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Project Summary

Difficulty	6
Time required	Average (about one week)
Prerequisites	None
Material Availability	Readily available
Cost	Low ($20 - $50)
Safety	No issues

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Abstract

Objective

The purpose of this project is to see if yeast will reproduce using various sugar substitutes.

Introduction

Did you ever wonder how bread gets its "spongy" structure? If you've 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 molecules. They can also break down larger carbohydrate molecules (like starches present in flour) into simple sugar molecules, which are then processed 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 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? This project is designed to find out. You will prepare several different yeast solutions, some "fed" with sugar, others "fed" with sugar substitutes and still others "fed" with only warm water. To measure the metabolism of the yeast under the different conditions, you will collect the carbon dioxide gas from each solution.

Terms, Concepts and Questions to Start Background Research

To do this project, you should do research that enables you to understand the following terms and concepts:

• yeast metabolism,
• simple sugar, compound sugar, starch.

• Investigate the simple sugar molecule glucose and each of your sugar substitutes to find out about their chemical properties. How are the substitutes similar to glucose? How are they different from glucose?
• Now that you have researched yeast metabolism and the chemical properties of the various sugar substitutes, 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 sugar?

Bibliography

• You may wish to choose different sugar substitutes than the examples we chose. This Wikipedia Category page has links to many possible choices:
  Wikipedia contributors, 2005. "Category: Sweeteners," Wikipedia, The Free Encyclopedia [accessed January 10, 2006] http://en.wikipedia.org/w/index.php?title=Category:Sweeteners&oldid=33822221.
• Here are Wikipedia articles on several different sweeteners (including both sugars and sugar substitutes):
  • Wikipedia contributors, 2006. "Acesulfame potassium," Wikpedia: The Free Encyclopedia [accessed January 10, 2006] http://en.wikipedia.org/w/index.php?title=Acesulfame_potassium&oldid=34490285.
  • Wikipedia contributors, 2006. "Aspartame," Wikpedia: The Free Encyclopedia [accessed January 10, 2006] http://en.wikipedia.org/w/index.php?title=Aspartame&oldid=34422683.
  • Wikipedia contributors, 2006. "Glucose," Wikpedia: The Free Encyclopedia [accessed January 10, 2006] http://en.wikipedia.org/w/index.php?title=Glucose&oldid=34642934.
  • Wikipedia contributors, 2005. "Saccharin," Wikpedia: The Free Encyclopedia [accessed January 10, 2006] http://en.wikipedia.org/w/index.php?title=Saccharin&oldid=33009492.
  • Wikipedia contributors, 2006. "Sucralose," Wikipedia: The Free Encyclopedia [accessed January 10, 2006] http://en.wikipedia.org/w/index.php?title=Sucralose&oldid=34548918.
  • Wikipedia contributors, 2006. "Sucrose," Wikpedia: The Free Encyclopedia [accessed January 10, 2006] http://en.wikipedia.org/w/index.php?title=Sucrose&oldid=34571913.

Materials and Equipment

For this project you will need the following items:

• dry yeast (buying a whole jar is probably more economical than individual packets),
• sugar,
• sugar substitutes, for example:
  • saccharin,
  • sucralose,
  • aspartame (commercial name: NutraSweet),
  • acesulfame potassium (also known as: Ace-K);
• warm water (typically 110°F–115°F, but consult the recommendations on your yeast package),
• thermometer to measure water temperature,
• at least 6 empty plastic bottles (1-pint water bottles are good),
• 1 cap (must fit all bottles)
• plastic tubing,
• epoxy or silicone sealant,
• graduated cylinder (an empty plastic bottle can be substituted, if necessary),
• plastic tub or bucket,
• packing tape,
• water.

Experimental Procedure

1. Do your background research.
2. You will be collecting CO₂ from the yeast by displacing water trapped in an inverted graduated cylinder. Here's 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.
3. Next, you need a way to bring the CO₂ from the yeast to your gas collection apparatus. You'll 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. 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 cm.
   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.
   4. You'll attach this cap to your yeast bottle, and place the other end of the tubing inside the inverted graduated cylinder. 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 blowing gently into the tube. The bubbles you create should be captured inside the cylinder. (You'll need to re-fill the cylinder before starting your experiment.)
4. When your gas collection apparatus is ready, you can start the actual experiment.
5. Label each of the bottles with the type of solution you'll be feeding the yeast (e.g., sugar, nothing, saccharin, sucralose, aspartame, acesulfame potassium).
6. You'll 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.
7. Dissolve 1 tablespoon of sugar in 1  cup of warm water (110°F–115°F). When the sugar is fully dissolved, add 2 teaspoons 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.
8. 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.
9. 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.
10. 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.
11. When the time is up, note how much CO₂ was collected.
12. 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.
13. 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 teaspoons). Use 1 tablespoon of the sugar substitute instead of sugar.

• 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?

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's 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.

• For more science project ideas in this area of science, see Microbiology Project Ideas.

Credits

original project by Scott L. Karney-Grobe

with additional material by Andrew Olson, Ph.D., Science Buddies

Last edit date: 2006-06-30 10:15:00

Career Focus

If you like this project, you might enjoy exploring careers in Microbiology.

Microbiologist Microorganisms (bacteria, viruses, algae, and fungi) are the most common life-forms on Earth. They help us digest nutrients; make foods like yogurt, bread, and olives; and create antibiotics. Some microbes also cause diseases. Microbiologists study the growth, structure, development, and general characteristics of microorganisms to promote health, industry, and a basic understanding of cellular functions.

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