Purpose: Students will learn about pasteurization by performing an experiment that involves calculating and interpreting results.

Procedural overview: Students will annotate and analyze “Louis Pasteur’s devotion to truth transformed what we know about health and disease” from Science News online. After learning about Pasteur’s discoveries and how he developed the pasteurization process, students will do a hands-on experiment. In this experiment, yeast solutions will be prepared at different temperatures and then monitored for gas production. Students will collect data on gas production by measuring how big the gas makes a balloon and will approximate the volume of gas produced. The data will be graphed and interpreted by students to identify the temperature that pasteurization occurs. As an optional activity, students will further study the importance of pasteurization in food production and in the prevention of foodborne illness.

Approximate class time: 2 class periods

Supplies:

Beaker

Bunsen burner

Thermometer

Whiteboard/chalkboard

Yeast (Make all packets the same brand)

Sugar

Scale (Alternative: tablespoon or disposable plastic spoon)

Stopwatches

Plastic bottles of the same size, (1 liter or 1.25-liter bottles recommended)

Funnel

Round balloons

Tape measures (Alternative: rulers and string/yarn)

Student worksheet

Directions for teachers:

The setup

Before the start of class, set up the experiment. Around the lab, set up stations with a Bunsen burner, a beaker to heat water, a funnel, a bottle, a thermometer, a yeast packet, a stopwatch, a tape measure and a balloon. Have one or two weighing stations where students can weigh sugar and yeast.

In class, have students read the introduction and the section call “How Pasteur developed pasteurization” in “Louis Pasteur’s devotion to truth transformed what we know about health and disease.” This article appeared in the November 19, 2022, print edition of Science News with the title “Louis Pasteur’s Long Legacy.” Ask students to annotate the article as they read and identify any new vocabulary and concepts.

Article analysis

After students have read and done their annotations, have them answer the following questions.

1. Who was Louis Pasteur?

Louis Pasteur was a French chemist and biologist, who was born in 1822.

2. How was tartaric acid important to Pasteur’s career?

Pasteur’s work on tartaric acid and wine got him started on work that eventually led to discoveries about microbes and diseases.

3. Why was it important that Pasteur showed yeast are living things?

Until Pasteur’s work, most scientists thought that fermentation was a “natural nonbiological chemical process.” Demonstrating that yeast are living organisms changed how scientists thought about fermentation.

4. Microorganisms are living things too small to see with the naked eye and include fungi and bacteria. What kind of microbe is a yeast?

Pasteur thought yeast was a “small plant,” but it really is a kind of microscopic fungus.

5. What do yeast do with sugar, and what is the process called?

Yeast can convert sugar to alcohol in a process called fermentation.

6. Why do yeast ferment?

Fermentation is how yeast meet their nutritional (energy) needs.

7. People use fermentation to make wine, beer and other products. But sometimes those products can become spoiled or get contaminated with microbes that are harmful. What method of food and beverage protection did Pasteur develop, and how does it work?

Pasteur created pasteurization, a method of heating that kills microorganisms that can spoil food items or cause disease.

8. What questions do you have about the fermentation process? What might you want to investigate about pasteurization?

Student answers will vary. When you make yogurt, how much can you change the flavor by changing the bacteria you add to the milk? I would like to do an experiment with raw milk. My question: If you start with raw milk, and it starts to sour because of the microbes that are present, can you reverse or minimize the souring by pasteurizing the sour milk?

Preparing to do the experiment

Review fermentation and introduce the chemistry behind it. Include any of the following information that you find useful.

Pasteur thought yeast was a “small plant,” but today we know that it is really a microscopic fungus. However, Pasteur was right in thinking that yeast metabolizes sugar in a process called fermentation. In this process, yeast consumes sugar to produce carbon dioxide, ethanol and energy (in the form of ATP).

C₆H₁₂O₆ –> 2C₂H₅OH + 2CO₂ + 2ATP

Sugar –> ethanol (alcohol) + carbon dioxide + energy

Humans have used yeast to ferment food items for thousands of years. Because yeast creates ethanol, a type of alcohol, it is used to make alcoholic beverages like beer and wine. Yeast is also important in breadmaking. When the yeast in bread dough ferments, carbon dioxide is produced. The carbon dioxide helps the bread rise.

