Materials Included In Kit

Additional Materials Required

Prelab Preparation

1. To prepare the yeast solution, dissolve 5 g of yeast and 5 g of glucose in 150 mL of warm (approximately 30 °C), distilled water.
2. A minimum of one hour is required to allow the yeast to rehydrate and be active. As time passes, activity will decrease. Make a fresh yeast suspension each day.
3. To prepare a 0.3 M glucose solution, dissolve 5.95 g of dextrose monohydrate (glucose) in 100 mL of distilled water.
4. To prepare a 0.3 M fructose solution, dissolve 5.40 g of levulose (fructose) in 100 mL of distilled water.
5. To prepare a 0.15 M maltose solution, dissolve 5.40 g of maltose in 100 mL of distilled water.
6. To prepare a 0.15 M sucrose solution, dissolve 5.13 g of sucrose in 100 mL of distilled water.

Safety Precautions

Although materials for the Introductory Activity are considered nonhazardous, wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. Evaluate all student-produced procedures for safety. Wash hands thoroughly with soap and water before leaving the laboratory. Remind students to wash their hands thoroughly with soap and water before leaving the laboratory. Please review current Safety Data Sheets for additional safety, handling and disposal information.

Disposal

Please consult your current Flinn Scientific Catalog/Reference Manual for general guidelines and specific procedures, and review all federal, state and local regulations that may apply, before proceeding. All solutions from the introductory activity may be rinsed down the drain according to Flinn Suggested Disposal Method #26b. Sugar solutions may be stored in a lab refrigerator for short periods of time. Sugar water is growth media for microbes, so keep refrigerated and bring to room temperature before resuming the lab. For any chemical not specifically listed in the materials, but used in the inquiry portion of the lab, please consult your current Flinn Scientific Catalog/Reference Manual for general guidelines and specific procedures, and review all federal, state and local regulations that may apply before students begin experimentation.

Lab Hints

• Enough materials are provided in this kit for 30 students working in pairs or for 15 groups of students. The prelaboratory assignment and introductory laboratory activity can reasonably be completed in one 50–minute class period. The inquiry laboratory activity, including time for designing the experiment and running multiple trials, can reasonably be completed in three to four 50–minute class periods.
• This lab kit is compatible with Lab 22 in Investigating Biology through Inquiry from Vernier Software & Technology. The teacher’s manual includes many sets of example data for several different variables.
• Students should be familiar with using the Vernier interface and data-collection software. If they are not, allow an extra day to instruct students on their use. For detailed information on how to use Vernier data collection software and sensors, visit www.vernier.com.
• To reduce error in this lab due to variations in concentration of yeast, keep the yeast suspension on a stir plate and have students only remove the amount they need for a trial. Note: The yeast must be removed from the middle (not top or bottom) of the stirred yeast suspension.
• The normal operating temperature range of the CO₂ Gas Sensor is 20–30 °C.
• When positioning the CO₂ Gas Sensor, it is important to get a good seal with the bottle. Gently twisting while placing the sensor in the bottle will seat it in place.
• The CO₂ Gas Sensor relies on the diffusion of gases into the probe shaft. Students should allow a couple of minutes between trials for the gases from the previous trial to exit the probe shaft. Alternatively, the students can use a firm object such as a book or notepad to fan air through the probe shaft.
• Test tubes with caps can be cleaned and reused for the guided-inquiry portion of the lab.

