An investigation to look at the affect of changing temperature on the speed of fermentation of yeast.

AS and A Level Science

An investigation to look at the affect of changing temperature on the speed of fermentation of yeast

Introduction

Yeast is a microbe that can respire anaerobically (without oxygen). The reaction for this experiment is:

Glucose (with yeast) = ethanol + carbon dioxide

C6HL2O6 = 2C2H5OH + 2CO2

Preliminary Plan

We set up the equipment as below:

We used the temperatures 10°C, 20°C, 30°C, 40°C, 50°C, 60°C and 70°C. We got the yeast to the right temperature by either melting ice in it to cool it down or putting it in a water bath to heat it up. We then placed three spatulas of glucose (sugar) into the beaker with the yeast. We then put the tube into the beaker to collect the carbon dioxide. We placed this tube into the water bath with the measuring cylinder full of water, upside down over the top of it. Then as the carbon dioxide filled the measuring cylinder the water level decreased. We timed this for 3 minutes with a stopwatch then read the amount of carbon dioxide that had been produced and recorded it. We did this seven times for each of the temperatures. When we examined our results we could see that they weren’t very accurate. This was because of the markings on the measuring cylinder. We then decided to use a new method.

Revised plan

In this revised plan we decided to use more accurate equipment. This is a diagram of the equipment we used:

We used the same temperatures of yeast as in the preliminary plan. We obtained these the same way also. We started with the yeast (at the right temperature) in a conical flask. We then added the three spatulas of glucose (sugar) and put the bung in the top. The bung had a thermometer and a rubber tube through it. We connected the rubber tube to the gas syringe and timed the experiment for 5 minutes with a stopwatch. We then read the amount of carbon dioxide that had been produced and recorded it. We did this with each temperature. We did two runs with each temperature so we could get an average. To make the experiment fair we made sure with each temperature we used the same amount of sugar and timed it for 5 minutes exactly. We made the experiment safe by wearing goggles to protect our eyes.

Prediction

This is a sketch of my predicted graph:

I researched this in biology by Ann Fullick. At 10°C there is very slow fermentation. This is because the glucose molecules haven’t got very much kinetic energy and so are moving extremely slowly leading to a small amount of Carbon dioxide being made. As the temperature begins to increase the amount of carbon dioxide increases also. This is due to the lock and key mechanism. ...

This is a preview of the whole essay

Prediction

This is a sketch of my predicted graph:

An enzyme is a globular protein (twisted on itself), which speeds up biological reactions. In order to react the glucose molecules need enough energy, known as the activation energy. Increasing the temperature increases the numbers of glucose molecules that have sufficient energy to react. Enzymes lower the activation energy needed for the reaction to occur. The diagram below is from Biology by Ann Fullick. It shows the difference in activation energy between a catalysed and an uncatalysed reaction.

Energy diagram

In the yeast enzyme there is an active site. This has a specific shape especially for use in fermentation. Only a glucose molecule is the right shape to be a substrate for the yeast enzyme active site. When the glucose molecule has enough kinetic energy it slots into the yeast enzymes active site (key fitting into lock). The reaction has then been catalysed and the products can’t stay in the active site so they are released. These products are ethanol and carbon dioxide.

Studying my predicted graph the optimum temperature for the reaction is 40°C because it is the activation energy. The glucose molecules have enough kinetic energy to collide and lock onto the yeast enzymes active sites.

After 40°C some of the enzymes begin to denature (change shape) but not all at once. When they denature the glucose molecules can’t lock onto the active sites anymore.

At around 70°C all of the enzymes become denatured and the active sites have changed shape so no glucose molecules can lock on therefore there is no fermentation.

Results table

Conclusion

My graph shows that the results of my experiment are what I predicted. The graph is similar to my predicted graph. Looking at zone (a) I can see that at 10°C there isn’t much carbon dioxide being produced. This shows that the fermentation is very slow. This is due to the glucose molecules having very little kinetic energy. The molecules are moving very slowly therefore there are few collisions and not many of the molecules lock onto the yeast enzymes active sites.

At zone (b) as the temperature begins to increase so does the amount of carbon dioxide. This means the amount of fermentation is increasing as the temperature does. This is because as the temperature increases the glucose molecules gain more energy and can collide and lock on to more yeast enzyme active sites.

Zone (c) is the optimum temperature, which is 40°C. This is the maximum yeast fermentation. This is due to the glucose having the perfect amount of kinetic energy to collide and lock onto the enzymes active sites. This is its activation energy.

Zone (d) shows that after 40°C denaturing begins. The graph shows us this because as the temperature increases past 40 °C the carbon dioxide production decreases therefore the amount of yeast fermentation is also decreasing. The scientific reasoning for this is that after 40°C some of the enzymes denature (change shape). The glucose molecules cannot lock onto these active sites that have denatured. They don’t all denature at the same time but around 50°C the process begins creating less active sites for the glucose to lock onto.

Zone (e) shows us that at 70°C the fermentation has stopped. We can know this because there is no carbon dioxide being produced anymore. Fermentation has stopped because all of the enzymes active sites have denatured and therefore none of the glucose molecules can lock onto them so there is no fermentation.

Evaluation

I was generally happy with my results. They followed the pattern of my predicted results. There were two anomalous results. These were 20°C and 60°C. For my results to gain in reliability I would need to find out what factors could have affected my final results and then remedy them. I don’t think the yeast was kept at the exact temperature all the way through the experiment. Once we had gotten the yeast to the temperature we wanted we took it out of the water bath and began the experiment. While we did the experiment we didn’t keep the temperature the same, it probably started to go to room temperature. This would have affected my results and could explain my anomalous ones. To prevent this from happening, while doing the experiment we should keep the yeast in the water bath and make sure it remains at the desired temperature. Another factor that could have made our results unreliable is the sugar. We added three spatulas of sugar but they weren’t at an equal level. To make sure this doesn’t affect our experiment if we did it again I would make sure that each spatula of sugar was either flat or the same level. Another improvement that I could make to my method is to seal the rubber tube and thermometer better into the bung. I felt that in the experiment carbon dioxide could have leaked out of the bung and therefore caused anomalous results.