In this experiment, students will determine the temperature at which pasteurization occurs for a yeast solution. Pasteurization occurs when the yeast have died. Ideally, you should not give the students hints about the temperature at which pasteurization occurs.

Before starting the experiment, discuss the concept of a control in a science experiment and what might be possible options for a control in this study. Remind students of the importance of multiple trials; ask that each group test at the control temperature and one of the other nine temperatures. Also, they should do at least two trials at each of their temperatures, if there are enough materials and time. Two separate student groups could also test at the same temperature.

Cover lab safety. At the highest temperatures, the students could burn themselves if they do not pour carefully.

After mixing yeast, water and sugar together in a bottle, the students will top the bottle’s opening with a balloon to capture any gas produced. The water used in the bottles will be varied in temperature so students can find out for themselves at what temperature yeast cells die and pasteurization occurs.

Students will need it to calculate the volume of gas produced during fermentation. Review this volume formula, where c is circumference, V=(c³)/(6π²) and note that the unit students will use is cubic centimeters cm³.

The amount of information you give students can vary depending on the time available. However, students will feel more ownership over their experiment and their findings if they help decide how to set up the experiment.

Use the following questions to guide students in setting up the lab and determining experimental variables.

1. Yeast produces carbon dioxide, a gas, as it ferments sugar. What do you expect will happen to the balloons?

The balloons will inflate with carbon dioxide as the yeast cells break down the sugar into carbon dioxide, alcohol and energy.

2. How do you think the water temperature will affect gas production?

Initially, as the temperature increases, the amount of gas produced will also increase. Once the temperature is hot enough for pasteurization to occur, gas production will begin to decrease or will stop entirely as the yeast cells die.

3. The data you collect in this experiment will be graphed. What units will you use on the x axis and the y axis?

The x axis will show temperature in degrees C. Depending on how students are measuring the gas produced, the y axis will show the amount of carbon dioxide, indicated by either the circumference or volume of the balloon.

4. What temperatures should the class test (starting at room temperature, approximately 22° C)? (Note that each group should run trials at the control temperature and one other temperature.)

Student answers will vary. We should test at 22° C, 28° C, 34° C, 40° C, 46° C, 52° C, 58° C, 64° C,  70° C and 76° C.

5. What should the control group be in this experiment?

A bottle with water, sugar and no yeast at room temperature; a bottle with water, no sugar and no yeast at room temperature; or a bottle with water, yeast and no sugar at room temperature.

6. What should be the length of time for each trial?

Twenty minutes.

7. If time permits, how many trials should ideally be run at each temperature?

Two or three trials.

8. How can we measure the amount of gas produced, and what scientific unit could we use?

Student answers will vary. We could figure out the amount of gas produced after measuring the balloon’s volume. To get to volume, we will need to know the circumference of the balloon to put into the volume formula. The unit to use is cubic centimeters (cm³).

Fermentation experiment

Ask the class to decide what temperatures they want to test. Try to have the students evenly space the temperatures. For example, temperatures could increase in 6-degree increments: 22° C, 28° C, 34° C, 40° C, 46° C, 52° C, 58° C, 64° C, 70° C and 76° C. Temperatures should not go above 80° C.

The students also must agree on how much sugar, yeast and water they want to use in their bottles and in what order they want to combine their materials, or you could suggest 250 ml of water, 40 grams (3 tablespoons) of sugar and the contents of one yeast packet. All yeast packets should be the same brand and at room temperature. Whatever the students decide, please emphasize that each group should use the same amount of sugar, yeast and water. Only the control will have different amounts of sugar and/or yeast.

Have the students form groups. Each group would ideally do multiple trials on the control temperature and one other temperature. In setting up, each group should put the sugar and yeast in the bottle, and then heat their water beaker over the Bunsen burner. As the water heats, carefully check the temperature. When the water reaches the group’s study temperature, students should carefully pour 250 ml of water through a funnel into their bottle, screw on the bottle’s lid and shake the bottle to mix the water, yeast and sugar. The temperature of the water might cool during its transfer into the bottle. This could result in some experimental groups showing carbon dioxide production when the yeast should have been killed by pasteurization.

