Class 12 Biology - Microbes in Human Welfare

Activity 8.1: Curd Formation and Kinetic Analysis of LAB

Aim: To study the conversion of milk to curd by Lactic Acid Bacteria (LAB) and the effect of temperature on this process.

Materials Required:

Pasteurized Milk (500 mL)
Inoculum/Starter culture (Fresh curd containing Lactobacillus)
Thermometer (0°C - 100°C range)
pH paper or Digital pH meter
Beakers (250 mL)
Incubator or Water bath

Procedure:

Heat the milk to 60°C for 1 minute (to kill unwanted microbes), then cool it down to exactly 40°C.
Divide the milk into three beakers: A (stored at 5°C), B (stored at 40°C), and C (stored at 70°C).
Add 1 teaspoon (approx. 5 mL) of the starter culture to each beaker and stir gently.
Record the initial pH of the milk.
Incubate for 6 hours.
Measure the final pH and observe the consistency (coagulation).

Scientific Note LAB (Lactobacillus) produces Lactic Acid, which lowers the pH of the milk. This acidity causes the denaturation and coagulation of the milk protein Casein. Additionally, LAB improves the nutritional quality of curd by increasing the content of Vitamin $B_{12}$ .

Observation:

Beaker B (40°C): Successfully forms thick curd; pH drops from approx. 6.6 to 4.5.
Beaker A (5°C): Milk remains liquid; metabolic activity of LAB is inhibited by cold.
Beaker C (70°C): Milk remains liquid; LAB are killed by high temperature.

Lab Best Practice The "inoculation temperature" is critical. 40°C is the physiological optimum for Lactobacillus. If the milk is too hot (above 50°C), the starter culture will be thermally inactivated.

Activity 8.2: Fermentation of Dough using Yeast

Aim: To demonstrate the production of $CO_2$ during anaerobic respiration by Saccharomyces cerevisiae.

Materials Required:

Refined flour (Maida, 100g)
Baker's Yeast (Dry active yeast, 5g)
Sucrose (Sugar, 10g)
Warm water (35°C)
Measuring cylinder (250 mL)

Procedure:

Dissolve 10g of sugar in 50 mL of warm water. Add 5g of yeast and wait for 10 minutes until it froths.
Mix this yeast solution with 100g of flour to make a soft dough.
Place the dough at the bottom of a 250 mL measuring cylinder and record the initial volume.
Keep the cylinder in a warm place (30°C - 35°C) for 2 hours.
Record the final volume of the dough.

Scientific Note Yeast performs Ethanolic Fermentation under anaerobic conditions. The enzyme Zymase converts sugars into Ethanol and Carbon dioxide ( $CO_2$ ). The $CO_2$ gas gets trapped in the gluten network of the flour, causing the dough to rise (leavening).

Safety First Ensure the water used for yeast activation does not exceed 45°C. High temperatures will kill the yeast cells, leading to experimental failure.

Observation: The volume of the dough increases significantly (e.g., from 50 mL to 150 mL) and develops a characteristic pungent alcoholic smell.

Class 12 Biology - Microbes in Human Welfare

Activity 8.1: Curd Formation and Kinetic Analysis of LAB

Aim: To study the conversion of milk to curd by Lactic Acid Bacteria (LAB) and the effect of temperature on this process.

Materials Required:

Pasteurized Milk (500 mL)
Inoculum/Starter culture (Fresh curd containing Lactobacillus)
Thermometer (0°C - 100°C range)
pH paper or Digital pH meter
Beakers (250 mL)
Incubator or Water bath

Procedure:

Heat the milk to 60°C for 1 minute (to kill unwanted microbes), then cool it down to exactly 40°C.
Divide the milk into three beakers: A (stored at 5°C), B (stored at 40°C), and C (stored at 70°C).
Add 1 teaspoon (approx. 5 mL) of the starter culture to each beaker and stir gently.
Record the initial pH of the milk.
Incubate for 6 hours.
Measure the final pH and observe the consistency (coagulation).

Observation:

Beaker B (40°C): Successfully forms thick curd; pH drops from approx. 6.6 to 4.5.
Beaker A (5°C): Milk remains liquid; metabolic activity of LAB is inhibited by cold.
Beaker C (70°C): Milk remains liquid; LAB are killed by high temperature.

Activity 8.2: Fermentation of Dough using Yeast

Aim: To demonstrate the production of $CO_2$ during anaerobic respiration by Saccharomyces cerevisiae.

Materials Required:

Refined flour (Maida, 100g)
Baker's Yeast (Dry active yeast, 5g)
Sucrose (Sugar, 10g)
Warm water (35°C)
Measuring cylinder (250 mL)

Procedure:

Dissolve 10g of sugar in 50 mL of warm water. Add 5g of yeast and wait for 10 minutes until it froths.
Mix this yeast solution with 100g of flour to make a soft dough.
Place the dough at the bottom of a 250 mL measuring cylinder and record the initial volume.
Keep the cylinder in a warm place (30°C - 35°C) for 2 hours.
Record the final volume of the dough.

Safety First Ensure the water used for yeast activation does not exceed 45°C. High temperatures will kill the yeast cells, leading to experimental failure.

Observation: The volume of the dough increases significantly (e.g., from 50 mL to 150 mL) and develops a characteristic pungent alcoholic smell.

Class 12 Biology - Microbes in Human Welfare (Activities)

Class 12 Biology - Microbes in Human Welfare

Activity 8.1: Curd Formation and Kinetic Analysis of LAB

Activity 8.2: Fermentation of Dough using Yeast

On this page

Class 12 Biology - Microbes in Human Welfare (Activities)

Class 12 Biology - Microbes in Human Welfare

Activity 8.1: Curd Formation and Kinetic Analysis of LAB

Activity 8.2: Fermentation of Dough using Yeast

On this page