Food Production » Anaerobic Respiration Practical

What you'll learn this session

Study time: 30 minutes

The process of anaerobic respiration in yeast
How to set up a practical investigation of anaerobic respiration
Variables that affect the rate of fermentation
How to measure carbon dioxide production
Applications of fermentation in food production
How to analyse and interpret experimental results

Introduction to Anaerobic Respiration in Food Production

Anaerobic respiration is a vital process in food production, particularly in the making of bread, beer, wine and yoghurt. In this practical, we'll explore how yeast carries out anaerobic respiration (fermentation) and how we can measure the rate at which it occurs.

Key Definitions:

Anaerobic respiration: The release of energy from glucose without using oxygen.
Fermentation: The type of anaerobic respiration that occurs in yeast cells.
Yeast: A single-celled fungus used in baking and brewing.

🍞 Yeast in Food Production

Yeast has been used for thousands of years in food production. It converts sugars into carbon dioxide and ethanol during fermentation, which is essential for making bread rise and producing alcoholic drinks.

🧮 The Science Behind Fermentation

During fermentation, yeast breaks down glucose in the absence of oxygen, producing ethanol and carbon dioxide as waste products. This process releases energy that the yeast cells use to survive.

The Chemistry of Anaerobic Respiration

The chemical equation for anaerobic respiration in yeast is:

Glucose → Ethanol + Carbon Dioxide + Energy

C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂ + Energy (ATP)

This process is less efficient than aerobic respiration, producing only about 2 ATP molecules per glucose molecule compared to the 38 ATP produced during aerobic respiration. However, it allows organisms to survive in oxygen-poor environments.

Setting Up the Anaerobic Respiration Practical

Materials and Equipment

To investigate anaerobic respiration in yeast, you'll need:

Fresh yeast or dried yeast
Glucose solution (various concentrations)
Conical flasks
Delivery tubes
Water bath (to control temperature)
Thermometer
Gas syringe or inverted measuring cylinder
Stopwatch
pH meter or universal indicator

🔬 Experimental Setup

Place yeast and glucose solution in a conical flask. Attach a delivery tube connected to a gas syringe or inverted measuring cylinder to collect the CO₂ produced.

📊 Taking Measurements

Record the volume of CO₂ produced at regular time intervals (e.g., every minute for 10 minutes). This allows you to calculate the rate of fermentation.

📝 Recording Results

Create a table to record your results and plot a graph of CO₂ volume against time. The gradient of the line represents the rate of fermentation.

Variables Affecting Fermentation Rate

Several factors can affect how quickly yeast ferments sugars. Understanding these variables is crucial for controlling food production processes.

🌡 Temperature

Yeast works best at around 37°C. Below 10°C, fermentation is very slow. Above 50°C, the enzymes in yeast denature and fermentation stops. You can investigate this by placing your fermentation setup in water baths at different temperatures.

🥤 Substrate Concentration

The amount of sugar available affects fermentation rate. More glucose generally means faster fermentation, up to a point where the enzymes become saturated. Test this by using different concentrations of glucose solution.

💧 pH Level

Yeast enzymes work best at slightly acidic pH levels (around 4-6). You can test this by adjusting the pH of your glucose solution using buffer solutions.

🧿 Yeast Concentration

More yeast cells mean more enzymes to catalyse the reaction. Investigate by varying the amount of yeast added to your glucose solution.

Practical Investigation: Effect of Temperature on Fermentation Rate

Let's focus on a specific investigation to demonstrate how temperature affects the rate of fermentation in yeast.

Method

Prepare 5 conical flasks, each containing 5g of yeast and 50ml of 10% glucose solution.
Set up water baths at 10°C, 20°C, 30°C, 40°C and 50°C.
Place each flask in a different water bath and attach a delivery tube leading to a gas syringe.
Allow 5 minutes for the mixtures to reach the water bath temperature.
Start the stopwatch and record the volume of CO₂ produced every minute for 10 minutes.
Calculate the rate of CO₂ production for each temperature by finding the gradient of the line on your graph.

Case Study Focus: Bread Making

Bakers have used yeast fermentation for thousands of years. The CO₂ produced during fermentation creates bubbles in the dough, making it rise. The ethanol evaporates during baking. Professional bakers carefully control temperature to ensure optimal fermentation rates. Too cold and the bread won't rise properly; too hot and the yeast will die. This is why recipes often specify "warm water" for activating yeast – typically around 40-45°C.

Analysing Your Results

After conducting your experiment, you should analyse your results to draw meaningful conclusions.

Expected Results

You should observe that:

At low temperatures (10°C), fermentation is slow with minimal CO₂ production.
As temperature increases, the rate of CO₂ production increases.
The rate reaches its maximum around 37-40°C.
Above 40°C, the rate decreases rapidly.
At 50°C or above, very little or no CO₂ is produced as the enzymes have denatured.

💡 Interpreting the Graph

Your graph of CO₂ production over time should show a straight line for each temperature (if the rate is constant). The steeper the gradient, the faster the rate of fermentation. Plot a second graph of rate against temperature to see the classic enzyme activity curve.

⚠ Common Errors

Watch out for gas leaks in your apparatus, which can lead to underestimated CO₂ production. Also ensure your water baths maintain a constant temperature throughout the experiment. Remember that yeast concentration should be kept the same across all tests.

Applications in Food Production

Understanding anaerobic respiration in yeast has numerous applications in food production:

🍞 Bread Making

Yeast fermentation produces CO₂ that makes dough rise. The ethanol evaporates during baking, leaving the characteristic bread texture and flavour.

🍺 Brewing

Beer production relies on yeast fermenting sugars from grains to produce alcohol and CO₂, which creates carbonation.

🍷 Wine Making

Yeast ferments sugars in grape juice to produce wine. Different yeast strains and fermentation conditions create different flavours.

Industrial Applications

Commercial food producers carefully control fermentation conditions to ensure consistent products. In large-scale bread production, temperature, humidity and fermentation time are precisely monitored. Some bakeries use slow fermentation at cooler temperatures (4-10°C) over 24 hours to develop more complex flavours in sourdough bread. Similarly, craft breweries might ferment beer at specific temperatures to encourage particular flavour profiles from their yeast strains.

Conclusion and Key Points

Anaerobic respiration in yeast is a fascinating biological process with significant applications in food production. Through practical investigation, we can observe how various factors affect fermentation rates and apply this knowledge to real-world scenarios.

Remember these key points:

Yeast fermentation produces CO₂ and ethanol from glucose without using oxygen.
The rate of fermentation is affected by temperature, substrate concentration, pH and yeast concentration.
The optimal temperature for yeast fermentation is around 37°C.
Measuring CO₂ production is a reliable way to track fermentation rates.
This process is essential for producing bread, beer, wine and other fermented foods.

In your practical work, always consider fair testing by controlling variables carefully and remember to repeat your experiments to ensure reliable results.

🧠 Test Your Knowledge!