IntroductionHave 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!
This activity is not appropriate for use as a science fair project. Good science fair projects have a stronger focus on controlling variables, taking accurate measurements, and analyzing data. To find a science fair project that is just right for you, browse our library of over 1,200 Science Fair Project Ideas or use the Topic Selection Wizard to get a personalized project recommendation.
BackgroundBacteria, 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.
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 ResultsWas 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.
More to Explore
Better Homemade Yogurt: 5 Ways to Make Thicker Yogurt from Emma Christensen at theKitchn
Teisha Rowland, PhD, Science Buddies
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