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Guided Inquiry • Real-World Lab
Chapter 9 Lab Comparing Fermentation Rates of Sugars
Problem
How does the type of sugar affect the rate of fermentation?
Introduction
Long ago, humans learned to harness the process of fermentation to preserve foods and to make new foods, such as cheese and bread. At an early stage in the bread-making process, yeast is added to the bread dough. Yeast are unicellular organisms that can break down carbohydrates in dough to produce carbon dioxide and ethanol. The bubbles of carbon dioxide gas that accumulate in the dough give bread its volume and texture. The alcohol evaporates as the dough rises and the bread is baked.
Sugars ferment at different rates. Commercial bakers use data on rates of fermentation to decide which sugars to use in different baked goods. In this lab, you will measure and compare rates of fermentation for five different sugars.
Skills Focus
Predict, Measure, Analyze Data, Infer
Materials
• probe interface
• gas pressure probe
• hot plate
• 400-mL beaker
• thermometer
• ring stand
• test-tube clamp
• medium test tube
• test-tube rack
• sugar solution
• yeast suspension
• 2 pipettes
• vegetable oil
• 1-hole rubber stopper
• plastic tubing with lock fitting
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Safety
Always wear goggles and an apron when using a water bath. If you have glass beakers or test tubes, check for cracks or chips. Alert your teacher if you break a glass object. Avoid contact with the surface of the hot plate. To avoid electric shocks, make sure cords, connections, and your hands are dry. Wash your hands thoroughly with soap and warm water at the end of the lab.
Pre-Lab Questions
1. Infer Why do you think you will add a layer of vegetable oil above the sugar and yeast mixture?
2. Relate Cause and Effect Explain why it is possible to compare the rates of fermentation by measuring gas pressure in the test tubes.
3. Predict Which of the sugars do you think will have the highest rate of fermentation, and why?
Procedure
As a class you will test glucose and fructose, which are simple sugars. You will also test sucrose, lactose, and maltose, which are disaccharides. Each group will test one sugar and share its data with the class.
Part A: Set Up the Water Bath, and Probe Interface
1. Put on goggles and an apron.
2. Plug in your hot plate and use a setting that will produce a temperature between 37°C and 40°C.
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3. Pour about 300 mL of water into a 400-mL beaker and place the beaker on the hot plate. Heat the water until it reaches a temperature between 37°C and 40°C. The yeast will need to be kept within this temperature range throughout the experiment. caution: Make sure your water bath is placed away from the edge of the lab bench or table.
4. While the water is heating, follow your teacher’s instructions for setting up your probe interface. Set the pressure scale on the vertical axis for 90 to 130kPa. Set the time scale on the horizontal axis for 0 to 15 minutes. Set the data collection rate to 6 samples per minute.
5. While the water is heating, follow your teacher’s instructions for setting up your probe interface and probe.
Part B: Prepare the Reactants
6. Use a test-tube clamp to attach the test tube to a ring stand, as shown in Figure 1. You will need someone with a steady hand to do Steps 6–8.
7. Use a pipette to add 1 mL of the sugar solution to the test tube.
caution: Do not allow the solution or the pipette to touch the sides of the test tube.
8. Use a second pipette to gently stir the yeast suspension. Then use the pipette to add 1 mL of the suspension to the test tube.
caution: Do not allow the solution or the pipette to touch the sides of the test tube.
9. Use a dropper to carefully construct a layer of vegetable oil to completely cover the yeast and sugar mixture as shown in Figure 2. Add enough oil to make a layer that is about 0.5 cm deep.
Figure 1 Overall setup Figure 2 Test tube with liquids
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9. Insert the rubber stopper with the post for connecting tubing to the stopper. caution: Don’t connect the tubing to the stopper at this time. You don’t want pressure to build up in the test tube before you are ready to measure the pressure.
10. Carefully lower the test tube into the water bath until the contents are fully submerged in the warm water.
11. You will need to incubate the test tube in the water bath until the yeast have used up all the oxygen that is dissolved in the yeast and sugar mixture. Make sure the temperature stays between 37°C and 40°C.
Part C: Collect Data
12. After 10 minutes, connect the plastic tubing to the rubber stopper. Collect data for 15 minutes. Keep monitoring the temperature during this time. caution: If the pressure exceeds 115 kilopascals, the stopper will pop off. Disconnect the tubing if the pressure reaches 113 kilopascals.
13. After 15 minutes, find the value of the slope, m, for the change in pressure in the test tube. This is the rate of fermentation.
14. Record the slope in the appropriate row of the data table. Post your data on the board and record data from other groups in your table.
15. Turn off the hot plate. Follow your teacher’s instructions for dismantling the setup and for cleanup. Wash your hands thoroughly with soap and warm water before leaving the lab.
Data TableSugar / Rate of Fermentation (kPa/min)
Glucose
Fructose
Sucrose (glucose + fructose)
Lactose(galactose + glucose)
Maltose(glucose + glucose)
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Analyze and Conclude
1. Interpret Tables Which sugar had the highest rate of fermentation? Which sugar had the lowest rate?
2. Draw Conclusions Use what you know about the structures of the sugars to explain the results for glucose, fructose, sucrose, and maltose.
3. Infer Why do you think little or no fermentation occurred with lactose? Hint: Recall the role that enzymes play in reactions that take place in cells.
4. Predict The solutions used in this lab contained only 5 percent of sugar. Would an increase in the concentration of sugar change the rate of fermentation? Explain.
5. Apply Concepts Why did all of the oxygen in the mixture have to be used up before fermentation could begin?
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Extend Your Inquiry
Find a recipe for bread and make one or more loaves. Record your observations at each stage in the process. Use as many of your senses as possible.
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