Teacher Tips

• This lab integrates two important concepts—artificial selection and respiration. Review both concepts before completing this lab.
• Disaccharides and monosaccharides are processed differently by yeast. Enzymes must cleave the disaccharides and convert them to glucose before glycolysis can begin. Students could investigate the differences in chemical structure of different sugars as an extension, integrating structure and function.
• Students could visit a bakery or a baker could speak about the characteristics of yeast and how yeast is controlled in the baking process.
• Students could research the history of fermentation as a way to preserve food prior to refrigeration and modern agriculture.
• Students designing their own procedures should have prior experience analyzing data and writing explanations based on structured laboratory experiences.
• Students proposing their own questions may need to first brainstorm in groups or as a class to come up with designs that integrate the appropriate content.
• Aerobic Respiration and Fermentation Made Easy (Flinn Catalog No. FB1750) explores yeast’s ability to carry out aerobic as well as anaerobic respiration and is a good companion experiment.
• Yeast—on the Job—Student Laboratory Kit (Flinn Catalog No. FB1436) is a good follow-up and comprehensive lab exploring four major life processes—maintaining homeostasis, respiration, enzyme-controlled reactions and reproduction.

Answers to Prelab Questions

1. What are the reactants and products of fermentation?

   Glucose (C₆H₁₂O₆) is the reactant. Carbon dioxide (CO₂) and ethanol (C₂H₅OH) are the products.

2. Define rate of respiration. How will this be determined in the experiment?

   The rate of respiration is the amount of glucose that is converted to carbon dioxide and ethanol in a given amount of time. This experiment measures the amount of carbon dioxide in a closed container. Since this is one of the two products of anaerobic respiration, the change in carbon dioxide is directly correlated to the respiration rate. The graph shows carbon dioxide as a function of time. The slope of the line of best fit is equal to the increase in carbon dioxide in a given amount of time.

3. The three strains of yeast have been selected for different purposes. Describe one adaptation you expect to find in each of the different strains of yeast.

   Student answers will vary. Make sure they have one reasonable answer for each strain. Baker’s yeast must withstand high temperatures, tolerate doughy substrates, metabolize sugars quickly and not produce byproducts that negatively affect the taste of the product. Brewer’s yeast must metabolize starch that has been steeped from barley malt, tolerate moderate levels of ethanol and substrates with a variety of chemical profiles and have a moderate metabolic rate. Wine yeasts must be able to metabolize high levels of sugar—including fructose—from grapes and tolerate high levels of ethanol. They generally are active under cool temperatures over a long period of time.

Sample Data

Data Table A. Respiration Rates of Yeast Strains

*Reported data is the average of several trials.

Determining Respiration Rate Guided-Inquiry Design

Investigating Sugar Type Investigating pH

Answers to Questions

1. Perform a linear fit on the 10–15 minute section of the graph. Record the slope of the line, m, as the respiration rate (in ppm/min) in the data table. Note: During the first few minutes of the data-collection period, the glucose and yeast mixture warmed to the temperature of the water bath. Glucose in the mixture had to enter the cell and be metabolized to produce carbon dioxide gas. Accordingly, the first ten minutes of the data-collection period will not be used to determine the respiration rate.

   See Sample Data graph.

2. Obtain data from two groups that investigated the type of yeast you did not. Obtain data from one group that investigated the same type of yeast that you did. For example, if you tested brewer’s yeast in your second trial, find two groups that investigated wine yeast and one that investigated brewer’s yeast. Record the respiration rates of their second trials in the data table.

   See Sample Data table.

3. Describe any differences in the respiration rates of the different types of yeast.

   Baker’s yeast has the highest respiration rate, followed by brewer’s yeast and wine yeast. Factors, such as time elapsed since first proofing the yeast, will impact the actual numbers, but the trends will be the same.

4. In the notes/observations section of the data table, record any interesting things you saw and anything done that was not in the written procedure.

   See Sample Data table.

5. Explain why there may be differences between your data and another group’s data for the same yeast-type using these observations.

   Accept all reasonable answers. Inherent differences in the population density of the yeast sample will lead to variation in answers. Individual organisms may vary in their metabolism, just as different humans have different metabolic rates. Carbon dioxide is measured in parts per million, which is a very small measurement. Therefore, variation of some degree is normal. Yeast is sensitive to temperature changes, so a small temperature change may impact the results.

