I finally got around to conducting two
experiments which I wanted to do for a while now: the titration of
wort and beer.
Titration is a process in which
increasing amounts of a strong acid and/or base are added to a sample
while the pH of the sample is monitored. This gives an indication of
the pH buffer capacity of the sample at various pH points. It can
also be plotted as a nice graph, a so called titration curve. More
about the basics of pH and titration can be found here : An Overview
of pH.
The set-up of the experiment was as
simple as this: A diluted solution (~0.64%) of hydrochloric acid was
prepared by mixing 37% Muriatic Acid with water. I don’t like working
with the concentrated form of this acid and the amounts of acid
needed tend to be so small that they would be difficult to measure if
the undiluted acid is used. In addition to that, a dilute solution
(1.25%) of sodium hydroxide (NaOH), which is a strong base, was also
prepared.
The wort, and later beer, sample was
weighed and placed into a glass cup along with a stir bar. The pH
meter probe was affixed to the glass cup using masking tape. (Masking
tape seems to be a very useful tool in my brewery). While being
stirred on a stir plate the pH was constantly measured. To determine
the amount of acid/base that has been added a small cup was filled
with the acid/base solution and placed onto a digital scale which
provided a resolution of 0.01 g. The scale was zeroed while the cup,
titration solution and a pipette was on the scale. By doing so the
total amount of titration solution removed from the scale would be
shown as a negative weight.
The wort sample had an extract content
of 12.7 Plato.
The initial pH was measured and
recorded. After that a small amount of acid was added and once the pH
reading stabilized this reading and the amount of acid added so far
was recorded as well. The process was repeated until the sample
reached a pH of less than 2.0.
Titration of a fresh sample using a the
strong base (NaOH) and acid/base titration of a beer sample followed.
The beer sample had an original gravity of 12.0 Plato and an apparent
extract of ~3.0 Plato. The beer and wort samples were from different
batches.
Using a spread sheet acid and base
additions were converted to milliequivalents of acid/base per extract
weight in the case of the wort sample and per original extract weight
in the case of the beer sample. This was seen as a suitable approach
since the pH characteristics of the samples are largely determined by
the dissolved substances.
Using this data the added amounts of
acid (negative mEq/kg) or base (positive mEq/kg) were plotted over
the pH of the samples which resulted in the following chart (click for larger version):
As expected the wort sample had a
higher initial pH (5.18) than the beer sample (4.62). But what is
also apparent is that around that pH the beer sample has a higher
buffer capacity than the wort sample. Buffer capacity is the amount
of acid that is needed to change the pH by a given amount. The unit I
like to use is mEq/(pH*kg) which is milliequivalents of acid/base for
each pH shift of 1 unit and for each kg of substrate. The latter
doesn’t count the water. For beer this buffer capacity was 92
mEq/(pH*kg) when adding the base and 76 mEq/(pH*kg) when adding the
acid. Ideally they should be the same but measurement errors could
have lead to this difference.
In the case of the wort sample,
however, there was a distinct difference between the buffer capacity
when adding an acid (29 mEq/(pH*kg)) and when adding a base (64
mEq/(pH*kg)). I don’t know how to explain this and this is not the
first time I noticed this discrepancy. It also appeared to me during
my mash pH experiments. In order to double check my titration
solutions I calculated how much of the NaoH solution I would have to
add to a sample of the HCl solution to neutralize all the acid. I
then performed that experiment and found that the actual amount I had
to add was within 1.5% of the expected amount. So my titration
solutions had their expected strength. At least in relation to each
other.
The beer sample also shows an area of
strong buffer capacity around pH 6.5 (where the curve is steepest
between the two flatter sections). It is possible that this is the
1st pKa of carbonic acid, which is at pH=6.4, since the
beer sample was slightly carbonated.
While the beer and wort samples were
not from the same batch it is very likely that fermentation creates
additional pH buffers which are the cause of stronger buffer capacity
of beer.
Conclusion
This experiment does little when it
comes to finding ways to make better beer, but it gives insight into
the pH buffer characteristics of both beer and wort. One thing to
take away is that around the normal pH for wort and beer there is a
nearly linear relationship between the amounts of acid/base added and
the pH shift. This means that if the addition of X amount of acid
drops the pH by Y the addition of 2X acid will also drop the pH by
2Y. Within the pH range that is practical in brewing there won’t be a
case where the addition of a little bit more acid causes the pH to
suddenly “fall off a cliff”.
