I finally completed an article than I promised when I wrote the Sep/Oct 2011 Zymurgy article on mash pH and beer color: Beer color to mash pH (v2.0). Don’t get too excited, though. This is very heavy on math and should merely be seen as a reference to how I arrived at the current SRM to mash pH formula I implemented in the most recent versions of Kaiser_water_calculator_US_units.xls and Kaiser_water_calculator.xls.

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Ever since I discovered that chalk, when simply added to water or mash, does not raise the pH as much as an equal amount will do when it is dissolved with CO2 (see these old bog posts: How much alkalinity does 1 ppm of CaCO3 (Chalk) really add? and Undissolved vs. dissolved chalk in the brewing water) I was wondering why.

Recently the discussion got some traction again and got more of A.J. deLange’s attention than before. He proposed that chalk dissolves only slowly at mash pH. Slow enough that the time it takes us to mash doesn’t dissolve all the chalk. In hindsight this looks like a logical explanation. It also breaks with the common (home) brewing wisdom that chalk added to water or mash will dissolve since the mash pH is low enough. And then there has been this “bug” in John Palmer’s water calculator which for a while has been seen as the only reference for mash pH calculations. His water spreadsheet assumes that chalk adds only half its alkalinity potential to the mash. While it may not have been intended it ended up being a close enough approximation of chalk’s behavior in the mash.

A few days ago I did a simple experiment. I rinsed another 500 ml of 7 Plato wort from the spent grain of a batch of IPA I was brewing. To 250 ml I added ~110 mg chalk and to the other I added ~130 mg baking soda. Based on mash titration experiments these salt additions should have been capable to raise the pH by more than 1. I was not really looking for quantitative results but for a qualitative measurement of the pH behavior when chalk or baking soda are added to wort.

After the salts were added I kept stirring and recorded the measured pH values over time. The resulting graph is shown below.

It is apparent that the pH change caused by baking soda is sudden and remains fairly constant over time while chalk causes a sudden initial rise and a further gradual rise that continues over 15-20 min. Even though the 110 mg chalk added about 1.57 mEq residual alkalinity and the 140 mg baking soda added about the same amount (1.55 mEq), baking soda caused 2x the pH change. This lines up with previous observations I made regarding undissolved chalk.

If chalk does not completely dissolve we also need to assume that it releases only part of its calcium to the mash. This has been corrected in the current version of my Kaiser_water_calculator.xls which assumes that undissolved chalk contributes only half its calcium and half its alkalinity.

I have evaluated the effect of calcium and magnesium on the mash pH before when I investigated the pH effects of various waters (The effect of brewing water and grist composition on the pH of the mash). This time I repeated these experiments but didn’t add the calcium and magnesium salts before dough-in but after dough-in. I was wondering if there is a difference or if I could repeat my observations.

The experiment set-up was fairly simple. 7 glasses were filled with 160 ml distilled water and heated in a ~75C water bath. 7 40 g samples of Rahr 2-row were weighed and milled separately with a ~0.75 mm mill gap resulting in a mash thickness of 4 l/kg. Strong calcium chloride and Magnesium sulfate solutions were prepared. The mash samples were doughed-in 3 min apart from each other. Each of the mashes had an initial mash pH of 63-64 C. 5 min after dough in different amounts of either the calcium brine or the magnesium brine were added. One mash remained unchanged. 15 min after the salt addition a sample of the mash was removed, cooled and its pH was recorded. Another sample was taken 60 min after the salt addition.

The results, along with data from previous mash experiments, are plotted in the chart below:

The first observation is that the distilled water pH for the Rahr 2-row is surprisingly low for a pale malt. I also observed this when I used this malt before. The earlieexperiment used pilsner malt which had a more typical distilled water mash pH of 5.7 and 5.8 respectively. Another observation is that magnesium is less effective than calcium in lowering the mash pH. A fact that is already known from the residual alkalinity equation where magnesium hardness is seen as half as effective in neutralizing alkalinity compared to calcium hardness.

As the calcium content increases the achieved pH drop gets smaller which suggests that the curve is approaching a saturation. However, this matters little to practical brewing since the amounts of calcium needed to drop the pH that low by far exceed the recommended amounts. At 42 mEq/kg, for example, the calcium content of the water in a 4 l/kg mash is already 212 mg/l. 50-150 mg/l is the recommended range for brewing water. In case of magnesium 40 mEq/kg mean ~110 mg/l magnesium in the mash water. This is way more than the magnesium levels commonly found in brewing water. Since magnesium is not as effective as calcium anyway it would not be a good choice for lowering the mash pH anyway. If the salts are only added to the mash water, their “flavor active” concentration can be spread over the total water volume used to brew that beer which will reduce the overall impact.

To put this in perspective ~2.1 g of gypsum (calcium sulfate) needs to be added for every kg of malt in order to drop the mash pH by 0.1 units. If we assume that for the average 12 Plato beer ~7.5 l water are needed for every kg of malt, this gypsum addition is equivalent to a water calcium increase by 65 mg/l and a sulfate increase of 155 mg/l.

For calcium chloride only 1.8 g are needed. The calcium content gets bumped by 65 mg/l (when spread over all the water even though the calcium is only added to the mash) and the chloride content gets bumped by 115 mg/l.

There is little change in between the 15 min and the 60 min pH measurement.

Continuing the titration experiments I got a chance to titrate some mash tonight. The titration procedure was the same as described in the previous post and I won’t repeat that here.

But here are the stats for the two mashes that were prepared. One for titration with HCl and the other for titration with NaOH:

The results are shown here. This time I followed convention and plotted pH over the amounts of titrant that was added:

This time the slopes leading up to the normal pH, which was 5.82, are pretty much the same. Based on A.J. deLange’s feedback the large discrepancy between these slopes in the wort titration experiment may have been the result of the fact that the normal pH for malt and wort is at the lower end of the pH where there is a fairly linear relationship between mEq/kg and pH

I assume a repeat of the wort titration experiment is in order.

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
1^st 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”.