6. List factors that could possibly affect the evolution of different strains of yeast.

   Accept all reasonable answers. Environmental (substrate) conditions including temperature, pH, presence of ethanol and presence of salt. Ability to metabolize various types of carbohydrates including monosaccharides, disaccharides and polysaccharides. This is usually done by enzymes, so the evolution of enzymes with the ability to break down varies types of carbohydrates is very crucial to yeast function.

References

Special thanks to Vernier Software & Technology for the use of Figure 1, instructions on using the carbon dioxide sensor and the Introductory Activity.

Melville, J.M., Collins, M., and D. Volz. Investigation 22, Evolution of Yeast in Investigating Biology through Inquiry. Vernier Software & Technology, 2012.

Introduction

Just as farmers select the most fit plants and animals to breed, bakers, brewers and vintners select specific strains of yeast Saccharomyces spp. to produce the best baked goods and fermented beverages. These strains of yeast are good fermenters, metabolizing glucose into carbon dioxide and ethanol, but each has specific traits to make it desirable for a particular product. The process of selecting individuals with particular characteristics for breeding is called artificial selection.

Concepts

• Fermentation
• Anaerobic respiration
• Evolution
• Artificial selection
• Rate of respiration

Background

Through both natural and artificial selection, hundreds of varieties of yeast have evolved. These yeasts vary in inherited traits that allow them to break down specific types of sugars under certain conditions to produce ethanol and carbon dioxide. Depending on the other ingredients and conditions for fermentation, yeast varieties may have other beneficial traits, such as a high tolerance for heat, acid, alcohol or salt.

Alcoholic fermentation is a form of anaerobic respiration, which is yeast’s preferred method of metabolizing glucose. The yeast get energy for metabolic processes when glucose (C₆H₁₂O₆) is broken down into carbon dioxide (CO₂) and ethanol (C₂H₅OH).

Yeast is used to make dough rise through the production of carbon dioxide gas. In beer and wine making, the ethanol stays in solution while most of the carbon dioxide gas comes out of solution and into the air.

Carbohydrates—including sugars—must be converted to glucose before respiration can occur. Enzymes facilitate this conversion by reducing the amount of energy required to start the hydrolysis reaction. Sucrose is a disaccharide that reacts with water to form glucose and fructose. Sucrose and water are both stable molecules that require a high amount of energy to react. An enzyme called sucrase binds to sucrose, changing its shape and destabilizing the molecule. Once destabilized, sucrose more easily reacts with water. An organism must have a set of enzymes to digest different types of carbohydrates into glucose. A specific enzyme breaks down a certain type of carbohydrate.

The evolution and artificial selection of organisms with differing sets of enzymes allows for the utilization of different food sources. This variety reduces competition by enabling organisms to occupy different niches in the same ecosystem. Therefore having a specific, but different, set of enzymes can be beneficial to survival where food is scarce. Yeasts are organisms that vary in the amounts and types of enzymes they have, which allows them to utilize different types of sugars or other food sources. Cultivators have capitalized on this variation and have selectively bred yeast varieties to accomplish different things, such as producing beer, wine or bread.

Experiment Overview

Three yeast varieties will be characterized by their ability to convert glucose into carbon dioxide and ethanol. The production of carbon dioxide will be measured over time, giving the rate of respiration. In the introductory activity, a CO₂ gas sensor will be used to determine the rate of respiration for baker’s yeast and for brewer’s or wine yeast as each metabolizes glucose. In the inquiry portion, students design and carry out experiments to investigate factors that may influence the respiration of each yeast type.

Materials

Prelab Questions

1. What are the reactants and products of fermentation?
2. Define rate of respiration. How will you determine this in the experiment?
3. The three strains of yeast have been bred for different purposes. Describe one adaptation one might expect to find in each of the different strains of yeast.

Safety Precautions

Although materials for the introductory activity are considered nonhazardous, wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. When designing the inquiry portion, include safety precautions and get approval before proceeding. Wash hands thoroughly with soap and water before leaving the laboratory. Please follow all laboratory safety guidelines.

