Lab 1: The Science of Cooking a Hamburger

Design The Lab

1. Divide the class into teams of 3 to 4. Challenge each team to form a hypothesis and design a lab that examines how temperature affects the bacteria in a hamburger.
2. Use the following guided inquiry to help the students design well-thought-out labs.
   • "What factors should be considered as you design the labs?" (Weight, size, thickness, temperature, consistency, etc.)
   • "How can you assure that all burgers are the same size?" (They should be weighed.) Why? (If the hamburgers vary in size, another variable is introduced.)
   • "Does thickness matter?" (Yes, burgers should be about .5 inches [1.3 cm] thick. It’s easier to accurately insert the thermometer in a burger of this thickness.)
   • "How should you take the temperature?" (Take the temperature in more than one place. Make sure the temperature probe reaches the center of the burger. Take the temperature through the side into the center of the burger. Follow the instructions on the thermometer package.)
3. Have teams present their hypotheses and lab designs to the class. Encourage everyone to discuss the merits of each suggested lab. (If students have not included that the recommended temperature for cooking hamburger is 160° F [71° C], challenge them to find out the temperature at which pathogens in ground meat will be killed.)
4. Distribute the Student Lab Sheet: Cooking Right. Have the students discuss this design. How is it similar or different to their design? Let each team decide if they want to use this lab design or their group design. Note: At least 2 teams should follow the Cooking Right design.

Conduct The Lab

1. Label the Petri dishes:
   • control
   • raw
   • 120° F (49° C)
   • 140° F (60° C)
   • 160° F (71° C)
2. Cook the burgers to 120° F (49° C), 140° F (60° C), and 160° F (71° C).
   • Spray the pans with a non-stick spray before cooking the hamburgers.
   • Use a food thermometer to measure the internal temperature of the hamburgers.
     • Ask students, "Why is it important to take the hamburger out of the pan to measure its temperature?" (The heat from the pan will interfere with getting an accurate temperature reading of the inside of the hamburger.)
   • Clean the thermometer with an alcohol wipe each time you take the temperature. Ask students: Why is this necessary? (If there are bacteria in the meat, they might get onto the thermometer and be transferred to the next hamburger you’re cooking.) What is this called? (cross-contamination)
     • Note: We’re using alcohol to kill any bacteria and prevent cross-contamination in this lab. However, when you’re cooking at home, you can thoroughly wash the thermometer with soap and hot water between uses.
3. Break the hamburger in half and swab the inside of the broken edge when the hamburger has reached the specified temperature. Ask students, "Why do you break hamburger rather than cut it in this scientific test?" (The knife might have bacteria on it and you might transfer the bacteria into the hamburger.)
4. Inoculate the appropriate Petri dish after each desired temperature is reached. Make sure students use sterilized swabs and the correct technique (see attached Lab Procedures sheet).

Incubate Petri Dishes

1. Seal the dishes with Parafilm (see Lab Procedures sheet).
2. Place the Petri dishes in the incubator at 95° F (35° C) for 1 to 2 days or let them sit at room temperature for the appropriate amount of time.

Observe, Record, and Summarize Results

1. At the beginning of the next day’s lab, have students observe, record, and graph colony numbers in the 4 samples. Ask students to discuss:
   • "Which temperature produced the most effective results in reducing colony numbers?" (The temperature of 160° F [71° C] should show the best results. This is the recommended temperature for safely cooking ground meat. See the “Apply the Heat” chart on page 58 of the Food Safety A to Z Reference Guide.)
   • "How did the amount of colonies from the raw hamburger compare to the cooked burgers?" (The raw hamburger will have many more bacteria than any of the cooked burgers.)
   • "What did the control dish show?"

Lab 2: A Chilling Investigation

1. Use the following scenario as an introduction to the lab, or ask students to come up with a scenario of when meat might be unintentionally left out of the refrigerator for too long.
   • Suggested Scenario: Last night, Mrs. Smith bought 2 packages of hamburger that she planned to cook for dinner the next evening. She put one package in the refrigerator. But then the phone rang, and other things occurred that distracted her. She forgot to put the other package of hamburger in the refrigerator. It sat out on the kitchen counter all night long. She woke up the next morning and placed the hamburger in the refrigerator, but wondered if the unrefrigerated hamburger was safe to eat.
2. Ask students, "Would you eat the unrefrigerated hamburger? Why or why not? Let’s test both packages of hamburger and see if there’s any difference between them."

Prepare a Hypothesis

1. Have the class form a hypothesis about the properly refrigerated hamburger versus the hamburger that was left out at room temperature.
2. Now ask, "How would you test your hypothesis?" Record their answers.
3. Discuss a good design for this lab.

