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Yogurt Cultures

Difficulty
Time Required Short (2-5 days)
Prerequisites None
Material Availability Readily available
Cost Very Low (under $20)
Safety Requires parental supervision when cooking on the stove.

Abstract

Have you ever wondered how yogurt is made and what makes some yogurts different from other yogurts? You may have noticed that most yogurt containers advertise that the yogurt contains "live cultures." This means that there are living bacteria in the yogurt! These amazing bacteria can turn plain old milk into a yummy yogurt treat. In this science project, you will investigate whether the bacteria affect what the yogurt feels, tastes, and smells like by making your own yogurt at home!

Objective

Investigate whether using different yogurt starter cultures affects the yogurt that is made.

Credits

Sara Agee, Ph.D., Science Buddies

Teisha Rowland, Ph.D., Science Buddies

Cite This Page

MLA Style

Science Buddies Staff. "Yogurt Cultures" Science Buddies. Science Buddies, 30 June 2014. Web. 30 Sep. 2014 <http://www.sciencebuddies.org/science-fair-projects/project_ideas/MicroBio_p010.shtml>

APA Style

Science Buddies Staff. (2014, June 30). Yogurt Cultures. Retrieved September 30, 2014 from http://www.sciencebuddies.org/science-fair-projects/project_ideas/MicroBio_p010.shtml

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Last edit date: 2014-06-30

Introduction

Yogurt is a yummy treat, but how is it made? With the help of microorganisms called bacteria, milk is turned into yogurt. Do not freak out though, these are not the kind of bacteria that cause you to get sick. The bacteria in yogurt are good bacteria that can actually help you! There are certain species of bacteria that are commonly used to make yogurt. If you look at the ingredients listed on the yogurt product's packaging, you can often figure out the exact species of bacteria that it contains. Some species you might find listed include: Streptococcus thermophilus (S. thermophilus); Lactobacillus bulgaricus (L. bulgaricus); L. acidophilus; L. casei; L. rhamnosus; Bifidobacterium animalis (B. animalis, or sometimes just "Bifidus"); and B. bifidum.

To turn milk into yogurt, these bacteria ferment the milk. Fermentation is when a substance gets broken down and turned into another substance. During fermentation to make yogurt from milk, small sugars in the milk (specifically lactose sugars) get turned into a different chemical, specifically lactic acid. The lactic acid is what causes the milk, as it ferments, to thicken and taste tart. Because the bacteria have partially broken down the milk already, it is thought to make yogurt easier for us to digest. Additionally, eating yogurt can help restore the good bacteria that normally lives in your stomach and intestines (your gastrointestinal tract) after they have been lost from, for example, taking antibiotics or having an upset stomach.

In this microbiology science project, you will investigate how using different types of yogurt to make your own yogurt cultures affects how those cultures turn out. You will try different yogurt products as starter cultures to test which factors are important to the fermentation process and how the yogurt you make smells, feels, and tastes. Do you think yogurts that use different bacteria will be different? What about other factors in the yogurt that might affect the resultant yogurt culture, such as food coloring or added sugar? You will also learn how to culture (grow) microorganisms, and how to use sterile techniques so that you will not contaminate your cultures.

Terms and Concepts

  • Yogurt
  • Microorganisms
  • Bacteria
  • Fermentation
  • Yogurt cultures

Questions

  • What is fermentation? How is it used to make yogurt?
  • What do you think affects the smell, taste, and firmness of a yogurt product?
  • If you made a yogurt culture using a starter that did not have live bacteria in it, what do you think would happen to the yogurt culture?

Bibliography

Have an adult help you do further research by visiting the following websites, which give information about making yogurt and the living bacteria in yogurt:

Materials and Equipment

  • 8-oz canning jars with lids (12)
  • Large pot
  • Whole milk (one gallon)
  • Stirring spoon
  • Large double boiler (or thick-bottomed pot) with lid
  • Candy thermometer with a range of 40 to 90°C (100 to 200°F)
  • Large pan or sink that can be plugged
  • Permanent marker
  • Different types of yogurt (4). Use only new, unopened containers. You will want to use four different types that have different features that you can compare. Try to specifically include:
    • At least three products with live and/or active cultures. Try to find products that list the specific species of bacteria in them and try to find products that use different bacteria species.
    • One product that does not have live/active cultures in it. You might find such a product labeled a "dairy snack" instead of "yogurt." This may be difficult to find, so if you cannot find it, just skip using it in this science project.
    • A product that has flavoring or coloring agents added, such as Red 40, and a product that does not have flavoring or coloring agents added.
    • A product that is sweetened (with sugar listed in the ingredients) and one that is unsweetened (such as a Greek yogurt).
    • A product with added stabilizers, such as gelatin or agar, and a product that does not have added stabilizers.
    • For example, you could use two yogurts to compare two of these features if you used one yogurt that is white and unsweetened and another that is artificially colored and sweet.
  • Clean forks (4)
  • Measuring tablespoon
  • Optional: Funnel
  • Cooler. You want a cooler big enough to fit all 12 8-oz canning jars inside of it. Alternatively, you could use two small coolers.
  • Lab notebook
  • Adult helper

