GOLD

The Bacteria Behind Thawing Meat

This project explores the bacteria that results from different methods of thawing meat and quantitatively and qualitatively determines the safest method for thawing meat.
Leandra Joshua
Grade 10

Hypothesis

If you defrost the same type of meat using different thawing methods then the meat that is thawed at a sustained colder temperature will develop the least amount of bacteria because meat bacteria will be unable to adjust to the cold environment and rapidly multiply, resulting in the least amount of bacterial growth.

Research

Types of Meat and their Common Bacteria

  • Fresh meat has a high water content => favourable for microorganism growth
  • Beef: Most common bacteria occurring in beef is E. coli => can cause severe damage to intestinal lining
    • Salmonella, staphylococcus aureus, and Listeria monocytogenes can also occur
    • These types of bacteria can be destroyed by cooking
  • Pork: E. coli, salmonella, S. aureus, Yersinia enterocolitica are the most common types of bacteria
    • Diarrheal illness (yersiniosis) can occur due to this bacteria
    • This bacteria can be destroyed by cooking to internal temperature of 145°F
  • Chicken: Salmonella and E.coli are the most common types of bacteria
    • Microbes killed at internal temperature of 165°F

Chicken and Bacteria

  • Campylobacter bacteria
    • Causes an infection called campylobacteriosis
    • Infects around 1.3 million people in the U.S. every year
    • Causes diarrhea and other serious complications
    • Affects people when they eat undercooked poultry or food that has touched raw poultry
  • Salmonella
  • Clostridium perfringens
  • These bacteria mostly affect those aged 65 or older, children younger than 5, those with weak immune systems, and pregnant women

Methods of Defrosting and Cooking

  • Refrigerator thawing
    • Thawing the meat in the fridge until ready to use
    • Considered to be the safest method of defrosting
    • Temperature must be 40°F or below
    • Time required is longer than other methods of defrosting
  • Cold water thawing
    • Placing meat under cold running water or submerged completely in cold water
    • Least preferred method of defrosting out of the safe methods => easy to reach warmer temperatures where bacteria can grow
    • Water should be under 60°F
    • All surfaces (such as the sink) should be cleaned prior to defrosting
  • Microwave thawing
    • Thawing food in the microwave until ready to use
    • Food should be cooked immediately afterwards
  • Room Temperature thawing
    • Highly dangerous
    • Worst method of defrosting
  • Hot Water thawing
    • Highly dangerous
    • Meats quickly reach danger zone
    • Really quick method of thawing
  • Cooking without thawing
    • Should only be used for foods designed to be thawed and cooked at the same time

Defrosting at Room Temperature

  • Defrosting at room temperature
    • Worst method of defrosting
    • Highly dangerous
    • Food becomes warmer than 40°F and bacteria present before freezing starts to multiply
    • Number of bacteria that causes illnesses can double every 20 minutes at room temperature
    • Bacteria that is present in the meat before freezing is able to grow at room temperature and cause illness
    • Some organisms can create toxins that can survive a cooking process even if the temperature of the cooking process kills that bacteria themselves

Temperature

  • 40° to 140° F => bacterial growth range

Foodborne Illnesses

  • In the U.S., the federal government estimates that around 48 million cases of foodborne illnesses occur annually
    • Results in an estimated 128 000 hospitalizations
    • Also results in an estimated 3 000 deaths
  • Common bacteria that causes serious illnesses:
    • Salmonella
    • E. coli
    • Clostridium botulinum
  • In Canada, Salmonella and Campylobacter are leading causes of bacterial food-borne illness
  • 2015: Canada had around 88 000 Salmonella cases and 145 000 Campylobacter cases
  • These illnesses can also be infectious

Food Spoilage vs Food Pathogens

  • Food spoilage
    • Yeasts, molds, fungi, or bacteria that grow on food
    • Often change the smell, appearance, and taste of food
    • Do not cause life-threatening infections
  • Food pathogens
    • Biological agents that cause disease or illness to the person ingesting the meat
    • Causes illnesses and possibly death
    • Bacteria, viruses, and parasites
    • Cannot be seen, smelled, or tasted
    • Can be caused by improper methods of thawing and cooking