Conduct the Lab

1. Ask for three volunteers.
   • Remind students to wear safety gloves, lab aprons, and safety goggles. Note that they should take their sample near the center of the meat and away from the surface where the hamburger was cut. If possible, get a drop of hamburger juice.
   • One student will label dish #1 “control.”
   • The second will label dish #2 “chilled,” swab the hamburger that was properly chilled, and inoculate dish #2.
   • The third will label dish #3 “room temperature,” swab the hamburger that was left out of the refrigerator, and inoculate dish #3.
2. Tell the students that one package of hamburger was cut in half to make two packages. Then ask, "Why did we cut the package in half rather than just buying 2 individual packages?" (To ensure that the meat tested is from the same batch, so as not to introduce another variable into the lab)

Incubate Petri Dishes

1. Seal the dishes with Parafilm (see attached Lab Procedures sheet).
2. Place the Petri dishes in the incubator at 95° F (35° C) or let the dishes sit at room temperature (away from the sun) for 1 to 2 days.

Observe, Record, and Summarize Results (This will be done at the beginning of Lab 3.)

1. Have the class discuss the results in relation to their hypothesis. Were there any surprises?
2. Ask students:
   • "Did the cold kill the bacteria in the refrigerated sample?" (There may be some bacterial growth, since cold doesn’t kill bacteria but keeps the bacteria from growing while the food remains chilled.)
   • "What did you observe in the unrefrigerated sample?" (Since the sample had remained in the “Danger Zone” for several hours, more bacteria grew than on the refrigerated sample.)
   • "What can you conclude about what went wrong along the Farm-to-Table Continuum in respect to this hamburger?" (The hamburger may have been contaminated with bacteria before Ms. Smith purchased it. However, she compounded the problem by mishandling the meat after she brought it home. She did not follow the “Chill” rule of the 4 Cs of Food Safety — she violated the 2-hour rule by not placing the hamburger in the refrigerator immediately.)
   • "Who has the final responsibility for the safety of this burger?" (It’s our responsibility to make sure that food stays safe after we purchase it.)
3. Could you just cook the unrefrigerated hamburger thoroughly and make it safe to eat? (No. If food is left unrefrigerated, bacteria cells will grow and more heat is required to kill the additional cells. Meat that is left out too long can accumulate bacterial toxins that may not be destroyed by additional cooking. Also, leaving the meat unrefrigerated invites the possibility of cross-contaminating surfaces, hands, etc. You should practice safe food handling habits and always handle your food defensively. If the hamburger was left out at room temperature for more than 2 hours, it should have been discarded.)

Lab 3: Don't Cross Me

Getting Ready

1. Let the students work in teams of 3 or 4. Have each team develop a hypothesis about cross-contamination and cleaning. Then, using cheese and hamburger and any of the other materials on the “Lab Bench,” have them design labs to support their hypotheses.
2. Have each team share its hypothesis and lab design with the class. Encourage class members to discuss their procedures and design.
3. Distribute Student Lab Sheet: Don’t Cross Me to each team. Have the students analyze this design. How is this similar to or different from your lab designs? Let students decide if they want to use this design or their group design.
   • Note: At least 2 groups should follow the Don’t Cross Me procedure.

Conduct the Lab

1. Have students conduct their labs using either the Don’t Cross Me lab sheet or their own design.

Observe, Record, and Summarize Results

1. At the beginning of Lab 4, have students observe the samples and record the results. Students can prepare data tables to record the results. Ask each team to share their test results and discuss how their results either proved or disproved their hypotheses. How would they change or improve their design?
2. Conclude by asking the students:
   • "What will happen to the bacteria in the hamburger?" (The burger can be cooked to 160° F [71° C] and the harmful bacteria will be killed.)
   • "What will happen to the bacteria on the contaminated cheese?" (The cheese will not be cooked, thus the bacteria will be eaten. The person eating the “contaminated” cheese might get sick.)

Lab 4: Instant Replay

1. Show video Module 4 (optional) and discuss the following with students:
   • What did Dr. X mean when he said, “. . . food safety depends on more than just technology?” (He meant food safety is also in your hands.)
   • What is one of the biggest sources of food contamination in restaurants? (People)
   • What did Dr. X say was so critical in keeping our food safe? (Handwashing)
   • What did each of the Barkleys do before preparing the food? (Alex washed the cutting board in hot, soapy water before and after food preparation, Mr. Barkley cooked the hamburgers to 160° F [71° C] and used a food thermometer, Olivia chilled the food before serving, and Mrs. Barkley prevented cross-contamination by placing the cooked burgers on a clean platter.)