Dependence of Fermentation on Temperature

Introduction

All cellular processes, like fermentation, consist of an interconnected series of chemical reactions.  Temperature can influence the rate of chemical reactions by affecting how quickly the reactants move, and therefore, how often they collide with each other- which gives them opportunity to react.  However, if the temperature gets too high, enzymes or other proteins involved with cellular respiration can unfold, or denature, rendering them inactive. Therefore, it is very important for cells and organisms to regulate their internal temperature to ensure that cellular respiration and other chemical reactions can continue at the proper rate.  In this Activity, we will immerse yeast solutions in graduated cylinders in water baths of different temperatures, to try and identify the optimum temperature for maximum yeast fermentation.

Safety Precautions

• Use protection when handling hot glass and materials.
• Do not mix very hot water with very cold water in glass containers.
• Clean up any spilled liquids to prevent slipping.

Materials:

• Graduated cylinders
• Water
• Ice
• Thermometer
• Beakers
• Sugar substitute solution
• Yeast solution (one packet of yeast with 100 ml of water; can use from Activity 1)
• Hot plate
• Droppers or pipettes
• Medium-size, un-inflated balloons
• Cloth measuring tape or pieces of string and a ruler

For this activity, you will work in pairs.

Procedure

Step 1:   Start with four water baths.  You can use ice or hot plates to reach the temperatures you would like.  Select four temperatures and describe them and how you plan to use ice and hot plates or other techniques to achieve them and keep them constant during the trial, in your lab notebook.  Set up the four baths, and continue below while they equilibrate to their intended temperatures.

Step 2:  Set up a table with seven columns (the first for labels, the other six for times of 15, 30, 45, 60, 75, 90 min) and seven rows (the first for labels, and six for different temperatures).  Leave a little space to the right of the table.

Step 3:  Because inverting graduated cylinders (as we did in Activity 1) would be difficult while immersing them in water baths, we will instead measure CO₂ production by the yeast by allowing the CO₂ to inflate balloons (rather than measuring froth inside the cylinder), and measuring their diameter with calipers. The balloons need to be pre-stretched in a consistent fashion so that they will yield comparable results.  Discuss with your partner what that stretching protocol should be, and describe it in your lab notebook.  Should the same person do the stretching, or should it be shared?  Pre-stretch six balloons.

Step 4:  Use which ever yeast “food” was identified in Activity 1 as producing the most fermentation.   Use the same procedure as in Activity 1, of adding 15 ml of yeast solution and then 10 ml of the food solution to the 50ml graduated cylinder, but do not top the cylinder up with water (leave as is).  Fasten a balloon to the top of the cylinder, and stand it upright in the water bath.  Note the start time of each, down to the second.

Step 5:  Measure the balloon diameters at 15 min intervals and enter into your table, and also note the times at which the balloons a) first lift enough  to not be leaning against their own cylinder; b) lift above horizontal.  Enter these latter two,  lto the right of the table.

Step 6:  Graph the lines on the board in front of the room.  Which temperature had the highest rate of balloon growth?

Step 7:  Is it possible that the optimum temperature was one that you did not test?  What additional two temperatures would you test to further refine your estimate?

Discussion

How might you improve on the methodologies used, with the same materials or others accessible in the lab?