Phases of Bacterial Growth – Bacterial growth curve

  • Represents number of live cells in a bacterial population over time
  • Factors like oxygen, pH, temperature, and light influence microbial growth
  • 4 distinct phases of growth
    • Lag
      • Cellular activity is occurring but no growth is happening
      • Bacteria adjusts to environment and prepares for growth and multiplication
    • Exponential (Log)
      • Cells dividing rapidly
      • Metabolic activity is high
      • Growth phase
      • At this point, bacteria is still able to be eliminated effectively
    • Stationary
      • Population growth declines as nutrients are depleted or waste products accumulate
      • Plateau is reached
      • No overall population growth
      • Pathogenic bacteria begins to generate substances (toxins) to survive harsh conditions
        • Causes disease
        • Even after undergoing the process of cooking at high temperatures, these toxins can remain in the meat and cause serious illnesses
    • Death
      • Number of living cells decreases
      • This may be due to a change in environment conditions, depleting nutrients, or increasing waste products
      • Spore bacteria can still possibly survive, however, and produces spores that can produce bacteria in a new, life-supporting environment

Bacterial foodborne illnesses related to meat are often overlooked but actually create serious problems in society. Thawing methods play a large role in preventing foodborne illnesses, and choosing the right method of defrosting meat is essential to preventing future illnesses and possible deaths. Even after cooking, toxins released from bacteria remain, posing a threat to the health of the person consuming the meat.

Variables

Manipulated Variable

  • Process in which each meat sample is defrosted

Responding Variable

  • Number of bacterial colonies that appear on the agar plates

Controlled Variables

  • Type of Agar plate used (Tryptic soy agar plates from Dalynn Biologicals)
  • Type of swabs used (sterile cotton swabs)
  • Locations of the different defrosting methods
    • Countertop, fridge, bowls of water, microwave
  • Location and environment of the bacteria on the agar plates
  • Amount of time the bacteria was allowed to grow
  • Temperature at which the plates were placed (27 degrees celsius)
  • Type of meat - boneless, skinless, extra lean, split chicken breast from Costco
  • Cutting board and knife used to initially slice the meat
  • Size and shape of meat (3.5 cm in width, 2.5 cm in height, and 0.75 cm in depth)
  • Freshness of meat (same batch fresh from the store)
  • Temperature of freezer (-18°C)
  • Length of time the meat was frozen prior to being thawed (1hr, 50min)
  • Room temperature (23°C)
  • Temperature of water in the water thawing method (4°C)
  • Temperature of refrigerator in the refrigerator thawing method (3°C)
  • Amount of time in the microwave in the microwave method (54 seconds)
  • Temperature and setting of microwave (def1, 0.1lb setting)
  • Type of microwave (LG Microwave Oven)
  • Mastercraft digital temperature reader
  • Type of ziploc bags used (Ziploc sandwich bags)
  • Size of area in which bacterial colonies were observed (5cm length, 0.5cm width)

Procedure

Day 1 (Preparation of Samples)

  1. Meat bought at store at 3:00 p.m.
  2. Left in car until 6:30 p.m. at a temperature of -5°C
  3. Brought inside at 6:30 p.m. and prepared into cubes until 6:50 p.m. at a temperature of 23°C
  4. Cut into 15 rectangular prisms
    1. 3.5 cm in width, 2.5 cm in height, and 0.75 cm in depth
    2. Cut on same cutting board with same knife
  5. Each cube placed into its own new Ziploc bag and sealed
  6. All the Ziploc bags placed into the freezer at the same time at 6:50 p.m.
    1. Temperature of -18°C inside freezer
  7. All samples were removed from the freezer at 8:40 p.m. when they were frozen solid
  8. The samples were placed in their different environments at the same time (8:40 p.m.) => 3 samples per thawing method in order to attain reliable results
    1. 3 in the fridge at 3°C
    2. 3 in cold water at 4°C (new water added every 15 minutes)
    3. 3 in warm water at 25°C (new water added every 15 minutes)
    4. 3 outside at 23°C
    5. 3 in the microwave at the predetermined thawing setting (def1, 0.1lb, 54 sec)
      1. Left frozen until 11:00 p.m., then microwaved and swabbed with other samples
  9. Samples swabbed onto refrigerated tryptic soy agar plates with refrigerated cotton swabs at 11:00 p.m.
  10. Agar plates placed in dark room at 11:30 p.m. at a temperature of 27°C
    1. Placed in room at same time