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Experimental Procedure

  1. To successfully make yogurt, good, sterile technique is needed. This means that the cookware used in this science project should be clean and handled properly to keep unwanted bacteria out of your yogurt cultures. Before starting, make sure that all of the cookware is clean and wash your hands with soap and rinse them thoroughly.
  2. With the assistance of an adult helper, sterilize the canning jars, their lids, and their rings in a large pot.
    1. Sterilize these pieces by separating them and putting them all in the large pot. Add about 2.5 centimeters (cm) (1 inch) of water, cover the pot with its lid, and boil the water for 10 minutes.
    2. Then turn off the heat and let the jars sit, still covered, in the pot.
    3. Safety note: Be careful when sterilizing the jars as the pot and everything inside of it will become very hot.
  3. Have an adult help you heat the milk to 85-90°C (185-195°F) in a large double boiler or thick-bottomed pot., keeping the pot covered to reduce evaporation.
    1. If you use a thick-bottomed pot, stir frequently to prevent the milk from sticking to the bottom.
    2. Be careful not to let the milk boil over!
  4. Remove the covered pot from stove and place the pot in either a large pan of clean, cool water or in a plugged sink that has been filled with about 3-5 cm (about 1-2 inches) of clean, cool water. Let the pot sit, still covered, until the milk is very close to 55°C (130°F).
    1. While the milk is cooling, prepare your jars by doing steps 5-11.
  5. Carefully remove the jars from the pot in which they were boiled and arrange them on a clean surface. Immediately put the lids and rings on each jar but leave the lids loosened.
    1. Safety Note: Be careful when handling the jars as they will be hot!
    2. Empty out any water in the jars.
    3. Do not touch the inside of the jars, as this could introduce unwanted bacteria into your yogurt cultures.
  6. Assign a letter, either "A," B," "C," or "D," to the four different types of yogurt. In your lab notebook, write down which number matches which type of yogurt.
  7. You will be making three jars for each of the four types of yogurt. Use the permanent marker to label three jars "A," three "B," three "C," and three "D." Label the A jars 1-3, and then do the same for the B, C, and D jars.
  8. Open the yogurt product you assigned the letter A to. Stir the yogurt with a clean fork to be sure it is mixed evenly.
  9. Add one tablespoon of yogurt A to each of the three A jars. Put the lids back on the jars. Thoroughly clean the measuring tablespoon.
  10. Repeat steps 8-9 using yogurts B, C, and D.
    1. All of the jars should now have one tablespoon of yogurt in them, as shown in Figure 1 below for three jars.


Microbiology science project Photo of three jars with different types of starter yogurt.
Figure 1. Add one tablespoon of yogurt to each sterile jar, as shown here. Note: This picture only shows three jars, each with a different type of yogurt, but in your science project you will be testing four different types of yogurt, with three jars for each type, making a total of 12 jars.


