A Chilling Investigation
Students will observe the difference in bacterial count between a hamburger that’s left out at room temperature and a hamburger that’s kept refrigerated. The lab reinforces the concept that food must be properly chilled in order for it to remain safe to eat. This lab will be conducted as a teacher demonstration. Grades 6-8
- .5 pound (227 grams) package of inexpensive, raw hamburger
- Dishwashing detergent for cleaning utensils and countertops
- Disinfecting bleach solution (20 ml of liquid household bleach in 1 L of tap water, see page 8)
- Knife for cutting hamburger package
- 2 self-sealing (zipper-style) plastic bags
- 2 plates for hamburger packages
- Paper towels
- Safety gloves and lab aprons for anyone handling hamburger
- 3 sterile Petri dishes with nutrient agar and covers
- Parafilm to seal Petri dishes
- Sterile cotton swabs
- Refrigerator or cooler with ice pack to keep the meat chilled
- Lab Report Outline
- Purchase hamburger.
- Disinfect the knife.
- Divide the hamburger package in half by cutting through the package, including the meat and the bottom of the Styrofoam™ tray.
- Put each half in a self-sealing bag and seal.
- Label one bag “chilled” and refrigerate it immediately.
- Label the other bag “room temperature” and leave it out at room temperature at least overnight.
- Be sure to put the packages on plates or in a bowl to prevent raw meat juices from leaking onto other food items in the refrigerator or onto the counter.
- Make one copy of the Lab Report Outline for each student.
danger zone: a temperature range for food where the risk of harmful bacteria growth is the highest
food safety: the practice of handling, preparing, and storing food in a way that prevents food-borne illness
foodborne illness: any illness resulting from the consumption of food contaminated with viruses, parasites, or pathogenic bacteria
Background Agricultural Connections
Chilling is a critical method for controlling microbial growth. It does not kill microorganisms; therefore, it’s important to handle meat properly when defrosting and cooking. The United States has one of the safest food supplies in the world, but there’s always room for improvement. The battle to prevent foodborne illness is waged every day because bacteria are everywhere. Food safety has to do with controlling bacteria. And since everyone eats, we all share the responsibility for keeping our food free from harmful bacteria.
One method used to prevent foodborne illness is to keep foods at the proper temperature. Under the right conditions, some bacteria can double their numbers within minutes and form toxins that cause illness within hours. To minimize bacterial growth in foods, it’s important to keep food temperatures below 40° F (4° C) or above 140° F (60° C). The level in between this temperature range is known as the Danger Zone.
To be as safe as possible, foods must be kept at the proper temperature in each step along the Farm to Table Continuum. Foods are transported in temperature controlled trucks, receiving areas at processing plants and grocery stores are maintained at cold temperatures of 41° F (5° C) or below, grocery stores must keep food at the proper temperature in storage areas and display cases, and finally the consumer also holds responsibility in keeping the food at the proper temperature to maintain the cold chain that started way back on the farm.
How the 4 Cs of Food Safety Control Bacteria
If bacteria can grow so rapidly under the right conditions, then how do we control them? It’s simple:
- Cleaning — removes bacteria from hands and surfaces.
- Cooking — kills bacteria by breaking down their cell walls and destroying enzymes, which they need to survive.
- Chilling — slows down the bacteria’s metabolism, thus slowing their growth.
- Combating Cross-Contamination (separating foods) — prevents bacteria from spreading from one food item to another, or between foods and hands or surfaces/utensils.
Science and our Food Supply
This lesson was developed as a portion of an entire unit of lessons focusing on food safety from farm to table. Use the following links to see the remaining lessons:
Module 1: Bacteria
Module 2: Farm
Module 3: Processing and Transportation
Module 4: Retail and Home
- Supermarket Smarts
- Cooking Right: The Science of Cooking a Hamburger
- A Chilling Investigation
- Crossed Up!
- Hands Off, Bacteria!
Module 5: Outbreak and Future Technology
Evaluation: Lose a Million Bacteria (The Game)
Interest Approach - Engagement
- Use the following scenario as an introduction, 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.
- 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."
Part 1: Conduct the Lab
- Have the class form a hypothesis about the properly refrigerated hamburger versus the hamburger left out at room temperature.
- Now ask, "How would you test your hypothesis?" Record their answers.
- Discuss a good experimental design for this lab.
- 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.
- Have one student label one Petri dish “control.”
- Have the second student label one dish “chilled.” Have them swab the properly chilled hamburger, and inoculate the “chilled” dish.
- 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.
- 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)
- Use Parafilm to seal the dishes (see the attached Lab Procedures sheet for more details).
- 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.
- Have the class discuss the results in relation to their hypothesis. Were there any surprises?
- Ask students:
- "Did the cold kill the bacteria in the refrigerated sample?" (There may be some bacterial growth, since cold doesn’t kill bacteria.)
- "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.)
- "Could I 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.)
- Students can use the attached Lab Report Outline to record the results.
Concept Elaboration and Evaluation
- What does the cold chain have to do with the things we learned in this lab? (We all need to continue the cold chain that started back on the farm in order to keep our food safe. Keep food chilled until it’s ready to be cooked or eaten.)
- Why do we freeze hamburger meat? (Freezing keeps food safe by causing foodborne illness microbes to enter a dormant stage.)
- Does freezing kill bacteria? (No, freezing slows down the growth of harmful bacteria.)
To freeze or not to freeze: that is the question! Well, the answer is simple. To keep harmful bacteria from growing and multiplying, always store foods that won’t be used right away in the refrigerator or freezer.
Relate what you’ve learned about bacterial growth and chilling to other foods such as chicken, fish, seafood, eggs, etc.
Visit a local fast-food restaurant or supermarket and interview the manager to find out how he/she maintains the cold chain.
The Science and Our Food Supply Curriculum was brought to you by the Food and Drug Administration Center for Food Safety and Applied Nutrition and the National Science Teachers Association.
- FDA Education Team Leader Food Safety Initiative: Marjorie L. Davidson
- FDA Science and Our Food Supply Project Director: Louise H. Dickerson
- FDA/NSTA Associate Executive Director and Science and Our Food Supply Program Director: Christina Gorski
- FDA/NSTA Science and Our Food Supply Program Assistant: Jill Heywood