Day 2 (Period of Growth)

  1. Swabbed plates left inside room for the whole day at a temperature of 27°C

Day 3 (Observation of Bacterial Colonies)

  1. Plates observed at 9:30 a.m. until 10:00 a.m.
    • I had 15.0cm by 0.5cm strips of paper that I placed in the exact same location on each typtic soy agar plate
    • The agar plates had markings on them which I utilized to ensure the strips of paper were in the exact same location on every plate
    • I counted the number of distinct bacterial colonies that were within the 15.0cm by 0.5cm areas
    • This data was used to analyze the bacterial growth
  2. Plates returned to room at 10:00 a.m.
  3. Plates observed again at 7:00 p.m.
    • Repeated the procedure of observation from the 9:30 a.m. period of observation
  4. Plates and bacteria disposed of accordingly

 


Further description of items used in procedure:

Meat

  • Boneless, skinless, extra lean, split chicken breast
  • Best before March 3, 2021
  • Pink, fleshy colour and soft texture
  • Fresh meat

Agar Plates and Cotton Swabs

  • Tryptic soy agar plates
  • Best before May 13, 2021
  • Bought from Dalynn Biologicals
  • Stored upside down in refrigerator at 3°C

Observations

Table 1: Internal Temperatures of Samples Immediately after Defrosting

(As observed using the Mastercraft Digital Temperature Reader)

Sample Number and Method of Thawing

Internal temperature of chicken samples when swabbing (°C)

1 – Fridge

18.6°

2 – Fridge

16.8°

3 – Fridge

24.4°

4 – Cold water

25.4°

5 – Cold water

23.7

6 – Cold water

24.3

7 – Warm water

23.9

8 – Warm water

23.9

9 – Warm water

24.3

10 – Outside

23.4

11 – Outside

23.4

12 – Outside

23.5

13 – Microwave

23.0

14 – Microwave

22.5

15 – Microwave

24.4

 

Table 2: Number of Bacterial Colonies on Tryptic Soy Agar Plates over Time

Observed on a constant 15cm by 0.5cm area

(As observed on Day 3)

Sample Number and Method of Thawing

9:30 a.m.

7:00 p.m.

1 – Fridge

24

29

2 – Fridge

37

39

3 – Fridge

8

9

4 – Cold water

29

32

5 – Cold water

24

25

6 – Cold water

27

29

7 – Warm water

197

197

8 – Warm water

210

211

9 – Warm water

98

98

10 – Outside

71

74

11 – Outside

15

16

12 – Outside

96

96

13 – Microwave

62

62

14 – Microwave

151

152

15 – Microwave

65

65

 

Table 3: Qualitative Observations

(As observed on Day 3)

Sample Number and Method of Thawing

9:30 a.m.

1 – Fridge

Slightly yellowish colonies, evenly spread around

2 – Fridge

Slightly yellowish colonies, somewhat evenly spread around

3 – Fridge

Very few colonies, slightly yellowish colonies, somewhat evenly spread around

4 – Cold water

Quite a few colonies, slightly yellowish colonies, evenly spread around

5 – Cold water

Few colonies, slightly yellowish colonies, evenly spread around

6 – Cold water

Few colonies, slightly yellowish colonies, evenly spread around

7 – Warm water

Lots of colonies (almost entirely covering the plate), colonies small in size, slightly yellowish colonies, spread out in streaks

8 – Warm water

Very large quantity of colonies (almost entirely covering the plate), slightly yellowish colonies, colonies seem to be very small in size, spread out in streaks but less visible because of the large quantity of bacterial colonies

9 – Warm water

Slightly yellowish colonies, colonies are a little bigger in size compared to the other colonies of other samples, colonies evenly spread around

10 – Outside

Slightly yellowish colonies, colonies somewhat evenly spread around

11 – Outside

Very few colonies, slightly yellowish colonies, somewhat evenly spread around, larger colonies

12 – Outside

Lots of colonies, colonies small in size, slightly yellowish colonies, evenly spread out

13 – Microwave

Few colonies, slightly yellowish colonies, evenly spread around

14 – Microwave

Lots of colonies (almost entirely covering the plate), colonies small in size, slightly yellowish colonies, spread out in streaks (some streaks are very distinct)

15 – Microwave

Few colonies, slightly yellowish colonies, evenly spread around

 

Table 4: Images of Tryptic Soy Agar Plates

(As observed on Day 3)

Sample Number and Method of Thawing

 

9:30 a.m.

