Part 1: Conduct the Lab

1. Have the class form a hypothesis about the properly refrigerated hamburger versus the hamburger left out at room temperature.
2. Now ask, "How would you test your hypothesis?" Record their answers.
3. Discuss a good experimental design for this lab.
4. Ask for three volunteers. Remind them to wear safety gloves and lab aprons. 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.
5. Have one student label one Petri dish “control.”
6. Have the second student label one dish “chilled.” Have them swab the properly chilled hamburger, and inoculate the “chilled” dish.
7. Have the third student label one dish “room temperature.” Swab the hamburger that was left out of the refrigerator, and inoculate the “room temperature” dish.
8. 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 experiment)
9. Use Parafilm to seal the dishes (see the attached Lab Procedures sheet for more details).
10. Place the Petri dishes in the incubator at 95° F (35° C) or let dishes sit at room temperature (away from the sun) for 1 to 2 days.

Part 2: Observe, Record, and Summarize Results

At the beginning of the next day, ask students to observe and record results.

1. Have the class discuss the results in relation to their hypothesis. Were there any surprises?
2. Ask students:
   • Question: Did the cold kill the bacteria in the refrigerated sample?
     Answer: There may be some bacterial growth, since cold doesn’t kill bacteria.
   • Question: What did you observe in the unrefrigerated sample?
     Answer: Since the sample had remained in the “Danger Zone” for several hours, more bacteria grew than on the refrigerated sample.

      

   • Question: What can you conclude about what went wrong along the Farm-to-Table Continuum in respect to this hamburger?
     Answer: 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.

      

   • Question: Who has the final responsibility for the safety of this burger?
     Answer: It’s our responsibility to make sure that food stays safe after we purchase it.
   • Question: Could I just cook the unrefrigerated hamburger thoroughly and make it safe to eat?
     Answer: 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.
3. Students can use the attached Lab Report Outline to record the results.