Class: BE210
Group: W5
Members:
FACILITATOR………………………..J. Cory Aceto
TIME & TASK KEEPER……………..Patrick Antkowiak
SCRIBE………………………………..Jimmy Sastra
PRESENTER………………………….Lillian Wang
Date: May, 2002
Abstract:
The effect of pH of the extracellular growth medium on the growth rate
of Saccharomyces cerevisiae (Bakers’ yeast) was explored in this project.
The yeast was grown in an aerobic batch culture with Difco Bacto YPD Broth
as the growth medium at a temperature of 37°C. The pH of the
extracellular medium was regulated using a manual “pH pump” by the addition
of 1M HCl or NaOH. Six targeted pH values ranging from 3-8 were chosen
and the extracellular medium was kept at that targeted pH value + 0.1 pH
units. Growth rate constants for each targeted pH value were determined
to be 0.00602 min-1 (pH =3), 0.00251 min-1 (pH =4), 0.00298 min-1 (pH =
5), 0.00228 min-1 (pH=6), 0.00246 min-1 (pH=7) and 0.002652 min-1 (pH =8).
Statistical analysis using t-tests on the linear regression for growth
rate constants vs. pH shows no significant difference between the data
points, indicating that extracellular pH has no effect on the growth rate
of the yeast cells. This conclusion is also strengthened by the 95%
confidence intervals for pH levels of 4, 5, 6, 7, and 8 that overlap with
each other. Interesting phenomena occurred at the pH extremes.
At the pH of 3, the growth rate constant was twice as high as the other
pH levels and a significant drop in absorbance occurred upon the addition
of HCl that could not be attributed solely to the dilution factor.
At the pH of 8, absorbance values showed a sequential decrease after reaching
a peak, indicative of the death phase.