Investigation of the Role of Yeast and Bacteria in the Fermentation of Flour and Milk

Abstract

This Grade 10 Biology practical activity explores the biological process of fermentation using yeast and lactic acid bacteria. Students observe microbial transformation in flour and milk, gaining insights into its importance in food preservation and processing.

Introduction

Fermentation is a metabolic process where microorganisms convert sugars into gases, acids, or alcohol. In this experiment:

• Yeast is used to ferment flour in the bread-making process.
• Lactic acid bacteria are used to ferment milk into curd or yogurt.

This experiment aligns with the Federal Board of Intermediate and Secondary Education (FBISE) standards for Grade 10 Biology.

Procedure

1. Take two clean jars: one with a flour-water mixture and the other with milk.
2. Add yeast to the flour mixture and lactic acid bacteria (from yogurt) to the milk.
3. Cover both jars with a cloth and leave them at room temperature for 24–48 hours.
4. Observe the smell, texture, and formation of bubbles or curd.

Observations and Conclusion

Flour + Yeast: The dough rises, smells sour or alcoholic, and forms bubbles due to CO₂ production.

Milk + Bacteria: The milk thickens into curd with a sour smell due to lactic acid fermentation.

Conclusion: This experiment confirms that yeast and bacteria are essential in food fermentation, each transforming substrates (sugar/lactose) into useful byproducts.

20 Short Questions with Answers

1. What is fermentation?
   Answer: Conversion of sugars into gases, acids, or alcohol by microorganisms.
2. Name one microorganism involved in flour fermentation.
   Answer: Yeast.
3. Which bacteria are used in milk fermentation?
   Answer: Lactic acid bacteria.
4. What gas is produced by yeast?
   Answer: Carbon dioxide.
5. What is the sour product of milk called?
   Answer: Curd or yogurt.
6. Why does dough rise?
   Answer: Due to carbon dioxide produced by yeast.
7. Role of lactic acid bacteria?
   Answer: Converts lactose into lactic acid.
8. How long was the mixture left to ferment?
   Answer: 24–48 hours.
9. Smell of milk after fermentation?
   Answer: Sour.
10. Conditions for fermentation?
    Answer: Warmth, moisture, and anaerobic environment.
11. Is oxygen required?
    Answer: No, it's an anaerobic process.
12. Scientific name for curd?
    Answer: Coagulated milk or yogurt.
13. What sugar does yeast feed on?
    Answer: Glucose.
14. What sugar is fermented in milk?
    Answer: Lactose.
15. Type of process (chemical/physical)?
    Answer: Chemical.
16. Can fermentation occur in cold?
    Answer: It slows down; warmth is better.
17. Importance in food processing?
    Answer: Preserves food and enhances taste and nutrition.
18. Which vitamin may form during fermentation?
    Answer: Vitamin B12.
19. Does yeast ferment milk?
    Answer: No, bacteria do.
20. Texture of fermented milk?
    Answer: Thick and gel-like.

5 Multiple Choice Questions (MCQs)

1. What does yeast produce during fermentation of flour?
   Ⓐ Oxygen Ⓑ Carbon dioxide Ⓒ Hydrogen Ⓓ Nitrogen
   Answer: Ⓑ Carbon dioxide
2. What is formed when milk is fermented by bacteria?
   Ⓐ Yogurt Ⓑ Butter Ⓒ Water Ⓓ Vinegar
   Answer: Ⓐ Yogurt
3. Which of the following causes dough to rise?
   Ⓐ Alcohol Ⓑ Lactic acid Ⓒ Carbon dioxide Ⓓ Water
   Answer: Ⓒ Carbon dioxide
4. Which microorganism is used in fermenting milk?
   Ⓐ Yeast Ⓑ Lactic acid bacteria Ⓒ Fungi Ⓓ Amoeba
   Answer: Ⓑ Lactic acid bacteria
5. What is the byproduct of milk fermentation that makes it sour?
   Ⓐ Ethanol Ⓑ Acetic acid Ⓒ Lactic acid Ⓓ Citric acid
   Answer: Ⓒ Lactic acid

🔗 Explore more Grade 10 Biology Practicals and FBISE Model Papers.