Lesson to Grow
Yogurt & Microorganisms
Grade Level: 3-12
Yogurt is easy to make and a fun hands-on activity to learn
about helpful microorganisms in the food supply. It is also a
Essential Skills: 3, 9 low tech way to observe microorganisms in action without
having to use a high-powered microscope.
Science Standards: 3.1 - 8.1
Yogurt History: Cultured milk products have been eaten since 2,000 BC. Yogurt is thought
Time: 1 class period to have developed in Central Asia and was probably fermented spontaneously, likely by
bacteria residing inside goatskin bags. People discovered when milk was left in a warm place,
Materials: 1¾ cups of it thickened and developed a different, tart flavor. More importantly, yogurt kept longer than
powdered milk; 4 cups very fresh milk.
warm water (45-50 °C or
110-125° F); 1/3 cup plain What is Yogurt? Yogurt (i.e. yoghurt) is a dairy product produced by bacterial fermentation of
yogurt (use yogurt with active milk. Fermenting lactose produces lactic acid, which changes milk protein to give yogurt its
cultures and no additives); wire
thick texture and tangy flavor. Yogurt is produced using a culture of Lactobacillus bulgaricus
whisk; mixing bowl; 5-6 one
cup containers with lids, like
and Streptococcus thermophilus bacteria.
old yogurt containers, cooler
(to use as an incubator) Once the bacteria are added to milk, it starts to consume the milk sugars and begins
fermentation, much like yeast in bread. The benefit of having a fermented milk product
Vocabulary: is that so much acid is produced by these organisms that few other potentially harmful
Live and active cultures microorganisms can grow in yogurt’s acidic environment.
refers to the living organisms,
Lactobacillus bulgaricus and Directions:
1) Begin this lesson by reviewing all the instructions with students ahead of time and stressing
that convert pasteurized milk
to yogurt during fermentation. the importance of working efficiently so the yogurt will work correctly. Time and temperature
Milk is pasteurized before are major factors in making yogurt and keeping the micro organisms healthy. Important:
culturing to remove any Students should wash their hands before handling ingredients. Also, handle yogurt with care.
harmful bacteria. It will not thicken or separate if it is disturbed or bumped during the incubation period.
Lactose is a sugar found 2) Have students taste commercial plain yogurt and record what they taste. Is it sweet, bitter,
mostly in milk. Lactose makes
lumpy, smooth, etc. Later they will taste their own yogurt and compare.
up around 2 - 8% of milk (by
3) Break students into groups and provide a set of materials. In a deep mixing bowl students
Coagulation in yogurt making combine 1¾ cup powdered milk into 4 cups very warm water. Whisk ingredients until
happens when the inoculated dissolved.
milk acidifies the sugars
enough to cause thickening of 4) Next, add 1/3 cup plain yogurt (active cultures and no additives yogurt) and whisk until
the proteins, making yogurt.
most of the clumps are dissolved. Work quickly; students do not want the mixture to cool.
AITC Free Library Resources:
Dairy Kit - books, DVDs, posters
5) Pour the mixture into the cups, cover and place the containers in an insulated cooler (do
and activities not add ice) for 6-8 hours. The cooler serves as an incubator and keeps the mixture warm
longer than if it was sitting out at room temperature. During this incubation time the bacteria
“ A cup of low-fat yogurt will multiply, ingest the milk sugar (lactose), and thicken the milk turning the mixture into
contains 448 mg. of yogurt.
calcium - 34% of daily
requirements for school- 6) Ask students to hypothesize about what will happen if the mixture cools? Why do they
aged children.” think the temperature matters? Have students record findings.
http://AITC.oregonstate.edu . Oregon Agriculture in the Classroom Foundation . 541-737-1318