Yeast Reproduction in Sugar Substitutes
AbstractThere 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!
Edited by Andrew Olson, PhD, Science Buddies
Edited by Svenja Lohner, PhD, Science Buddies
Recommended Project Supplies
ObjectiveInvestigate how much carbon dioxide yeast produces with different sugar substitutes.
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, as shown in Figure 1.
Figure 1. Baker's yeast (Saccharomyces cerevisiae) viewed with a scanning electron microscope.
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
- Fungus (plural: fungi)
- Yeast metabolism
- Carbon dioxide
- 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?
BibliographyYou 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:
- Wikipedia Contributors (n.d.) Category: Sweeteners. Wikipedia: The Free Encyclopedia. Retrieved April 14, 2011.
- McGee, Harold. (2004). On Food and Cooking: The Science and Lore of the Kitchen. New York: Scribner.
- Selim, Jocelyn. (2005, August 6). The Chemistry of Artificial Sweeteners. Discover Magazine. Retrieved April 14, 2011.
- Sohn, Emily. (2008, January 9). Sweeeet! The Skinny on Sugar Substitutes. Retrieved April 14, 2011.
Recommended Project Supplies
- Measuring Gas Production Kit, available from our partner
Home Science Tools. Includes:
- 250-mL graduated cylinder
- 100-mL graduated cylinder
- Wide-mouth, 8 oz. squirt bottles (4)
- Clear plastic tubing
- Waterproof thermometer
- You will also need to gather these items:
- Dry yeast. Tip: Buying a whole jar is probably more economical than individual packets.
- Plastic tub or bucket
- Optional: Plastic wrap
- Packing tape
- Permanent marker
- Measuring spoons
- Sugar (3 tsp.)
- Sugar substitutes (3 tsp. of each type), for example:
- Aspartame. The commercial name is NutraSweet.
- Acesulfame potassium, also known as Ace-K
- Measuring cup
- Warm water, typically 43-46°C (about 110°F–115°F), but consult the recommendations on your yeast package
- Clock or timer
- Lab notebook
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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: Project Involving Potentially Hazardous Biological Agents and Scientific Review Committee. You can also visit the webpage ISEF Rules & Guidelines directly.
Setting Up the Gas Collection Apparatus
Remove the small red cap from one of the squeeze bottles. Then connect the tubing to the tip opening, as shown in Figure 2. Make sure that you have a tight fit.
Figure 2. Tube connected to the bottle opening.
You will be collecting carbon dioxide from the yeast by displacing water trapped in an inverted graduated cylinder. Here's how to set it up:
- Fill your plastic dishpan (or bucket) about one-third full with water.
Fill the 100-mL graduated cylinder with water.
- If your dishpan is deep enough, fill the graduated cylinder by tipping it on its side inside the dishpan. 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 dishpan with packing tape (or have your helper hold it in place).
- If your dishpan is not deep enough, fill the graduated cylinder completely using the faucet and cover the top tightly with plastic wrap. Quickly invert the cylinder and place the opening in the dishpan, beneath the surface of the water. Remove the plastic wrap. Attach the cylinder to the side of the tub with packing tape (or have your helper hold it in place).
The graduated cylinder should now be upside down, full of water and with its opening under the surface of the water in the dishpan. Place the free end of the tubing from the plastic bottle inside the graduated cylinder. Your apparatus is now ready to trap carbon dioxide from the yeast (see Figure 3).
A graduated cylinder is placed upside-down in a tub of water and a plastic tube enters the cylinder from underwater. The plastic tube is connected to the nozzle of a squeeze bottle. Air from the squeeze bottle will be funneled into the upside-down cylinder and will be trapped by the water below.
Figure 3. Picture of the inverted graduated cylinder gas collection apparatus.
- You can test your gas collection apparatus by removing the tube from the bottle top and blowing gently into the tube. The bubbles you create should be captured inside the cylinder. (You will need to reconnect the tube to the bottle and re-fill the cylinder before starting your experiment.)
Running the Experiment
- 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). If you need more than four bottles, you can re-use them. Make sure to rinse them out thoroughly between experiments.
- Dissolve 1 teaspoon (tsp.) of sugar in ½ cup of warm water (110°F–115°F). When the sugar is fully dissolved, add ½ teaspoon of yeast, mix and pour into the appropriate bottle. Be sure to note the actual temperature of the water in your lab notebook.
- 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.
- 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.
- 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.
- 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.
- Decide how long to collect CO2 (somewhere between 15–30 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.
- Note: Do not let the graduated cylinder become completely filled with CO2, but instead stop it before this point. If you let it become completely filled, and the next condition you test makes even more CO2, this could lead to poor and inaccurate results because your graduated cylinder may fill up before your test time is over.
- Tip: If your solution makes a large amount of CO2 very quickly, you can try to make it produce less CO2 by using less sugar and possibly less yeast. For example, you could repeat this step using ½ tsp. sugar (instead of 1 tsp.) and ¼ tsp. yeast (instead of ½ tsp.).
- When the time is up, note how much CO2 was collected by observing how much water was displaced from the graduated cylinder.
- 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.
- 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 (½ tsp.). Use 1 tsp. of each sugar substitute instead of sugar.
Analyzing Your Data
- Calculate the average volume of the CO2 produced for each condition you tested and write this in your lab notebook.
- Make a graph of your results.
- Write the different conditions (e.g., plain sugar, saccharin, no sugar, etc.) on the x-axis (the horizontal axis).
- Plot the corresponding average volume of CO2 produced on the y-axis (the vertical axis).
- 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?
- 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?
Ask an Expert
- 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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