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Semisolid Science: Growing Yogurt

A live culture challenge from Science Buddies



George Retseck

Key concepts
Microorganisms
Bacteria
Food science
Fermentation

Introduction
Have you ever wondered how yogurt is made, and why some yogurts differ from others? As most yogurt containers advertise, yogurt contains "live cultures." This means that there are living bacteria in the yogurt! These are not the harmful kind of microbes that cause you to get sick. Instead, these cultures have the amazing ability to turn plain old milk into a yummy yogurt treat. Do the bacteria affect what the resultant yogurt culture looks, feels, tastes and smells like? In this activity you'll find out!

Background
Bacteria, which are a type of microorganism, turn milk into yogurt. There are certain species of bacteria that are commonly used to make yogurt, and these species are good bacteria that can actually help you! 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, turning the lactose sugars in the milk into 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 replenish the necessary populations of bacteria in your gastrointestinal tract (stomach and intestines) after they have been lost from, for example, taking antibiotics or having an upset stomach.
 
Materials
•     Six canning jars with lids, eight-ounce (235-milliliter) size or larger
•     Large pot
•     Water
•     Half gallon of whole milk. Other types of milk can be used instead.
•     Candy thermometer with a range of 100 to 200 degrees Fahrenheit (40 to 90 degrees Celsius)
•     Stirring spoon
•     Large double boiler (or a thick-bottomed pot) with lid
•     Large pan or sink that can be plugged
•     Permanent marker
•     Two different types of yogurt. Try to pick types with multiple features that differ, such as one kind that is white and unsweetened (such as a Greek yogurt) and another that is artificially colored (such as by the food dye Red 40) and sweet. Use new, unopened containers.
•     Two clean forks
•     Measuring tablespoon
•     Cooler
•     Adult help and supervision with heating and handling hot liquids

Preparation
•     Wash your hands with soap and rinse them thoroughly.
•     With an adult's assistance, sterilize the canning jars, their lids and rings. Do this by separating these pieces and putting them all in a large pot, adding about one inch (2.5 centimeters) of water, covering the pot, and boiling the water for 10 minutes. Then turn off the heat and let the jars sit, still covered, in the pot.
•     Be careful when sterilizing the jars—the pot and everything inside of it will become very hot. Also be careful when handling the hot milk later in the activity.
•     To successfully make yogurt, a good, sterile technique is needed. Make sure that all cookware is clean and handled properly to keep unwanted bacteria out of the yogurt cultures.

Procedure
•     Pour the half gallon (two liters) of milk into the large double boiler or thick-bottomed pot.
•     Heat the milk at 185 to 195 degrees F (85 to 90 degrees C), keeping the pot covered. If you use a thick-bottomed pot instead of a double boiler, stir frequently. Be careful not to let the milk boil over!
•     Remove the pot from the stove and place it in a pan of clean, cool water, until the milk is close to 130 degrees F (55 degrees C ). Alternatively, you can cool the pot in a clean, plugged sink with water.
•     While the milk is cooling, prepare your jars. Carefully remove them from the pot in which they were boiled and arrange them on a clean surface. Be careful, they will be hot! Empty out any water. Do not to touch the inside of the jars. Immediately put the lids and rings on each jar.
•     You will be making three jars for each type of yogurt. Use the permanent marker to label three jars with the name of one of the yogurt types, and label the other three jars the other yogurt type.
•     Open the first yogurt container and stir it with a clean fork. How does the yogurt look and smell?
•     Add one tablespoon of the yogurt to each of the three appropriate jars. Put the lids back on. Thoroughly clean the measuring tablespoon.
•     Open the second yogurt container and stir it with a new clean fork. How does the yogurt look and smell?
•     Add one tablespoon of the second yogurt to each of the three remaining jars. Put the lids back on.
•     Once the milk has reached 130 degrees F, carefully pour it into the jars, filling them to about one half inch (1.5 centimeters) from the top. Cover the jars immediately with their lids and tighten them. If you are using canning jars that are larger than eight ounces (235 milliliters) in size, only fill them up to about six ounces (175 milliliters). (Note: The yogurt bacteria can be killed if exposed to temperatures above 130 degrees F, so be careful not to add milk that is too hot!)
•     Place the jars in a cooler and seal it.
•     Quickly heat up about one gallon (3.8 liters) of water until it is at 122 degrees F (50 degrees C).
•     Add the hot water to the cooler so that the jars are surrounded, but the water is well below the lid rims.
•     Put the cooler in a warm location and do not disturb it for three hours.
•     After three hours the yogurt cultures should be done if the temperature does not drop below about 100 degrees F (38 degrees C). Check on the jars. How do the yogurt cultures look? Have they solidified?
•     Refrigerate the jars overnight.
•     The next day, open and examine the yogurt cultures in each jar. Compare their appearance, firmness, smell and taste to the original yogurts.
•     Did the yogurt cultures all gel? Are they firm or runny? Do they smell good or bad? How do they look, smell and taste compared with the original yogurt that was used? How are the cultures of each type of yogurt similar or different from each other? Is this the same way the original yogurts differed from each other?
•     Extra: In this activity you may have focused on how the taste and color of the original yogurt affects a yogurt culture based on it, but you can explore how other aspects of the yogurt affect the resultant yogurt culture. How do added stabilizers (such as gelatin), using organic yogurt compared with regular yogurt or other factors, like fat content, affect what a yogurt's culture is like?
•     Extra: You can test if the amount of starter used in the yogurt 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?
•     Extra: Try testing which type of milk makes the tastiest yogurt. Try using whole, 2 percent, 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"?
•     Extra: In this activity 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?


Observations and results
Was the yogurt in all of the jars firm and white? Did the yogurt cultures taste and smell like a very mild version of the original yogurts used?

The yogurt cultures in the jars will probably seem very similar in several ways, with some subtle differences based on the original type of yogurt used to make them. They should also be relatively firm, or firm enough so they do not slosh when tipped, and all have a similar texture. If the yogurt is not firm at all, but is actually fluid or runny, something may have gone wrong in the process and killed the bacteria—most likely the milk was too hot when added to the yogurt starters.

The yogurt cultures, however, may have small differences in taste and color based on the original yogurt used to make them. For example, if the original yogurt was really sweet, the yogurt culture should be only mildly sweet. Likewise, if the original yogurt was a bit sour (like Greek yogurt), the culture should also be a little sour. If you used one yogurt with artificial coloring (such as with Red 40) and one that was white, both resultant yogurt cultures should look white, just like the milk used to make them. If you put them side by side, however, you may notice that the culture whose original yogurt had artificial coloring is slightly off-white with a tinge of color. Overall, multiple factors affect the yogurt culture, including: the presence of some nonliving diluted ingredients from the original yogurt such as diluted Red 40 coloring, the exact process used to make the culture such as the amount of time in the cooler, and the types and amount of bacteria that were in the original yogurt.

Cleanup
If the 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 lactic acid) helps preserve it and prevent potentially harmful bacteria from growing. 

More to explore
Better Homemade Yogurt: 5 Ways to Make Thicker Yogurt from Emma Christensen at theKitchn
Live and Active Culture (LAC) Yogurt FAQ's from AboutYogurt.com
Yogurt Making Illustrated from David B. Fankhauser, PhD, University of Cincinnati Clermont College
Yogurt Cultures from Science Buddies



This activity brought to you in partnership with Science Buddies
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