Lab Part 1: Teacher Demonstration with Guided Inquiry

Use the Demonstration Lab Sheet: Cooking Right as a guide for conducting the lab. Use the following suggestions for guided inquiry to help students analyze and design the experiment as you demonstrate.

1. Ask volunteers to assist you in labeling 5 Petri dishes: control, raw, 120° F (49° C), 140° F (60° C), 160° F (71° C); see the attached Lab Procedures sheet for more details.
2. Demonstrate how to set up a control dish.
3. Show students how to swab the raw hamburger, inoculate the “raw” dish, and seal the dish with Parafilm.
4. Ask students:
   • "What factors should be considered as we conduct this lab?" (Weight, size, thickness of hamburgers, temperature, consistency, etc.)
   • "How can we assure that all the hamburgers are the same size?" (They should be weighed.) "Why?" (If the hamburgers vary in size, another variable is introduced.)
   • "Does thickness of the hamburgers matter?" (Burgers should be about .5 inches [1.3 cm] thick. It’s easier to accurately insert the thermometer in a burger of this thickness.)
5. Demonstrate how to prepare 3 hamburgers — weigh and measure them to assure that they are all the same weight and thickness.
   • Option: Have student volunteers make, weigh, and measure the hamburgers. Make sure students wear safety gloves and follow safety procedures for handling raw meat.
6. Cook one hamburger to 120° F (49° C). Ask students:
   • "Why is it important to take the hamburger out of the pan to measure the temperature?" (The heat from the pan will interfere with getting an accurate temperature reading of the inside of the hamburger.)
   • "How should you take the temperature?" (Take the temperature through the side and into the center, making sure the temperature probe reaches the center of the burger, not just the outer edge. Follow the instructions on the thermometer package.)
7. Clean the thermometer with alcohol after every temperature reading. Ask:
   • "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.
8. When the hamburger has reached 120° F (49° C), break it in half and demonstrate how to swab the inside. Ask, "Why do you break it rather than cut it in this scientific test?" (The knife might have bacteria on it and you might transfer that bacteria into the hamburger.)
9. Have a volunteer swab inside the broken edge of the hamburger and inoculate the “120° F” dish. Seal the dish with Parafilm.
10. Cook the two remaining hamburgers: one to 140° F (60° C) and the other to 160° F (71° C). Have volunteers inoculate the dishes after each hamburger has reached the desired temperature. Seal dishes with Parafilm. Place the inoculated 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.

Lab Part 2: Observe, Record, and Report

1. At the beginning of the next day’s lab, have students observe, record, and report bacterial numbers in the 4 samples. Ask students to discuss:
   • "Which temperature produced the most effective results in reducing bacterial numbers?" (The temperature of 160° F [71° C] should show the best results. This is the recommended temperature for safely cooking ground meat.)
   • "How did the amount of bacteria in the raw hamburger compare to the cooked burgers?" (The raw hamburger will have much more bacteria than any of the cooked hamburgers.)
   • "What did your control show?"