6 – To Investigate the Conditions Necessary for Fermentation (LC 2027) Revision Notes for Leaving Cert Biology

6 – To Investigate the Conditions Necessary for Fermentation

Introduction

Fermentation is a type of anaerobic respiration where yeast breaks down glucose to produce ethanol and carbon dioxide without using oxygen. This investigation helps you understand what conditions yeast needs to carry out fermentation effectively.

Setting up the fermentation experiment

Equipment needed

Conical flasks
Glucose solution
Dried yeast
Airlock (fermentation lock)
Water bath set to 30°C
Limewater
Oil layer
Boiled glucose solution

Step-by-step procedure

Preparing the solution:

Dissolve glucose in water to create your glucose solution
Boil the solution in a conical flask for 5 minutes - this removes dissolved gases and creates anaerobic conditions
Allow the solution to cool, then add dried yeast and swirl gently to dissolve

Creating anaerobic conditions: 4. Cover the liquid with a thin oil layer - this prevents oxygen from entering the solution and maintains anaerobic conditions 5. Set up the airlock system as shown in the diagram

chatImportant

The oil layer is crucial for creating anaerobic conditions. Without it, oxygen will enter the solution and yeast will perform aerobic respiration instead of fermentation, preventing ethanol production.

Monitoring the experiment: 6. The airlock allows carbon dioxide to escape while preventing oxygen from entering 7. Place the apparatus in a water bath at 30°C - this is the ideal temperature for maximum yeast respiration 8. Look for bubbles of carbon dioxide in the water or limewater, indicating active fermentation 9. The limewater will turn cloudy or milky if carbon dioxide is present, showing fermentation is occurring

Control experiments

To make your investigation scientifically valid, you need to set up control experiments:

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Why Control Experiments Are Essential

Control experiments prove that each component (yeast, sugar, anaerobic conditions) is necessary for fermentation. Without controls, you cannot scientifically prove what caused your results.

Control without yeast cells:

Use the same apparatus but don't add any yeast (or use boiled yeast which kills the enzymes)
No bubbles should form and the limewater should remain clear
This proves that yeast is necessary for fermentation

Control for sugar requirement:

Repeat the process but leave out the glucose (sugar)
Add the same amount of yeast to boiled and cooled water
This shows that sugar is necessary for fermentation

Control for anaerobic conditions:

Repeat the process but don't add oil and don't cover the sugar and yeast solution
This investigates what happens under aerobic conditions

Testing for ethanol production

The main product of fermentation (besides carbon dioxide) is ethanol. To test for ethanol:

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Worked Example: Testing for Ethanol

Step 1: Philtre the solution to remove yeast

Step 2: Place some filtrate in a test tube

Step 3: Add an equal volume of potassium iodide

Step 4: Add sodium hypochlorite solution (the colour turns brown-orange then becomes colourless)

Step 5: Place in a water bath at 50°C for 5 minutes

Step 6: Pale yellow crystals indicate ethanol is present

infoNote

The formation of pale yellow crystals is the positive test result that confirms ethanol has been produced during fermentation. This chemical test is specific for ethanol and will not give a positive result with other alcohols.

Investigating temperature effects on fermentation

To investigate how temperature affects fermentation rate:

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Worked Example: Temperature Investigation Setup

Step 1: Set up identical fermentation apparatus

Step 2: Use water baths at different temperatures (e.g. 10°C, 20°C, 30°C, 40°C, 50°C and 60°C)

Step 3: Count the bubbles produced per minute at each temperature

Step 4: Record results and plot a graph of temperature vs. bubble rate

Higher temperatures initially increase the rate, but very high temperatures (above 50°C) will denature the yeast enzymes and stop fermentation.

Understanding the science

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The Fermentation Process

Yeast contains enzymes that break down glucose in the absence of oxygen. The main products are ethanol and carbon dioxide. This process releases energy for the yeast cells to use, though fermentation is less efficient than aerobic respiration, producing much less energy per glucose molecule.

Why anaerobic conditions are essential:

In the presence of oxygen, yeast will carry out aerobic respiration instead
Aerobic respiration produces carbon dioxide and water (not ethanol)
The oil layer and airlock system ensure oxygen cannot enter the solution

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Aerobic vs Anaerobic Respiration in Yeast

When oxygen is present, yeast preferentially performs aerobic respiration because it produces much more energy (ATP) per glucose molecule. However, this produces water instead of the ethanol we want for fermentation studies.

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Key Points to Remember:

Fermentation requires four key conditions: glucose (sugar), yeast, warm temperature (around 30°C), and anaerobic (oxygen-free) conditions
The airlock system is crucial - it allows carbon dioxide to escape while preventing oxygen from entering
Temperature affects fermentation rate - too cold and enzymes work slowly, too hot and enzymes are denatured
Two main tests prove fermentation occurred: limewater turns cloudy (proves CO₂ production) and pale yellow crystals form with potassium iodide test (proves ethanol production)
Control experiments are essential to prove that yeast, sugar, and anaerobic conditions are all necessary for fermentation to occur

6 – To Investigate the Conditions Necessary for Fermentation (LC 2027) (Leaving Cert Biology): Revision Notes