Question

Food Microbiology

Purpose

To utilize the process of fermentation to make yogurt

To describe organisms responsible for food borne illnesses and summarize a recent outbreak

Introduction

Microorganisms have been used for centuries for food preservation and to improve or change its taste. Evidence exists that yogurt, which is milk fermented by bacteria, has been around for over 4000 years. Today, many of the foods we eat are the result of microorganisms acting on foods for a specific and desired effect. Examples of other fermented foods include wine/beer, sausage, sauerkraut/kimchee, bread, and cheese. The industrial use of microorganisms in food production began in earnest in the late 19^th century, when pure cultures of bacteria were grown specifically for that purpose.

Bacteria and Health Hazards

While there are a wide number of foods that are produced with microorganisms, the introduction of harmful organisms to food presents a serious health hazard. Even bacteria that normally colonize the human digestive system (such as E. coli) can cause severe illness or even death if ingested. The most commonly encountered microbial pathogens are Listeria monocytogenes, Staphyloccus aureus, Bacillus cereus, Bacillus anthracis, Clostridium botulinum, Clostridium perfringens, members of the Salmonella and Campylobacter genus, Escherichia coli (especially type O157:H7), Hepatitis and Rota viruses, Prions, different species of tape-worms and roundworms, and protozoa . Food-borne illness can be limited to a single person who ingests the contaminated food, or can lead to a more widespread outbreak of the same illness when multiple people consumed the same contaminated product. Many outbreaks are local in nature, such as when food from a restaurant is not held at the proper temperature, allowing bacteria to grow to disease-causing levels. However, increasingly there have been food-borne illness outbreaks that affect many geographical areas due to improper food handling. On April 24, 2013, the U.S. Food and Drug Administration placed two cucumber suppliers on Import Alert. Cucumbers from these two firms were denied admission into the United States until the suppliers demonstrated that their cucumbers were not contaminated with Salmonella. A total of 84 persons infected with the outbreak strain of Salmonella Saintpaul were reported from 18 states. 28% of ill persons were hospitalized, and no deaths were reported. (http://www.cdc.gov/foodsafety/outbreaks/multistate-outbreaks/outbreaks-list.html)

Bacteria and Fermentation

Fermentation is important for the production of a variety of dairy products. Cheeses are typically made using lactic acid-producing bacteria that aid the coagulation of the milk protein casein into the curd. The curd is then further treated with a different bacteria (depending on the desired final cheese) to produce distinctive tastes and aromas. The characteristic holes in Swiss cheese are produced by specific bacteria, that generates carbon dioxide and creates gas bubbles in the cheese.

Yogurt is made by the fermentation of lactose (milk sugar) by bacteria. Lactose is a disaccharide that consists of the simple sugars glucose and galactose. During the making of yogurt, the lactose is broken down by the enzyme lactase into glucose and galactose, which are fermented generating lactic acids and acetaldehyde. These two products lower the pH of the milk giving it a sour tart taste. The lowed pH also effects the milk proteins (caseins) causing coagulation, precipitating the proteins into a solid curd that forms the yogurt. The left over watery liquid is the whey. There are a variety of bacteria that may be utilized in this process, the two most commonly used to are Lactobacillus bulgaris and Streptococcus thermophilus

In this experiment we will start with milk. To increase the shelf-life, milk that is sold commercially is pasteurized. Pasteurization does not sterilize the milk, but will most of the microorganisms present. To manufacture yogurt these organisms must be destroyed so that they will not compete with the added organisms essential for the fermentation process. Commercially available yogurt containing live bacterial cultures will be used to inoculate the milk after heating

Experiment: Yogurt Preparation

MATERIALS

Thermometer

(1) 250 mL Beaker

1 Plastic, sterile fork

4 pH indicator strips / centrifuge tubes

Hot pad

*1 - 2 Dish towels

*4 C. ( 950 mL) and 1 tablespoon Fresh pasteurized milk (whole, 2%, 1%, or fat-free)