Procedure

Part A. Introductory Activity

1. Set up the CO₂ Gas Sensor.
   1. If the CO₂ Gas Sensor has a switch, set it to the low (0–10,000 ppm) setting. Connect the CO₂ Gas Sensor to the datacollection interface.
   2. Connect a Temperature Probe to the data-collection interface (optional).
   3. Start the data-collection program (e.g., Logger Pro^® 3 or LabQuest^® App).
   4. Set the data-collection duration to 15 minutes and the data-collection rate to 10 samples/minute.
2. Set up the equipment as shown in Figure 1.
   {11309_Procedure_Figure_1}
   1. Prepare a 29–31 °C water bath to ensure that the respiration reaction occurs at a constant and controlled temperature. Combine a total of 600 mL of warm and cool water in a 1000-mL beaker until it reaches 29–31 °C. Place the beaker on the base of a support stand.
   2. Immerse the respiration chamber in the water bath and fasten it in position with a buret clamp.
   3. Use a second buret clamp to attach a Temperature Probe or thermometer to the support stand and suspend in the water bath.
   4. Keep the temperature of the water bath constant in the indicated range by adding more hot or cold water. Before adding water, first remove an equal amount of water. A syringe or pipet may be used for this purpose.
3. Add 3.0 mL of glucose solution to a 15-mL test tube with screw cap.
4. Add 3.0 mL of baker’s yeast suspension to the tube for a total of 6.0 mL of solution. Note: The baker’s yeast suspension must be removed from the middle of the teacher-prepared yeast source that is being stirred by a magnetic stirrer at a constant stirring speed.
5. Cap the tube, invert it three times, and then pour its contents into the respiration chamber.
6. Using a gentle twisting motion, quickly place the CO₂ Gas Sensor into the opening of the respiration chamber and start data collection.
7. When data collection has finished, remove the CO₂ Gas Sensor from the bottle and store the run.
8. Rinse the bottle with water and empty it. Repeat this rinsing procedure two more times. Make sure that all yeast has been removed.
9. Thoroughly dry the inside of the chamber with a soft paper towel.
10. Wait a couple of minutes for the carbon dioxide to diffuse out of the sensor and the readings to get close to the starting readings of the first trial. Fanning paper across the bottom of the sensor will speed up this process. Note: Do not blow on the sensor; your breath has a high amount of carbon dioxide.
11. Repeat steps 3–9 using either brewer’s yeast or wine yeast and new graduated pipets and test tubes.
12. Complete the Evolution of Yeast worksheet before continuing with Part B, Guided-Inquiry Design.

Part B. Guided-Inquiry Design

1. Consider the following questions while reflecting upon your knowledge of respiration and yeast.
   1. How might changing a component, such as type of sugar, temperature or environmental conditions affect the rate of respiration?
   2. What are the environmental conditions the yeast encounter when making bread? How are these different from making fermented foods or beverages?
   3. What are the differences among types of sugar?
   4. What is the optimal temperature for yeast?
2. Plan, discuss, execute and evaluate an experiment to determine the respiration rate of yeast under selected conditions.
   1. Develop a testable hypothesis.
   2. Discuss and design a controlled experiment to test the hypothesis. Include a detailed procedure based on the introductory activity that collects data to test your hypothesis.
   3. List any safety concerns or precautions that will be taken to protect yourself, your classmates and your instructor during the experiment. Make changes to increase protection and decrease risk.
   4. Determine how you will collect and record raw data.
   5. Determine how you will analyze the data to test your hypothesis.
   6. Review your hypothesis, safety precautions, procedure, data tables and proposed analysis with your instructor prior to beginning your experiment.
   7. Run your experiment. Once the experiment and analysis are complete, evaluate whether the experimental evidence supports, refutes or provides no information concerning the hypothesis.
   8. Make suggestions for revisions to the experiment or hypothesis.
3. Consult your instructor for proper disposal of materials used in both the introductory and inquiry portions of the lab.

Student Worksheet PDF

11309_Student1.pdf