Ph and Fermentation
Location: Outside U.S.
Date: January 2009
I just completed an experiment testing how pH affects
fermentation of glucose by yeast cells in anaerobic conditions. The
three different pH values used were: 4, 7, and 9. The most efficient
fermentation occurred at pH 7. Is there a reason as to why the rate of
fermentation decreased as the carbohydrate became more acidic or
Remember that pH is a logrithmetric scale. So each change of one pH unit is a
factor of 10 in the concentration of hydrogen (or hydroxyl) ion. Going from 7
to 4 is an increase of 1000 in the hydrogen ion concentration. Similarly,
going from 7 to 9 is a factor of 100 decrease in the hydrogen ion
concentration. These are not small changes!!
Most microbes thrive at near - neutral pH (i.e. 7). In addition, because
microbes can absorb or give off, hydrogen ions, it is necessary to supply
a salt buffer to maintain a relatively constant pH. This is too involved to
go into detail here.
I suspect there are websites that can provide growth media at less drastic
pH differences that will let you formulate a kinder environment for your
bacteria to thrive. One of the common methods of disinfection is to rapidly
alter the pH of the microbes. That is what you are observing.
When you change the pH of a growth medium, you are, by definition, changing
the Hydrogn ion concnetration of the medium. The Hydrogen ion concentration
of a pH 4 solution is 1000 the concentration of a pH 7 solution. The shape
of proteins and enzymes is affected by the Hydrogen ion concentration because
Hydrogen ions can break the bonds that maintain the tertiary (folded up)
configuration of the enzymes. The bonds affected are primarily Hydrogen bonds
and ionic bonds between oppositely charged amino acids. When you change the
shape of an enzyme it catalyzes less efficiently, and this catalysis is what
causes all metabolic reactions to occur.
Ron Baker, Ph.D.
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Update: June 2012