*1/2 C. (120 mL) Plain yogurt

*Sauce pan

*Clean mixing bowl

* Glass jar

*Stove or hot plate

* non-fat dried milk

*You must provide

PROCEDURE

1. Place sauce pan on stove or hot plate. Add 950 mL (approximately 1 pints) milk to the pan. Reserve a small amount, approximately 10 milliliter, of the milk. Place in centrifuge tube labeled “before heating”

2. Measure and record the pH of the reserved milk using pH indicator paper.

3. Insert thermometer into milk and gently heat while stirring. Bring the milk to 85 °C.

4. Remove the pan from the heat and allow the milk to cool to 43 °C.

5. Remove a small amount , approximately 10 milliiter, of the milk. Place in centrifuge tube labeled “after cooling”.

6. Measure and record the pH of the reserved cooled milk using pH indicator paper.

7. Measure and pour 120 mL plain yogurt into a mixing bowl. (This yogurt must be marked as containing live yogurt cultures)

8. Slowly stir in the cooled milk, 120 mL (1/2 C.) at a time, mixing the ingredients with a plastic fork. Be sure to thoroughly mix the ingredients to create a homogenous solution. Add nonfat dry milk, if desired. Adding about 1/4 cup to 1/2 cup nonfat dry milk at this time will increase the nutritional content of the yogurt. The yogurt will also thicken more easily. This is especially helpful if you're using nonfat milk.

9. After all the cooled milk has been added to the yogurt, remove a small amount approximately 10 milliliters, of the milk. Place in centrifuge tube labeled “ after yogurt”.

10. Measure and record the pH of the reserved mixture using the pH indicator paper.

11. Place the mixture into a glass jar..

12. Incubate the mixture in a warm place (30 - 32 °C) for 5 - 6 hours. If you don’t have a warm place for incubating the yogurt, wrap the bowl with a heating pad and wrap the heating pad with 1 - 2 towels. Set the heating pad to low or medium heat. Be sure to check the temperature.

HINT - It is a good idea to test the temperature with the glass jar filled with water prior to starting the experiment. If the heating pad is too high, a towel can be wrapped around the jar to decrease the temperature.

13. Check the yogurt’s consistency after the recommended incubation time. It should be “set” and have a consistency similar to store bought yogurt. If it is not yet set, incubate the yogurt for an additional 1 -hours.

14. Remove a small amount approximately 10 milliiter, of the milk. Place in centrifuge tube labeled “after incubation” measure and record the pH using pH indicator paper.

15. Place the yogurt in a refrigerator for 24 hours.

TABLE 1: Yogurt pH Results
Sample
1 – before heating
2 – after cooling
3 – after yogurt
4 – after incubation

QUESTIONS -

2. What is the function of heating the yogurt in step 3.

3. What is the purpose of adding the yogurt the cooled milk in step 8?

2. What attributes does lactic acid confer to yogurt?

3. How does the consistency of the milk change during the production of yogurt? What facilitates this change?

4. What are the breakdown products of lactose?

5. Go to the CDC website (http://www.cdc.gov/foodsafety/outbreaks/) and describe one of the Selected Outbreak Investigations that occurred over the last 2 years.

6. Some species of bacteria cause disease in humans, other animals, plants, and fungi. Perform an Internet search using the scientific name of each of the bacteria below to learn the common name of the disease. Include a brief description of the disease in the space provided under each name.

Genus and species                                                                                                              Treponema pallidum          Name of Disease _________________________       Description of disease: Helicobacter pylori             Name of Disease _________________________ Description of disease:                 Borrelia burgdorferi          Name of Disease _________________________ Description of disease:                     Clostridium botulinum       Name of Disease _________________________ Description of disease: Yersinia pestis                      Name of Disease _________________________ Description of disease:

Answer 1

2) The milk is heated to a temperature of 85 degree celsius. The purpose of heating the yoghurt is to kill the common bacteria and viruses present in it.

3) The purpose of adding the yoghurt is to increase the nutritional content of the mixture and also to thicken it.

4) The lactic acid confers the following attributes to yoghurt,

• Taste
• Texture
• Consistency
• Ability to solidify

5) The consistency of milk thickens during the production of yoghurt and gradually the texture hardens. This is due to the fermentation action of thermophilic bacteria.

6) Lactose consists of galactose and glucose

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