How is yogurt made and what are active cultures? Find out in this experiment how good bacteria can make plain old milk into a yummy treat!
In this experiment you will investigate the optimal starter cultures for making yogurt from whole milk.
Yogurt is a yummy treat, but how is it made? With the help of microorganisms called bacteria, milk is turned into yogurt. Don't 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! Dr. David B. Fankhauser, Professor of Biology and Chemistry at U.C. Clermont College explains about yogurt making, fermentation, and good bacteria:
"Yogurt is a fermented milk product which originated in Turkey in which a mixed culture of Lactobacillus bulgaricus (or occasionally L. acidophilus) and Streptococcus thermophilus produce lactic acid during fermentation of lactose. The lactic acid lowers the pH and makes it tart and causes the milk protein to thicken. The partial digestion of the milk when these bacteria ferment milk makes yogurt easily digestible. In addition, these bacteria will help settle GI upset including that which follows oral antibiotic therapy by replenishing non-pathogenic flora of the gastrointestinal tract.
"Several factors are crucial for successful yogurt making:
good sterile technique (i.e., proper cleansing and heat treatment of glassware, and keeping out unwanted bacteria)
proper incubation temperature. Lactobacillus is killed if exposed to temperatures over 55°C (130°F), and does not grow well below 37°C (98°F). We will incubate at 50°C, a temperature on the high side of its preferred growth temperature (122°F), a temperature which inhibits the growth of pathogenic bacteria. (Note that many recipes call for cooler temperatures than this. We find the results less dependable when incubation temperatures are lower.)
protection of the starter from contamination. Do not open the starter (either Dannon Plain yogurt, or 8 oz starter from the previous yogurt batch) until you are ready to make the next batch.
"Yogurt is preserved by its acidity which inhibits the growth of putrefactive or pathogenic bacteria. With lids intact, this yogurt will keep at least a month or two in the refrigerator. After that time, especially if your refrigerator is on the 'warm' side, a layer of non-pathogenic white mold may form on the top. Merely lift off the mold with a fork, discard, and use the yogurt for cooking." (Fankhauser, 2000)
In this experiment you will investigate fermentation by making your own yogurt. You will try different yogurt products as starter cultures to test which factors are important to the fermentation process. You will also learn how to culture (grow) microorganisms, and how to use sterile techniques so that you won't contaminate your cultures.
Terms, Concepts, and Questions to Start Background Research
To do this type of experiment you should know what the following terms mean. Have an adult help you search the internet, or take you to your local library to find out more!
How is yogurt made?
Do the cultures need to be live and active?
Do you need pasteurized or unpasteurized yogurt as a starter culture?
Willenberg, B. J.; Vollmar Hughes, K.; and Konstant, L. 1999. "Making Yogurt at Home: Country Living Series," Department of Food Science and Human Nutrition, University of Missouri-Columbia. [accessed June 23, 2006] http://chetday.com/howtomakeyogurt.htm
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Heat the milk to 85–90oC (185–195oF) in a double boiler, keeping the pot covered to reduce evaporation. You can also use a heavy-bottomed pot, but stir frequently to prevent the milk from sticking to the bottom.
Remove from stove and place the covered pot in a pan of clean, cool water until the stirred milk is very close to 55oC (130oF).
While the milk is cooling, prepare your yogurt starters.
Arrange several pre-sterilized canning jars (one for each starter) and loosen the lids.
Open each yogurt product you will use as a starter and stir with a clean fork to be sure it is mixed evenly.
Add one tablespoon of each starter to each jar. Label the jar with the brand name using a permanent marker.
After the milk cools to 55oC (130oF), it will still be warm to the touch. Carefully pour it into the canning jars filling the jars almost full, with about 1/2 inch from the top. Cover immediately with the sterile lids.
Place filled bottles in cooler, add enough 50oC (122oF) water so that bottles are surrounded, but the water is well below the lid rims.
Do not disturb the yogurt and it will be finished in 3 hours, provided the temperature does not drop below 40oC (104oF). Refrigerate overnight.
The next day open and examine each yogurt. Did the product gel? Is it firm or runny? Does it smell good or bad?
Write your observations in a data table:
Does it have active cultures? (Y/N)
Did it gel? (Y/N)
Observations of smell, taste, color, etc.
Evaluate your results. Which starter makes the best yogurt? Should you use pasteurized or unpasteurized? What happens if your starter does not have active cultures?
You can also test the effects of flavorings on yogurt cultures. Will 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. Is it better to use more or less? Does it take a longer or shorter time?
You can test which kind of milk works best. Try using whole, 2%, and skim milk. Also try using alternative products to cows milk: can you use goat milk, Lactaid, or soy milk?
Food Science Technician
Good taste, texture, quality, and safety are all very important in the food industry. Food science technicians test and catalog the physical and chemical properties of food to help ensure these aspects.
Food Scientist or Technologist
There is a fraction of the world's population that doesn't have enough to eat or doesn't have access to food that is nutritionally rich. Food scientists or technologists work to find new sources of food that have the right nutrition levels and that are safe for human consumption. In fact, our nation's food supply depends on food scientists and technologists that test and develop foods that meet and exceed government food safety standards. If you are interested in combining biology, chemistry, and the knowledge that you are helping people, then a career as a food scientist or technologist could be a great choice for you!
Microorganisms (bacteria, viruses, algae, and fungi) are the most common life-forms on Earth. They help us digest nutrients; make foods like yogurt, bread, and olives; and create antibiotics. Some microbes also cause diseases. Microbiologists study the growth, structure, development, and general characteristics of microorganisms to promote health, industry, and a basic understanding of cellular functions.
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