Once the bottle’s contents are combined, students should unscrew the bottle, place a balloon on the top of the bottle and start a 20-minute timer. As each group’s timer ends, the group should measure the circumference of their balloon at the widest point using a tape measure and record the circumference on their worksheet and on the board. Then use the formula V=(c³)/(6π²), to calculate the volume of carbon dioxide produced and graph the volume of gas produced using metric units for the volume and degrees Celsius for the temperature.

Ask the students to answer the following questions about their experimental results.

1. Measure the circumference of your group’s balloon and record it in your chart and on the board. Calculate the volume of your group’s balloon and add the data to the chart using the formula: V=(c³)/(6π²), where c represents the circumference. Continue to fill in the chart as other groups add their data to the board. (Students will use the chart in the student worksheet.)

Student answers will vary, but should follow this trend: Circumference and volume should go up for 28° C and 34° C. For high temperatures, circumference and volume should trend downward.

2. Graph the volumes from each group. Remember to label your graph and axes. (The graph is available in the student worksheet.)

Graphs will vary. The x axis should be for temperature in degrees Celsius; the y axis is for volume of carbon dioxide in cubic centimeters. Temperature should be marked every six spaces; the line on the graph climbs until about 34 degrees or a little higher, and then moves steadily downward until the volume of carbon dioxide tapers to zero. A possible label for the chart could be “Volume of Carbon Dioxide Produced from Yeast Fermentation.”

3. Why is it important to record what happens with the control group?

If the control group shows that gas has been produced, then there might be other microorganisms that could skew the data.

4. What is the relationship between gas volume and temperature?

Initially, the volume of carbon dioxide produced increases as temperature increases. However, eventually the volume of carbon dioxide produced begins to decrease.

5. At what temperature did the most fermentation occur? How could you tell?

Student answers will vary. For example, the temperature at which most fermentation occurred was 34° C because the largest volume of carbon dioxide was produced at 34° C.

6. At what temperature did yeast stop fermentation and gas production? How could you tell?

Student answers will vary. I think fermentation started slowing down when temperatures got into the 60s. As temperatures went higher and higher, there was less carbon dioxide produced. The highest temperatures killed the yeast, preventing them from fermenting the sugar and producing carbon dioxide. There was less carbon dioxide in the balloons at higher temperatures.

7. How might we know that the yeast died due to pasteurization?

Student answers will vary. We know that carbon dioxide is produced during fermentation. As temperatures rise, there will come a point when fermentation slows as yeast cells die. If the balloon does not inflate much, it suggests that carbon dioxide production is declining and that yeast are likely dying. Assuming nothing went wrong with the experiment, pasteurization probably occurred if the balloon does not inflate at all.

8. How has learning about Pasteur’s discoveries influenced your views about what you eat?

Student answers will vary. I am more aware of how important pasteurization is for protecting our food supply.

9. What other question would you like to answer about microorganisms in food?

Student answers will vary. Do different microorganisms get killed through pasteurization at different temperatures?

Activity extension: After students perform the experiment, you can ask them to learn about foodborne illnesses that can be prevented by pasteurization and to create a poster explaining how pasteurization protects people from these illnesses.

The following questions can serve as prompts. If students want to, they can ask their own questions.

1. What is a foodborne illness?

A foodborne illness is a disease that is spread by the consumption of contaminated food.

2. Name a foodborne illness that can be caused by drinking contaminated milk.

Student answers will vary. Listeriosis and tuberculosis are two diseases that can be spread in unpasteurized milk.

3. What are the symptoms of this disease?

Student answers will vary. Listeriosis can cause fever, muscle aches, loss of balance and seizures.

4. Have there been any recent outbreaks of this disease? If so, what food products caused this outbreak?

Student answers will vary. In 2022, people developed listeriosis after eating deli meats and cheeses and ice cream contaminated with Listeria.

5. Even pasteurized food can cause foodborne illnesses. Why is this the case?

Microorganisms could be introduced after the food was pasteurized or the disease was caused by a microorganism that was not killed by pasteurization.

6. Create a poster that shows how pasteurization protects people from your foodborne illness.

Student products will vary but should all mention pasteurization.