 

7:00 p.m.

1 – Fridge

2 – Fridge

3 – Fridge

4 – Cold water

5 – Cold water

6 – Cold water

7 – Warm water

8 – Warm water

9 – Warm water

10 – Outside

11 – Outside

12 – Outside

13 – Microwave

14 – Microwave

15 – Microwave

Analysis

Graph 1

(3 samples of meat were placed through each thawing method, and the average bar shows the average number of bacterial colonies on a 15cm by 0.5cm area in each method of thawing.)

Graph 2     

(This graph shows the average number of bacterial colonies on a 15cm by 0.5cm area for each thawing method along with the standard of deviation.)

Graph 3      

(3 samples of meat were placed through each thawing method, and the average bar shows the average number of bacterial colonies on a 15cm by 0.5cm area in each method of thawing.)

Graph 4      

(This graph shows the average number of bacterial colonies on a 15cm by 0.5cm area for each thawing method along with the standard of deviation.)

Analysis

In this experiment, two data collection sets were created in order to ensure the reliability of these results and also to account for human error in the counting of the bacterial colonies. As can be clearly seen from all the graphs, the most bacteria was observed in the warm water. The second largest number of bacterial colonies was observed in the microwave samples, the third largest number of bacterial colonies was observed in the room temperature samples, and the fourth largest number of bacterial colonies was observed in the cold water samples. The least amount of bacterial colonies were observed in the fridge samples.

Based on these results, the thawing method that resulted in the least amount of bacterial colonies was the fridge thawing method. This is because of the sustained cold temperature which prohibited the bacteria in the meat from rapidly multiplying due to the inhospitable conditions. However, the warm water thawing method and the microwave thawing method were the two worst methods according to the graph. This is due to the warmer temperatures of using these methods that place the bacteria in a favorable environment for bacterial growth.

An analysis of the results in this experiment demonstrates that altering the method of defrosting meat significantly affects the amount of bacteria that grows on the meat during the thawing process. This proves the importance of safely defrosting meat in order to ensure the quality and safety of the food being consumed. 

Conclusion

  • After completing this experiment, I was able to better compare the different methods of thawing meat, specifically focusing on chicken in this experiment.
  • When comparing different methods of thawing meat, thawing meat in warm water, at room temperature, or in the microwave significantly increased the amount of bacteria created when compared to the fridge method and the cold water method. The best method of thawing meat to reduce the amount of bacteria formed, based on this experimental procedure, is thawing meat in the refrigerator where a constant cold temperature of 3°C is maintained.

  • According to Table 2 and Graphs 1 to 4, the warm water method of thawing chicken had the highest average number of bacterial colonies observed on a 15cm by 0.5cm area (an area that remained constant when observing all the different samples) with 197, 211, and 98 bacterial colonies respectively on the tryptic soy agar plates by 7 p.m..
  • The fridge samples had the lowest average number of bacterial colonies with 29, 39, and 9 bacterial colonies respectively in the three samples by 7 p.m. on day three.

  • There is a very clear correlation between temperature and bacterial growth because the environments with warmer temperatures had more bacterial growth on the plates.
  • This is further emphasized by the fact that even though there was a large time difference between the two readings of the bacterial load, the order of the safest thawing methods remained the same.  

  • My hypothesis was correct because the cold, sustained temperature of 3°C in the fridge prevented the bacteria from entering the danger zone of 40°F to 140°F, the temperature range in which bacteria multiplies rapidly.

Based on the results of this experiment, the safest method of thawing meat is thawing it inside the refrigerator until it is ready to use. Thawing in cold water is also relatively safe; however, thawing meat in warm water, at room temperature, or in the microwave leads to the rapid growth of bacteria on the meat which can cause serious health issues and possibly even death.

