Science Buddies: "Ask an Expert"

We conducted an experiment to test the effect of pH levels of water on yeast growth. Group 1 started with a pH of 7.0, and ended with a pH of 4.0 - Group 2 started with 4.5 and ended with 4.0 - Group 3 started with 3.5 and ended with 4.0!! How can that be? They all started with a temperature of 35 degrees Celsius - however, Group 1 and 2 had an ending temperature of 21 degrees, while Group 3 had a temperature of 23 degrees. The yeast species was Saccharomyces cerevisiae and it was undergoing anaerobic respiration as the test tubes were covered. Each group had the same amount of sugar and water.

Can someone explain how to pH increased for group 3? I read an article talking about how ammonium is produced from "protein destruction" and how that can increase pH, but why did it not for the others? If anyone has an idea it would be much appreciated!!

Hi and welcome to Scibuddies,

Unexpected results are always more interesting because they challenge us to figure out what happened and why.

I guess my first question would have to be how accurate are your pH determinations? How did you measure pH? If you used pH test strips, were they reliable? Did you do each measurement at least three times and average the result? What was the standard deviation of your data? These are all necessary things that a scientist needs to know in order to have confidence in the data. Speculating about a particular set of results is going to be a waste of time if the measurements are inaccurate. Half a pH unit from 3.5 to 4.0 may be hard to determine accurately without using a pH meter.

Anaerobic fermentation by yeast will produce acid but fermentation stops at a pH of about 4. Did you measure carbon dioxide production? That might be a better indicator of yeast activity. How long did you allow the yeast to work before you measured the pH? How did you get the water to a starting pH of 4.5 or 3.5 to begin with?

If you explain a little more about your experiment, maybe we can help you understand the results better. Post again with more details.

Sybee