Yeast Reproduction in Sugar Substitutes
Published on May 15, 2017
There's nothing quite like the smell of fresh-baked bread to make your mouth water! As any baker can tell you, you can't bake bread without yeast. This project makes clever use of bread dough to measure yeast reproduction three different ways, and investigates how well yeast grow with sugar substitutes as a food source. Pass the butter, please!
The purpose of this project is to see if yeast will reproduce using various sugar substitutes.
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.
Materials and Equipment
• dry yeast (buying a whole jar is probably more economical than individual packets),
• sugar substitutes, for example:
o aspartame (commercial name: NutraSweet),
o 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,
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:
a. Fill your plastic tub (or bucket) about one-third full with water.
b. Fill the graduated cylinder with water.
i. 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.
ii. 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.
c. 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.
a. Make a hole in your bottle cap, just big enough to insert the plastic tubing. Get help from an adult if needed.
b. Insert the plastic tubing through the hole in the cap so that it sticks out about 2 cm.
c. 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.
d. 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.
e. 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.
• 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.
Yeast can reproduce in most sugar substitutes, but not all of them.
Yeast Reproduction in Sugar Substitutes by Alice Gallo, Margaret L. Jones