Application

  • The purpose of this project was to determine the safest method of thawing meat. For these purposes, I used raw chicken and found that thawing meat in the fridge is the safest method of thawing meat.
  • The importance of thawing meat is often overlooked since thawing meat is a very common routine in our everyday lives. There are many different methods of thawing meat, and the differences each method has and the consequent impact these differences have in our lives is momentous.
  • However, many people do not thaw meat properly, and this has led to many illnesses and even some deaths. In my own life, I know that my family has been thawing meat the more dangerous way for many years. A lot of this ignorance is due to the lack of awareness about frozen meat and safe thawing methods.
  • This project can raise awareness about the actual importance of thawing methods and help many people understand the significance of thawing meat properly.

  • Even after meat has undergone the cooking process, the bacteria that grew on the meat prior to cooking can release heat-resistant toxins that survive the high temperatures of cooking. This has effects on the health of the person consuming the meat and can cause food poisoning.
  • Furthermore, when handling meat that has a lot of bacteria on it in the kirchen, kirchen utensils and surfaces can become contaminated with the bacteria which can lead to future illnesses.
  • Proper thawing knowledge should become more widespread and its importance should be emphasized. This project demonstrates the need to raise awareness regarding food safety and can help society be safer in regards to food preparation.

  • In the future, I would like to further explore this concept of thawing meat by testing the effects of thawing on different types of meat.
  • I would also like to further explore the extent of the harm that can be caused by bacterial growth from improper methods of defrosting.
  • Furthermore, in order to deepen my understanding of the concepts explored in this experiment, I would also like to develop an algorithm for different methods of meat defrosting and its consequent results. 

Sources Of Error

  • One source of error in this experiment is found in the microwave-thawed samples. Although the microwave-thawed samples were placed under the pre-set microwave setting, this setting may differ between different models of microwaves. This could affect the safety of thawing meat in the microwave because of the difference in heat and time duration in the pre-set meat defrosting settings on the microwaves.
  • In the future, I can improve the results for the microwave samples by using multiple different microwave brands in order to properly measure the average affect of predetermined microwave thawing settings on bacterial growth. 

  • Another source of error can be found in the bacterial colonies. Some colonies partially merged with other colonies, skewing the number of single colonies in the plates. The size of the colonies can also skew the results because some of the samples had larger bacterial colonies but these colonies counted as the same as one of the smaller colonies. This would skew the results because when counting the bacterial colonies, the large colony would count the same as a small colony.
  • In the future, this can be improved by further dividinng up the bacterial count into different sections based on size and merging colonies. This would accurately represent this data. 

Citations

Works Cited

Bailey, Regina. “The Bacterial Growth Curve and the Factors Affecting Microbial Growth.” ThoughtCo, 19 Sept. 2018, www.thoughtco.com/bacterial-growth-curve-phases-4172692#:~:text=Bacterial%20growth%20cycles%20in%20a,)%2C%20stationary%2C%20and%20death.

Michelle Jarvie, Michigan State University Extension. “Food Spoilage and Food Pathogens, What's the Difference?” MSU Extension, 4 Oct. 2018, www.canr.msu.edu/news/food_spoilage_and_food_pathogens_whats_the_difference.

National Center for Home Food Preservation. “General Freezing Information.” National Center for Home Food Preservation | How Do I? Freeze, nchfp.uga.edu/how/freeze/thawing.html.

Shaffer, Dr. Catherine. “Microbes in Raw Meat.” News-Medical.net, 21 Jan. 2021, www.azolifesciences.com/article/Microbes-in-Raw-Meat.aspx#:~:text=The%20most%20common%20pathogenic%20bacteria,also%20common%20contaminants%20in%20beef.

U.S. Food and Drug Administration. “Refrigerator Thermometers: Cold Facts about Food Safety.” U.S. Food and Drug Administration.

United States Department of Agriculture. “FSIS.” Safe Defrosting Methods, www.fsis.usda.gov/wps/portal/fsis/topics/food-safety-education/get-answers/food-safety-fact-sheets/safe-food-handling/the-big-thaw-safe-defrosting-methods-for-consumers/bigthaw2.

Acknowledgement

I would like to acknowledge the following people who helped me throughout this project:

Dr. Garcia-Diaz who helped guide me in developing a project idea and planning out the process.

My family who inspired me to do this project and helped me in the experimental procedure.

Ms. Gierus who inspires me to participate in the Science Fair and is supportive throughout the entire process. 

Thank you to all those who helped me during this project!