how to set it up:
    1. Fill your plastic tub (or bucket) about one-third full with water.
    2. Fill the graduated cylinder with water.
      1. If your tub is big enough, fill the graduated cylinder by tipping it on its side inside the tub. Allow any bubbles to escape by tilting the cylinder up slightly, while keeping it under water. Keeping the opening of the cylinder under water, turn it upside down and attach it to the side of the tub with packing tape.
      2. If your tub is not big enough, fill the graduated cylinder completely and cover the top tightly with plastic wrap. Quickly invert the cylinder and place the opening in the tub, beneath the surface of the water. Remove the plastic wrap. Attach the cylinder to the side of the tub with packing tape.
    3. The graduated cylinder should now be upside down, full of water and with its opening under the surface of the water in the tub. It is ready to trap CO2 produced by your yeast.
Gas collection apparatus setup.
Medical Biotechnology science project
Figure 1. To create your gas collection apparatus, first fill a bucket or tub about one-third full with water. Then fill a graduated cylinder completely full with water and invert it in the bucket, making sure that there is no air inside the graduated cylinder, and then tape it to the bucket's side. Next, feed the empty end of the plastic tubing from the yeast bottle (on the right) into the opening at the bottom of the graduated cylinder.
  1. Next, you need a way to bring the CO2 from the yeast to your gas collection apparatus. You will attach some plastic tubing to the bottle cap to do this.
    1. Make a hole in your bottle cap, just big enough to insert the plastic tubing. Use a drill or a nail and a hammer. Get help from an adult if needed.
    2. Insert the plastic tubing through the hole in the cap so that it sticks out about 2 centimeters (cm) on the inside of the cap, as shown in Figure 2, below.
    3. Seal the tube to the cap with epoxy or silicone sealant so that it is air-tight. Allow the epoxy or silicone to cure fully before conducting your experiment.
      1. After the epoxy or silicone has completely cured, which should take approximately one day at most, check to make sure the tube and cap are sealed together with an airtight seal or you may have a leak that could affect your results. If there is a leak, apply more epoxy or silicone sealant and let it completely cure again.
    4. After the epoxy or silicone sealant has completely cured, attach the cap with the tubing to one of your 500 mL bottles, and place the other end of the tubing inside the inverted graduated cylinder.
      1. There should be water in the tubing as soon as it is submerged in the water. The CO2 gas will push some water out of the tubing before the graduated cylinder starts to fill with CO2 gas.
      2. Any CO2 produced by the yeast will bubble up inside the cylinder, where it will be trapped. You can measure how much CO2 is produced by seeing how much water is displaced.
    5. You can test your gas collection apparatus by removing the cap from the bottle and blowing gently into the tube. The bubbles you create should be captured inside the cylinder. (You will need to re-fill the cylinder before starting your experiment.)
Gas collection apparatus setup.
Medical Biotechnology science project
Figure 2. To bring the CO2 from the yeast bottle to your gas collection apparatus, make a hole in the bottle cap and insert the tubing about 2 cm through the top of the cap and into the side that will be inside the bottle. Seal the tubing to the cap airtight with epoxy or silicone sealant and allow it to cure.
  1. When your gas collection apparatus is ready, you can start the actual experiment.
  2. Using a permanent marker, label each of the bottles with the type of solution you will be feeding the yeast (e.g., sugar, nothing, saccharin, sucralose, aspartame, acesulfame potassium).
  3. Dissolve 1 tablespoon (Tbsp.) of sugar in 1  cup of warm water (110°F–115°F). When the sugar is fully dissolved, add 2 teaspoons (tsp.) of yeast (this is about the same amount as 1 packet of yeast), mix and pour into the appropriate bottle. Be sure to note the actual temperature of the water in your lab notebook.
    1. You will be making one solution at a time (unless you decide to set up more than one gas collection apparatus). It is important to use the same water temperature each time you make a solution, since yeast activity is temperature-dependent.
  4. Cap the bottle tightly with your "tube cap," and place the open end of the tube inside your gas collecting cylinder. Note the starting time in your lab notebook.
  5. Within 5–10 minutes, the yeast solution should start foaming, and you should see bubbles collecting in the graduated cylinder. Note the time when you first start seeing bubbles in your lab notebook.
  6. Decide how long to collect CO2 (somewhere between 30–60 minutes is probably good, but you may need to adjust for your particular conditions). Use the same amount of time for all of your tests.
  7. When the time is up, note how much CO2 was collected by observing how much water was displaced from the graduated cylinder.
    1. If you used a bottle instead of a graduated cylinder to collect the CO2, you can figure this out by marking where the CO2 level is on the bottle using a permanent marker, and then measuring the amount of water that is needed to fill up the bottle to that mark. Record the amount of water in your lab notebook.
  8. Re-fill your gas collection cylinder, and carefully rinse out the yeast solution from the bottle. You should run at least three separate trials for each food source.
  9. For each of the sugar substitutes, use the properly labeled bottle. When preparing your yeast solution, use the same temperature for the warm water and the same amount of yeast (2 tsp.). Use 1 Tbsp. of each sugar substitute instead of sugar.
  10. Calculate the average volume of the CO2 produced for each condition you tested and write this in your lab notebook.
  11. Make a graph of your results.
    1. Write the different conditions (e.g., plain sugar, saccharin, no sugar, etc.) on the x-axis (the horizontal axis).
    2. Plot the corresponding average volume of CO2 produced on the y-axis (the vertical axis).
  12. How much CO2 did the yeast produce when given the sugar substitutes compared to plain sugar? Could the yeast grow and reproduce using sugar substitutes? Did some work better than others? Can you explain your results?
    1. Note: Many commercial sugar substitutes are mixtures, not pure compounds. Check the labeling of your sugar substitute packaging carefully, and examine the ingredients. How might the additional ingredients affect the outcome of your experiment?

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Variations

  • For another method of measuring the products of yeast fermentation, see the Science Buddies project: Rise to the Occasion: Investigating Requirements for Yeast Fermentation. You could use the method described in "Rise to the Occasion" to test yeast's ability to use sugar substitutes as a food source.
  • The procedure for making your yeast solutions is very similar to what many bakers do when making homemade bread. It is called "proofing" the yeast. Before the yeast is added to the dough, it is suspended in warm sugar water. If the yeast foams after a few minutes, it is added to the dough. If not, the baker tries another packet of yeast. If one of your sugar substitutes fails to produce CO2 during the allotted time, is the problem the food or the yeast? To test if the yeast is the problem, you could try adding sugar to the solution. If the yeast starts to foam after a few minutes, you've proved that the yeast was not the problem.

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