Dr. X and the Quest for Food Safety Video

1. Prepare students for a video clip by explaining that after food leaves the farm, it is typically processed in some way. This could include washing, packaging, cooking, and/or other means of preparing food for retail sale and eventually consumption.
2. Inform students that "Dr. X" will beam them into the research lab of one of his scientist friends who looks at new ways to reduce the bacteria in our food through processing. Here are some things they should think about while they watch the video:
   • What do cows, astronauts, and elephants have to do with food safety and food processing? (Cows relate to pasteurization, astronauts relate to irradiation, and elephants relate to ultra-high-pressure treatment.)
   • What is pasteurization? (Pasteurization uses heat to kill harmful bacteria in foods.)
   • How can an egg be pasteurized in the shell without cooking it? (Manufacturers use a time/temperature relationship to pasteurize eggs in the shell without cooking them. Heating eggs above 140° F [60° C] will cook them. Thus, using a lower temperature of 135° F [57° C] for a longer time [1 hour and 15 minutes] kills bacteria without cooking the egg.)
   • What process keeps food safe in outer space? (Irradiation)
3. Show video Dr. X and the Quest for Food Safety Module III: Processing and Transportation (Time: 7 minutes).
4. Discuss the role of refrigeration in keeping foods safe. Ask students how some types of milk can stay safe without being refrigerated. (UHT milk contains fewer bacteria than conventionally pasteurized milk because it’s heated to a higher temperature. It’s also packaged in sterile, airtight containers. Therefore, unopened UHT milk can be stored without refrigeration for up to 90 days. Once opened, bacteria can get into the UHT milk, and it must be refrigerated.)

Part 1: Design and Conduct Lab

1. Ask students to form teams of three or four and encourage each team to develop a hypothesis on how temperature affects bacterial growth. Then ask them to design a lab experiment to test the hypothesis.
2. Introduce the three materials teams must use for their lab: regular pasteurized milk, ultra-high-temperature (UHT) milk, and methylene blue, which indicates the oxygen level in a liquid.
3. Ask, "How might you use methylene blue to help with your lab?" Students can research and discuss how oxygen levels in the milk might be related to the presence of bacteria. Tell them they can use any of the other materials on the lab table. Also, tell them that there’s a refrigerator and freezer they can use if you have one available.
4. Let teams discuss their hypotheses and experimental designs for 10 to 15 minutes. Then, begin posing the following questions and discussion points to help students design well-thought-out labs:
   • What do you know about the effect of temperature on bacteria? What are some ways you could test the effect of temperature on bacteria? (Heating is a way to kill bacteria; chilling and freezing are ways to slow the growth of bacteria.)
   • Explain that one container of milk came from the refrigerated dairy case of the supermarket and the other from an unrefrigerated shelf. Let students examine each container.
   • What’s an important difference between the two milk products? Is there any information on the labels that relates to our question about the effect of temperature on bacterial growth? (Students should discover that one is pasteurized and the other is UHT treated)
   • What are the similarities and differences between pasteurized and UHT milk? (Both pasteurization and UHT treatment use heat to kill bacteria. UHT methods use higher temperatures than regular pasteurization. Also, UHT products are packaged in special airtight containers to prevent bacteria from getting into the product.)
   • Could there be differences in the growth of bacteria between the two milks? What do you think the differences might be? (The regular pasteurized milk should show bacterial growth sooner than the UHT milk because the pasteurized milk has more bacteria in it.)
   • Should you consider these differences when you design your labs? Why? (Yes, both milks should be tested in all conditions.)
   • How can you tell if bacteria are growing in the test samples? (Add methylene blue to each sample. If bacteria are growing, the methylene blue will become colorless, and the milk will change from blue to white. This is not immediate but happens over time.)
5. Have each group present its hypothesis and experimental design to the class. Encourage students to discuss the merits of each suggested test. (One effective experimental design is to test pasteurized milk and UHT milk at three temperatures — room temperature, chilled, and frozen.)
6. After the group discussions, give the teams time to revise their hypotheses and experimental designs.
7. Let teams conduct the labs according to their designs.
   • Note: The test tubes must be checked each day after the lab is started. Since the color change happens over time, you could miss important findings if you don’t check every day.
   • Allow students to design their experiments, but the following steps can be used as a teacher reference:
     1. Assemble the test tube rack, and line up the tubes in the rack.
     2. Using the pipette or graduated cylinder, add 10 ml of the UHT milk into three test tubes. Repeat with the 2% pasteurized milk in the remaining three test tubes.
     3. Place a single drop of the methylene blue solution into each test tube. Be extremely careful with the solution—it will stain clothing and surfaces blue. Secure the caps and gently swirl the tubes to diffuse the dye evenly.
     4. Label the samples “UHT” or “2% Pasteurized.”
     5. To measure the effect of temperature on the rate of spoilage, keep one of each (UHT and 2% pasteurized) in the stand at room temperature, put one of each in the refrigerator, and one of each in the freezer.
     6. Observe the color of the milk over the next few days or weeks. The blue tint will begin to fade as the milk spoils. Record the color of each of the tubes according to the following scale:
     7. As the milk in the tubes becomes completely white students can begin to make a comparison and draw conclusions about how processing treatments deter bacterial growth.

Part 2: Observe and Record Lab

1. Pass out one copy of the Blue’s the Clue Data Table to each team or student.
2. Beginning on day two, students should observe and record the visual changes they see in their milk samples.
   • Option A: If observation time is short, you can speed the results by taking the refrigerated and frozen samples out of the freezer on day two and then observing them after sitting at room temperature for another day. Ask:
     • "How did the data support or reject your hypothesis?
     • "What might happen if the chilled and frozen samples were left out at room temperature for several hours or overnight?"
     • "Should we test them to find out?" (Yes, let the chilled and frozen samples stand at room temperature until the following day. As they reach room temperature and remain in the Danger Zone for several hours, the bacteria will begin to grow. As this happens, the methylene blue will become colorless, and the milk will change from blue to white. Observe and record the results.)
     • "What might happen if the UHT samples were left out at room temperature for another day?" (If you let the UHT samples sit out at room temperature for another day or more, the color will change to white. Observe and record the results.)
   • Option B: If you have a longer period of time to observe results, continue daily observation of samples for up to 10 days or until the samples are all spoiled (white). This will allow students to see that milk (and other foods) can spoil in the refrigerator or after being frozen depending on the length of time they are there. Ask the following questions:
     • "How did the data support or reject your hypothesis?"
     • "How do time and temperature relate to food spoilage?" (The warmer the temperature, the quicker the spoilage. The cooler the temperature, the slower the spoilage.)

Part 3: Observe, Record, and Report Lab

1. Observe and record findings on the final observation day. Ask students, "What happened to the frozen and chilled samples? What happened to the UHT samples?"
2. Give students 5 to 10 minutes to complete their Blue's the Clue Data Tables.
3. Have teams present their findings to the class. They should report both positive and negative results and discuss ways they would improve their experimental designs. Remind students to include the relationship of their findings to food safety.