Conducting a pre mash PH check

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aarong

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I am having some issues hitting my PH. Using a calibrated PH meter and Bru'n water. I have my local water profile and use 50% distilled water. I am getting a reading of 5.5 and was shooting for 5.4 as per bru'n water. Reading through the book Water, it is mentioned that you can take a mini mash to check PH.

Has anyone conducted a mini mash before mashing in? I would think I would scale down my mash in to around a 1 qt mash at my mash temp(scale down malt, water, salts, acid malt). Then mash for 10 minutes checking my PH. Then adding the amount of LA needed to fix my mash PH. Then scaling LA back up to full scale.

I know it might be a waste of time... thoughts?
 
You have run into, what I believe to be, the source of nearly all the pH discrepancies in water chemistry calculators, namely the DI pH of the recipe's grains.

The acid content of different grains will vary from grain type to grain type and from maltster to maltster, even when they are producing the same type of grain. The acid content of a roasted malt can have a DI pH of 4.71 and the acid content of a pilsner malt can a much higher DI pH of 5.75. It is also true that 2-Row malt produced by Rahr can have a stated DI pH value of 5.56 while a 2-Row malt produced by Briess can have a DI pH value of 5.70. Some maltsters provide inaccurate information regarding the DI pH of their grain while other maltsters provide no information at all, which confuses the interpretation of DI pH values even further.

Determining the DI pH of any grain can be accomplished by crushing 40 grams of malt and then stirring in 100 milliliters of distilled or reverse osmosis water to produce a 1.2 qt/lb ratio mash. Allow the mash to reach equilibrium by letting it settle for at least 20 minutes. During this time the pH of the mash will change. The darker the malt is the higher its acid content and the lower the pH value will be. Conversely the lighter the malt is the lower its acid content and the higher the pH value will be.

Use a recently calibrated pH meter to take a reading of the test mash and then record the pH value as the DI pH value of the grain tested. Then plug that number into your favorite brewing water calculator and you will see the results that you expected.
 
It is not sufficient to know just the DI mash pH. You must also know the buffering capacity of the malt. The reason for this is that robust mash pH algorithms need to know the amount of acid given off by (or absorbed) by the various malts in transitioning from the DI mash pH (the intrinsic pH) of the malt to the mash pH. To hit a certain mash pH you must determine these acid (proton) surfeits and/or deficits and adjust the sum of all of them (including the deficit of the water) to 0. If a prediction is desired it is found by computing the sum of the deficits and surfeits at several values of pH. The predicted pH is the one that brings that sum to 0.

The proton deficit of a malt at pH is well expressed as a1*(pH - pHdi) + a2*(pH - pHdi)^2 + a3*(pH-pHdi)^3. For most malts a1 is insufficient to specify the malt's buffering (the buffering is a function of pH) but as it is not a strong function you probably don't need a3. If mash pH is near pHdi a1 should be sufficient. a1 averages around 40 but can be as low (in magnitude) as -30 mEq/kg•pH or as high as -90 (and even higher for sauermalz) thus we cant really do much with just the pHdi.

Determining the DI pH of any grain can be accomplished by crushing 40 grams of malt and then stirring in 100 milliliters of distilled or reverse osmosis water to produce a 1.2 qt/lb ratio mash. Allow the mash to reach equilibrium by letting it settle for at least 20 minutes. During this time the pH of the mash will change.

So far so good but we'd add the admonition to use water warmed to dough in temperature, to put the sample in a metal beaker and to put the beaker in a water bath. Stir frequently and measure pH at 5 minute intervals for at least 30 minutes.

NOW do the same thing again but add acid in the amount of say 5 mEq/kg. Repeat from NOW (with a new malt sample each time) until your 30 minute pH readings are about 4.

Do the same thing again but add acid in the amount of -5 mEq/kg (IOW add + 5 mEq/kg base) and repeat until mash pH's are above 6.

Now pick a time (say 30 minutes) and plot the data with pH as the independent variable. You will have a curve of mEq/kg vs pH. Use a curve fitting routine to find a1, a2 and a3 that minimize chi-squared summed over the 30 minute data. These go into the model

acid = a1*(pH - pHdi) + a2*(pH - pHdi)^2 + a3*(pH-pHdi)^3



The darker the malt is the higher its acid content and the lower the pH value will be. Conversely the lighter the malt is the lower its acid content and the higher the pH value will be.
It seems to be true that the darker the malt the more acidic but the DI mash pH of 150L caramel is 4.48 while that of 600L roast barley is 4.70. Thus is as good as example as any for illustrating that the buffering as well as the DI mash pH must be taken into account.

Use a recently calibrated pH meter to take a reading of the test mash and then record the pH value as the DI pH value of the grain tested. Then plug that number into your favorite brewing water calculator and you will see the results that you expected.
Plug in your measured DI pH and at least a1 (the linear buffering terms in the pH region near pHdi. Your program won't accept a1? Then it can't do a very good job of estimating mash pH unless it does a good job of guessing buffering from color or malt type.
 
AJ thank you for taking the time to provide your concise, detailed explanation and advice on how malt DI pH values should be derived. I honestly never even thought about the buffering capacity of the malt. I have read your reply twice now and bookmarked it as well for reading through again. Admittedly it takes several reads before I am able to absorb all of the information that you provide and then several more reads before I am able to actually able apply it.