  1. Put the original yogurt products back in the refrigerator, sealing them as best you can. You will want to compare them to your yogurt cultures tomorrow!
  2. After the milk cools to 55°C (130°F), have an adult help you carefully pour it into the canning jars, filling them to about 1.5 cm (1/2 inch) from the top. Cover the jars immediately with their lids and tighten them.
    1. Note: The yogurt bacteria can be killed if exposed to temperatures above 55°C (130°F), so be careful not to add milk that is too hot!
    2. Tip: You may use a clean funnel to help you pour the milk into the jars.
  3. Place the filled jars in a cooler and seal it. Have an adult help you quickly heat up about one gallon (3.8 liters) of water until it is at 50°C (122°F). Open the cooler and carefully add this hot water to the cooler so that the jars are surrounded, but the water is well below the lid rims.
    1. Tip: You can rinse out the used milk container and use it to measure out a gallon of water.
    2. If you are using a large cooler and one gallon of water is not enough water to reach about 5 cm (2 inches) from the jars' lids, heat up more water to 50°C (122°F) and add it until the water is about 2.5-5 cm (1-2 inches) from the lid rims.
    3. 50°C (122°F) is a temperature that yogurt bacteria grow well at. If it is much hotter (above 55°C [130°F]) the bacteria die, and if it is much cooler ((below 37°C [98°F]) the bacteria do not grow so well. This temperature (50°C [122°F]) also helps prevent potentially harmful bacteria from growing in the yogurt.
  4. Put the cooler in a warm location and do not disturb it for three hours.
  5. After three hours, the yogurt cultures should be finished, provided the temperature does not drop below 38°C (100°F). At this time, check on the jars. How does the yogurt look? Does it look like the yogurt has solidified? Do not open the jars yet. Instead, put them in the refrigerator overnight.
  6. The next day, open and examine the yogurt cultures in each jar. Compare their appearance, firmness, smell, and taste to the original yogurt products.
    1. In your lab notebook, make a data table like Table 1 below. Record your observations in the data table in your lab notebook.
      1. Under the "Yogurt Container" column, the numbered cultures refer to the numbered jars you used for that type of yogurt. (For example, "Culture #1" refers to the yogurt culture in jar #1 for that type of yogurt.)
      2. "Original product" refers to the original yogurt you used.

      Yogurt Type Yogurt Container Appearance Firmness Smell Taste
      A Culture #1        
      Culture #2        
      Culture #3        
      Original Product        
      B Culture #1        
      Culture #2        
      Culture #3        
      Original Product        
      C Culture #1        
      Culture #2        
      Culture #3        
      Original Product        
      D Culture #1        
      Culture #2        
      Culture #3        
      Original Product        

      Table 1. In your lab notebook, create a data table like this one. Record your observations in the data table in your lab notebook.

    2. How do the yogurt cultures look? Are they all white (like the whole milk was) or are some of them slightly off-white in color? How do they look compared to the original products?
    3. Did the yogurt cultures gel? Are they firm or runny? How does this compare to the original product?
      1. Figure 2 below shows a picture of a relatively firm yogurt culture. When tipped on its side, the yogurt does not slosh at all. Note: No matter how firm the yogurt is, there will probably be a little bit of liquid on the top, as shown in Figure 2. This liquid is actually nutritious whey!
    4. Do the yogurt cultures smell good or bad? Do they smell sweet or sour? How does this compare to the original product?
    5. How do the yogurt cultures taste? Are they as sweet or sour as the original yogurt product?
    6. How do the yogurt cultures from the same original product compare to each other? Are there any differences in the yogurt cultures in the three jars made from the same yogurt?
    7. Overall, how do the yogurt cultures you made differ in appearance, firmness, smell, and taste compared to the original product? How do the yogurt cultures compare to each other, regardless of the original product used to make them?


Microbiology science project Photo of a jar with a firm yogurt culture and whey.
Figure 2. This photo shows a relatively firm yogurt culture. When tipped, the yogurt stays in place. The little bit of liquid on the top is whey.


  1. Analyze your results.
    1. Based on your results, what do you think affects how a yogurt culture turns out? Do you think it is the live and active bacteria cultures? Do you think other ingredients in the original yogurt product also affect it?
    2. Did any of the cultures not solidify? Why do you think this might be?
    3. Which starter made the "best" yogurt?
  2. If your yogurt cultures were made correctly, you should be able to enjoy your jars of yogurt as a tasty, healthy snack! You should also be able to refrigerate the sealed yogurt for one to two months. The acidity of yogurt (from the lactic acid) helps preserve it and prevent potentially harmful bacteria from growing.

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Variations

  • You can also test the effects of flavorings, preservatives, and other substances on yogurt cultures. Does flavored yogurt work as a starter culture? What about preservatives? Does organic yogurt work better than regular?
  • You can test if the amount of starter used in the culture makes a better product. How does using more or less yogurt affect the yogurt culture? Does it take a longer or shorter time to solidify?
  • Try testing which type of milk makes the tastiest yogurt. Try using whole, 2%, skim, soy, goat, or other types of milk. How does the type of milk affect what the resultant yogurt is like? Which milk works the "best"?
  • In this science project you cultured the yogurt for three hours in the cooler, but varying the amount of time that the yogurt is cultured for can affect its flavor. Try culturing the yogurt in the cooler for a longer amount of time, such as seven hours. How does increasing the culture time affect the yogurt culture? Does it look, smell, or taste different?

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