"Yeast Beasts In Action" Investigation

Problem: How do certain compounds (a compound that is primarly acidic, a compound that is primarily basic, and a compound that is primarily neutral) affect the pressure of yeast activity?

Hypothesis: The pressure will die down when exposed to the diet coke, since acidic compounds normally hinder "life" and growth, both of which are yeast's primary function. The yeast will be the most active when exposed to the peroxide, as previously proved in the Elephant Toothpaste/Monster Foam experiment. It will have a normal amount of pressure when exposed to the skim milk-- the neutral liquid.

Results: To summarize it simply, we put three different liquids-- Diet Coke, skim milk, and hydrogen peroxide-- in three different test tubes. We mixed yeast with water and then put a few drops of the solution into the Coke's test tube. We then capped the test tube with a rubber stopper that connected directly to a pressure sensor, which in turn was linked to the Logger Lite data collection program. We then recorded the data, and repeated the experiment with the skim milk, and then the hydrogen peroxide when we were finished with the skim milk.

The above images show our results for the second Coke experiment-- we had to redo it, since the cap popped completely off the first time due to pressure, causing a large "dent" in the graph. (This makes it obvious that my hypothesis was unsupported.) It popped somewhat off the second time, leaving a slightly smaller "dent" in the graph.

The next ones show the hydrogen peroxide's pressure over the course of two minutes. Contrary to my original prediction, hardly anything happened. The only possibility was that the yeast was mixed with water-- a single chemical added or subtracted can change a chemical reaction drastically!

These images show the skim milk, which unexpectedly rose more than the others.

So, the highest pressure, lowest pressure, and pressure raise (from beginning to highest point) for each substance was:

Coke: 110.07 and 98.81 kPa, with 11.26 kPa being its highest pressure raise (considering it only got a chance to raise 7.1 kPa again after the cap popped.)

Peroxide: 105.89 and 98.90 kPa, with 6.99 kPa being its entire pressure raise.

Skim milk: 118.34 and 98.73 kPa, with 19.61 being its entire pressure raise.

Conclusion: My hypothesis was completely unsupported. I expected the hydrogen peroxide to have the most pressure because yeast creates an "exploding" foam substance when exposed to it, but the water mixed in with the yeast may have stopped this reaction. As a result, the hydrogen peroxide had the lowest pressure raise (6.99), and the lowest high pressure point (105.89). I expected Coke-- with all of its acids-- to make the pressure die down, but its pressure did increase-- it was neither the highest of lowest of the three, with 11.26kPa, and with 110.07. (This was probably due to all of the sugar in the Coke.) Had the cap not popped, the pressure would probably have increased a bit higher, and probably would be around the same amount as the skim milk by the time the data had stopped collecting. The skim milk, which I expected to be neither the lowest or highest in increase and highest pressure point, ended up having both the highest pressure point and highest increase. The results would have been much different if our lab group had held the cap down more tightly during the Coke's data collection-- that is something we would definitely do different next time.