Minimizing Bacteria in the Thawing and Cooking of Meat

Test 1 - Thawing Meat at Room Temperature vs. In Refrigerator

1. Cut, measure, and weigh 14 pieces of meat (approx. 5 cm x 5 cm x 2 cm, about 65 g each).
2. Set aside two pieces of meat for a baseline bacterial count.
   1. Do the bacterial counts as described in the "Measuring the Number of Bacteria Present in a Meat Sample" section below.
3. Divide the remaining meat pieces into two equal piles, wrap each in plastic wrap & foil, and freeze (up to 14 days).
4. Thaw one package of meat pieces at room temperature until the internal temperature of meat the reaches room temperature. Record how long this takes and the room temperature in your lab notebook.
   1. Create a data table in your lab notebook to record this data, and other data you collect, while doing your experiment.
5. Thaw the other package of meat pieces in the refrigerator until soft. Record how long this takes and the temperature in your lab notebook.
6. Measure the bacterial count for two samples from each condition.
   1. Do the bacterial counts as described in the "Measuring the Number of Bacteria Present in a Meat Sample" section, below.

Test 2 - Cooking Meat in Microwave

1. Take two samples of meat thawed at room temperature and two samples thawed in the refrigerator and cook each of them separately in the microwave until the internal temperature of each meat sample reaches the recommended temperature: beef, 72°C; pork, 77°C; chicken, 85°C.
2. Measure the bacterial counts of the cooked meat samples as described in the "Measuring the Number of Bacteria Present in a Meat Sample" section, below.

Test 3 - Cooking Meat in Standard Oven

1. Take two samples of meat thawed at room temperature and two samples thawed in the refrigerator and cook them each separately using a standard oven heated to 190°C (375°F). Recommended cooking times are: beef, 20 min.; pork, 17 min.; chicken, 15 min.
2. Measure the bacterial counts of the cooked meat samples as described in the "Measuring the Number of Bacteria Present in a Meat Sample" section, below.

Measuring the Number of Bacteria Present in a Meat Sample

1. Puree a piece of meat (sample) in a sterilized blender with a small amount of sterile water (use the same amount of water each time).
   1. Note: To sterilize the blender, clean the jar and blade assembly with warm, soapy water and allow to dry. Wrap the blender jar and blade assembly only (not the base with the electric motor) in aluminum foil, and heat in an oven at 250°F for 30 minutes. Use oven mitts when removing it from the oven. Unwrap the foil just before use.
2. Using sterile technique, do 10-fold serial dilutions (9–11 times) of the pureed meat sample and then plate a 100 μL sample. You will have to determine how many times to dilute. We suggest using the sterile pipettes, a sterile 10 mL graduated cylinder, and sterile 15 mL test tubes for preparing the dilutions, and then plating 100 uL of solution from each of the last three dilutions (on three separate tryptic soy agar plates) to see which dilution works best for counting colonies.
3. Incubate the plates, inverted (agar on top), at 37°C, overnight or longer. Use a longer incubation time if necessary (for example, for incubation at lower temperature). Once you have determined the procedure (serial dilution and incubation time) that works best, stick with it for all of your counts.
4. Divide the plate into 4 quadrants, and use the stereo microscope to count colonies in one quadrant. Multiply count by 4 to get total CFU/100 μL for the diluted sample. Then multiply by the total dilution factor to get CFU for each piece of pureed meat. You can then divide by the original weight of the meat to get CFU/g of meat.

Bacterial Safety

Bacteria are all around us in our daily lives and the vast majority of them are not harmful. However, for maximum safety, all bacterial cultures should always be treated as potential hazards. This means that proper handling, cleanup, and disposal are necessary. Below are a few important safety reminders.

• Keep your nose and mouth away from tubes, pipettes, or other tools that come in contact with bacterial cultures, in order to avoid ingesting or inhaling any bacteria.
• Make sure to wash your hands thoroughly after handling bacteria.
• Proper Disposal of Bacterial Cultures
  • Bacterial cultures, plates, and disposables that are used to manipulate the bacteria should be soaked in a 10% bleach solution (1 part bleach to 9 parts water) for 1–2 hours.
  • Use caution when handling the bleach, as it can ruin your clothes if spilled, and any disinfectant can be harmful if splashed in your eyes.
  • After bleach treatment is completed, these items can be placed in your normal household garbage.
• Cleaning Your Work Area
  • At the end of your experiment, use a disinfectant, such as 70% ethanol, a 10% bleach solution, or a commercial antibacterial kitchen/bath cleaning solution, to thoroughly clean any surfaces you have used.
  • Be aware of the possible hazards of disinfectants and use them carefully.