Biology
Name:
Preparation of Yogurt Lab
Introduction: The production of many dairy products, such as yogurt, is
dependent upon the fermentative action of microorganisms on the milk
sugar, lactose. In the production of yogurt, two specific bacteria are
extremely important, Streptococcus thermophilus and Lactobacillus
bulgaricus. Freshly prepared yogurt contains 109 bacteria per gram.
Purpose: Observe the fermentative action of microorganisms in the production of
yogurt and then compare the properties of yogurt made in the lab to commercial
yogurt.
Background Questions: Answer the following questions in complete sentences. Use
the sentence starters to answer each of the questions.
1. Write out the equation for cellular respiration.
The equation for cellular respiration is….
2. Differentiate between aerobic and anaerobic respiration?
Aerobic respiration is different from anaerobic respiration because…
3. Explain the fate of pyruvate in anaerobic respiration.
During aerobic respiration pyruvate…
Procedure:
Day 1:
1) Pour 100 mL of skim or low fat milk into the 400 mL beaker.
2) Using a hot plate bring the milk to the boiling point, being careful not to boil
the milk. Allow the milk to cool to 60°C.
3) Weigh out 3.5 grams of the nonfat dry milk powder and add to the milk. Stir
vigorously with a glass rod to dissolve the powder. Allow the mixture to cool
to 45°C.
4) Add 1-2 tsp of the commercial yogurt. Mix well.
5) Cover the beaker with aluminum foil and incubate at least 6 hours at 45°C.
The mixture should become firm.
Day 2:
1) Measure the pH of your yogurt and of the commercial yogurt using a stirring
rod and pH paper. Record the results in your data table.
2) Observe both your yogurt and the commercial yogurt. Record your
observations in your data table.
Data Table
Characteristics of Yogurt
Type of Yogurt
pH
Yogurt made in lab
Commercial yogurt
General Observations
Questions for Discussion: Answer the following questions using data
(observations), make sure all of your answers are complete thoughts!
1. How does the pH of your yogurt compare with the pH of the commercial yogurt?
The pH of my yogurt...
2. How does your yogurt look compared to commercially prepared yogurt? What
might account for any similarities and/or differences?
Compared to commercially prepared yogurt, my yogurt...
3. Describe 3 other food products that are made with the aid of bacteria or yeast.
(You might have to do a little research to answer this.)
Three other food products made with the aid of bacteria or yeast are...
Conclusion: Write as one paragraph, use observations from your lab to complete the
conclusion. Details….details….and more details.
1. Restate the purpose
2. Did you meet the purpose… explain.
3. What are two things you learned?