GCSE Science/Carbon dioxide production of yeast as affected by temperature

Introduction

The purpose of this experiment is to discover the effect of temperature upon the rising of bread dough. The process of dough rising is based on yeast. Yeast is a type of fungus which breaks sugar down into smaller components. The type of yeast used for these tests is Saccharomyces cerevisiae, which was domesticated for wine, beer and bread production thousands of years ago. S. cerevisiae is commonly referred to as baker’s yeast or brewer’s yeast for this reason.

Yeasts used for leavening bread can be either caught from the environment or produced commercially. In the environment, yeast can easily be found in fruits and berries (such as apples, peaches, and grapes), as well as in plant exudates (such as sap).

When mixed with bread dough, the yeast converts sugar molecules into carbon dioxide (CO₂), alcohol and water. The CO₂ expands in the dough to produce gaseous bubbles. These bubbles cause the bread to rise - the more CO₂ present, the faster the bread will rise. If temperature is related to the amount of CO₂ produced by yeast, then higher temperatures will result in increased production of CO₂.

Procedure

Materials:

• 1 Erlenmeyer Flask
• 1 One-hole Rubber Stopper
• 1 hir
• 1 Striker
• 1 Glass Beaker of any size (large enough to contain flask)
• 1 Thermometer
• Yeast Solution
• Petroleum Jelly
• ice
• Glass Tubing
• Hot Plate
• Table for the recording of data (Figure 2)
• Access to tap water and natural gas will also be necessary.

1. Using the bunsen burner, bend the glass tubing so that it will reach from the inside of the Erlenmeyer flask to the bottom of the glass beaker, leaving enough room for gas to escape while in the beaker. (See Figure 1)

2. Add 75 ml of yeast solution to the flask.

3. Add 500 ml of water to the beaker.

4. Place the rubber stopper in the flask and apply petroleum jelly around the hole of the stopper to stop gas from escaping. Insert the glass tubing so that it will receive gas from the flask and carry it underneath the water in the beaker, and place the beaker and flask on a lab counter. (See Figure 3)

5. Observe the beaker for ten minutes and count the number of bubbles escaping into the water. Record this information and the current room temperature in a data table. (See Figure 2)

6. Find the average number of bubbles per minute and record this in the data table.

7. Remove the glass tubing from the apparatus.

8. Repeat steps 2-7 at a temperature of 83 degrees Celsius, placing the flask in a hot water bath prepared using the hot plate and waiting for five minutes to allow the yeast to reach the appropriate temperature before inserting the glass tubing. (See Fig. 4)

9. Repeat steps 2-7 at a temperature of 0 degrees Celsius, placing the flask in a container full of snow or a cold water bath and waiting for five minutes to allow the yeast to reach the appropriate temperature before inserting the glass tubing. (See Fig. 5)

10. Disconnect the materials, clean them and replace them in their proper locations.

Conclusions

The data collected (as seen in Figure 10) shows an average of 0 carbon dioxide bubbles per minute at 0 degrees Celsius, an average of 11.5 bubbles per minute at 19 degrees Celsius, and an average of 76.8 bubbles per minute at 83 degrees Celsius.

The test at 83 deg. C showed a near-constant decline in the amount of bubbles as time passed. This mostly occurred because enzymes denatured and could no longer catalyse the reaction. The high number of bubbles at the beginning is probably due to the rapid expansion of gas at high temperatures and does not actually represent carbon dioxide (See Fig. 6). The test at 0 degrees Celsius showed no production of carbon dioxide, presumably because the low temperatures froze the yeast. (See Fig. 7). The room temperature test (19 deg. C) showed a fairly even level of carbon dioxide production, but not enough to be useful in the making of leavened bread. (See Fig. 8)

We incurred two experimental errors in the first testing at 0 degrees Celsius. By beginning the testing immediately following the insertion of the yeast solution into the cold water bath, the warm air molecules in the flask were not given an opportunity to cool before being attached to the rest of the apparatus. This caused a vacuum to form in the glass tubing, pulling water up the tubing rather than pushing the gas out. Attempting to remedy the situation, we moved the flask into a slightly warmer area, skewing the results by raising the temperature enough to allow carbon dioxide production. Applying this knowledge to the second test eliminated these anomalies.

This experiment shows that carbon dioxide production from yeast increases with increased temperature, as the number of carbon dioxide bubbles increased as the temperature rose. (See Fig. 9). Then as temperatures rose too high the enzymes denatured and could not catalyse the reaction. Because the rising of bread dough is dependent on the amount of carbon dioxide produced, increased temperatures slightly will result in bread rising farther than it would at room temperatures but once at 83 degrees Celsius the yeast will die. It can also be concluded that bread will not rise at freezing temperatures, as the carbon dioxide necessary to make the bread do so is not present.

Bibliography

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Wikipedia contributors. Yeast [Internet]. Wikipedia, the free encyclopedia; 2004 Dec 1, 20:21 UTC [cited 2004 Dec 11]. Available from: http://en.wikipedia.org/wiki/Yeast

Filson, Richard. In Search of Real Science [Internet]. Access Excellence (from the National Health Museum); [cited 2004 Dec 12]. Available from: http://www.accessexcellence.org/LC/TL/filson/.

Elizabeth Botham & Sons LTD. Yeast [Internet]. Botham's of Whitby; [cited 2004 Dec 15]. Available from http://www.botham.co.uk/bread/yeast.htm