13: Food and Milk Microbiology
- Page ID
- 107300
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Learning Objectives
- Identify the importance of food microbiology in our everyday lives.
- List ways by which food can become contaminated with microbes.
- Define pasteurization and it's importance in the milk industry.
- Recognize common causes of foodborne illness.
- Describe measures that reduce risk of foodborne illness.
Pathogens can be introduced into foods at any stage: during growth/production at the farm, during processing (grinding, chopping, milling, etc.), during handling and packaging, and when the food is prepared in the kitchen. In many cases, small numbers of pathogenic bacteria are not dangerous, but improper storage and/or cooking conditions can allow these bacteria to multiply to dangerous levels.
Fecal contamination of water (and through water, contamination of food materials) is another one of the ways in which pathogens can be introduced. Coliform bacteria are Gram-negative non-spore forming bacteria that are capable of fermenting lactose to produce acid and gas. A subset of these bacteria are the fecal coliforms, which are found at high levels in human and animal intestines. Fecal coliform bacteria such as E. coli, are often used as indicator species, as they are not commonly found growing in nature in the absence of fecal contamination. The presence of E. coli suggests feces are present, indicating that serious pathogens, such as Salmonella species and Campylobacter species, could also be present.
In this lab, you will examine bacteria found in milk, chicken, and in other assorted food materials.
Milk
Milk contains carbohydrates, minerals, fats, vitamins, and proteins, and is therefore susceptible to breakdown by a wide variety of microorganisms. Several different kinds of bacteria may be present in milk, most commonly the genera Lactobacillus, Micrococcus, and Streptococcus. As discussed in Lab 7, regular pasteurization is the process used to reduce microbial loads to acceptable levels in foods like milk and fruit juices. Milk that has undergone a regular pasteurization procedure can still contain bacteria. If this milk is stored at 4ºC (refrigerator temperature) the bacteria are prevented from multiplying, but if the milk is left out at room temperature, the bacteria will reproduce and the milk will spoil.
If done aseptically, UHT (ultra high temperature) pasteurization can sterilize foods. This is why UHT milk can be left unopened at room temperature for long periods of time.
Microorganisms of concern in milk include Campylobacter jejuni, Escherichia coli O157:H7, Listeria monocytogenes, Mycobacterium bovis, Mycobacterium tuberculosis, Salmonella species (most common: S. enteritidis and S. typhimirium), and Yersinia enterocolitica.
Chicken
Raw poultry products are frequently contaminated by pathogens (e.g. Salmonella, Staphylococcus aureus, Clostridium perfringens, Campylobacter jejuni, etc.). Salmonellosis is one of the most common serious foodborne infection caused by contaminated poultry. The prevalence of Salmonella in raw fresh and frozen poultry approaches 80% in some countries.
Fruits and Vegetables
Fruits and vegetables may become contaminated with potential pathogens from the soil or water they come in contact with while growing. They may also pick up harmful bacteria during processing, storage and preparation. While the use of potable (uncontaminated, drinkable) water to wash and freshen harvestable fruits and vegetables is useful both commercially and in the home, residual water may support extensive microbial growth during further storage.
Reducing the Risk of Contracting Foodborne Illness
While we can't completely remove all microbes from our foods, we can take steps to reduce the risk of foodborne illness. Read the pamphlet below to learn best practices for handling and preparing common foods (provided by the CDC).
Key Terms
CFU, indicator bacteria, coliform bacteria, fecal coliform bacteria, UHT pasteurization
Procedures
A. The Microbiology of Milk
In this exercise, you will compare the amount of bacteria found in regular pasteurized milk with the amount found in unpasteurized milk.
Per pair of students:
- 2 TSA plates
- 1 tube of unpasteurized milk
- 1 tube of pasturized milk
1. Using the 20-200 µl pipettor (also called the P200), transfer 200 µl of the refrigerated milk on a TSA plate.
2. Use a sterile spreader to spread the the milk across the plate.
3. Incubate plates upside-down overnight at 37degC.
Predictions
What type of bacteria will grow on the TSA plates? ________________________________________
Predict which type of milk will have the most bacteria. Explain the rationale for your prediction. ___________________________________________________________________________________
B. The Microbiology of Poultry
In this exercise, you will compare the amount of bacteria found in raw and cooked chicken breast.
Per pair of students:
- 2 TSA plates
- Cooked chicken breast
- Uncooked chicken breast
1. Flame sterilize forceps.
2. Pick up a portion of your chosen food product, gently rub it over the surface of the TSA plate, then discard the food.
3. Incubate the plate until the next lab.
Predictions
What type of bacteria will grow on the TSA plates? ________________________________________
Predict which specimen will have the most bacteria. Explain the rationale for your prediction. ___________________________________________________________________________________
C. Microbiology of Food
Per student: 1 TSA plate
Choose a food item from home to culture in lab. Common examples include: fruits, vegetables, meats, dairy, bread, etc.
Chosen food material: _________________________________________
1. Flame sterilize forceps.
2. Pick up a portion of your chosen food product, gently rub it over the surface of the TSA plate, then discard the food.
3. Incubate the plate until the next lab.
Results
A. The Microbiology of Milk
Record the results of your experiment in the table below. Record growth as 0, +, ++, or +++. Describe any notable growth characteristics.
| Amount of Growth | Growth Characteristics | |
| Regular pasteurized milk | ||
| Unpasteurized milk |
Which type of milk had the largest number of bacteria? ___________________________________
Was your earlier prediction correct? ______________
B. The Microbiology of Poultry
| Amount of Growth | Growth Characteristics | |
| Cooked chicken breast | ||
| Raw chicken breast |
Predictions
Which type of chicken had the largest number of bacteria? ___________________________________
Was your earlier prediction correct? ______________
LAB ASSIGNMENT
1. What is the importance of pasteurization?
2. What are the potential dangers of eating raw or undercooked meats?
3. Why do you think humans don't get sick every time they eat if there are so many microbes growing on food (regardless of whether it is cooked).
4. What are some common causes of foodborne illness?
5. In 2022, an outbreak of Campylobacter jejuni occurred at a wedding event in Virginia. Over 60% of those who attended became ill with gastrointestinal upset, including diarrhea and vomiting. The caterer, Mrs. Jacob's Fried Chicken, provided fried chicken, french fries, and coleslaw at the wedding, as well as all glassware and disposable plastic utensils. None of the cooked food was found to have abnormally high quantities of C. jejuni, however, it was noted that the dishwasher was unavailable and all glassware had to be hand washed in the kitchen sink used to prepare the food.
A. What do you think could be the source of contamination at this wedding? Explain your rationale.
B. How could you test your above theory on the source of contamination?