As you pointed out in your reply, the water calculator that I referred to, allows for user entry of known DI pH values associated with the different grain types. Whereas the '40 grams of malt to 100 milliliters of water mash test' was the suggested method for obtaining the DI pH of the grain sample tested. I do not question your methodology or calculations, in fact your calculations require the use of a much sharper pencil point than do my own.

With that said, it will be much appreciated if it were possible for you to explain the differences in accuracy between the two approaches. When calculating pH measurement by entering the test mash DI pH of a grain. As opposed to entering the DI pH of a grain and using the linear buffering term a1 to calculate pH?
 
This is great information thank you! I was thinking of a mini mash of all the grains at once. I read that malt is a major factor and that alot of software doesn't take into account the different malts. Screwy and aj, how do you get your mash ph and conduct adjustments? The typical bru'n water software or do you use a similar method you mentioned to get a more accurate PH?
 
With that said, it will be much appreciated if it were possible for you to explain the differences in accuracy between the two approaches. When calculating pH measurement by entering the test mash DI pH of a grain. As opposed to entering the DI pH of a grain and using the linear buffering term a1 to calculate pH?

There aren't really two approaches. You must include the buffering in calculating the amount of acid required to reach a certain pH or to estimate what the pH of a mash will be. Suppose you have a small mash with 1 kg of some malt in it which malt has a DI pH of 5.68. How much acid do you have to add to that mash to get to pH 5.4? If the buffering of that grain is -41 mEq/kg•pH (Weyermann floor pils with pHdI = 5.62) then you will need -41*(5.4 - 5.62) = 9.02 mEq. OTOH if you have Rahr pils with a1 = -46.3 and pHDI = 5.8 you will need -46.3*(5.4 - 5.8) = 18.5 mEq. This assumes that the curve is linear for simplicity but the point is that the amount of acid required is dependent on both pHDI and buffering. You must have pHDI and that leaves the choice as to one of where to get the buffering information. The values for a1 in these examples were measured either by me or someone else. If you can't get measured data then you can just assume that a1 = 43 and not be too far off in either case. Thus the difference in accuracy between methods which take only pHDI as input and those which consider actual buffering depend on how close the simpler methods estimate of a1 is to the actual value of a1 for the malt in question.
 
There aren't really two approaches. You must include the buffering in calculating the amount of acid required to reach a certain pH or to estimate what the pH of a mash will be. Suppose you have a small mash with 1 kg of some malt in it which malt has a DI pH of 5.68. How much acid do you have to add to that mash to get to pH 5.4? If the buffering of that grain is -41 mEq/kg•pH (Weyermann floor pils with pHdI = 5.62) then you will need -41*(5.4 - 5.62) = 9.02 mEq. OTOH if you have Rahr pils with a1 = -46.3 and pHDI = 5.8 you will need -46.3*(5.4 - 5.8) = 18.5 mEq. This assumes that the curve is linear for simplicity but the point is that the amount of acid required is dependent on both pHDI and buffering. You must have pHDI and that leaves the choice as to one of where to get the buffering information. The values for a1 in these examples were measured either by me or someone else. If you can't get measured data then you can just assume that a1 = 43 and not be too far off in either case. Thus the difference in accuracy between methods which take only pHDI as input and those which consider actual buffering depend on how close the simpler methods estimate of a1 is to the actual value of a1 for the malt in question.

Thank you for the examples A.J. I am still at a loss as how to arrive at the a1 coefficient. I have been reading through your publication on the very subject "Some Observations on Mash pH Prediction/Control". Interestingly enough, and to my surprise on slide 25 Weyermann Pils malt was shown to have a DI pH value of 5.65.

The example on slide 27 'Example Malt Measurements', using the same Weyermann Pils malt, a graph shows the intersection point of 0 acid, and 0 buffering, with the pH value 5.485. I am still wrestling with Fit:mEq/kg = -37.503*(pH - 5.485) formula because it is 0.165 lower than the previously stated 5.65 pH value for Weyermann Pils malt. A pH value was not stated on slide 27, that I could find. On that graph a pH 5.485 test measurement intersects with the 0 acid added line and that I understand. How to get the coefficient for a1, it is the key to unlocking the value of the grains buffering.

Let's say that Weyermann Pils has a DIpH of 5.62 and I expect my mash pH to be 5.40. I have to find out how much acid is needed to lower the pH from 5.62 to 5.40, in doing so I will find out the buffering of the Weyermann Pils malt. Please bear with me as I stretch my mind around the following example.

"If the buffering of the Weyermann Pils is -41 mEq/kg•pH (with pHdI = 5.62) then -41*(5.4 - 5.62) = 9.02 mEq.". Here I do not know how to arrive at -41. When subtracting DIpH from pH, or 5.4 - 5.62 = -0.22 it seems easy enough, but when multiplying -41 * -0.22 I get -41.22 which doesn't seem to have a thing to do with the 9.02 mEq answer in the example.
 
That set of slides is a good place to be looking for fuller understanding of all this.

There are two things going on here that I have glossed over in the previous discussions here. First is that the actual pH measurements are made at 50°C or thereabouts. In brewing and brew planning we use pH at room temperature. Thus the DI mash pH we measured at near 50° is shifted to 20 ° as are all pH values from the titration curve. At the time I did those slides I assumed the temperature glide was 0.0055 pH/°C. Since then I have become aware that this is not a constant but another property of each malt which needs to be considered separately for each malt in a grist. Note that this assumed shifting of the pH assumes that all pH's shift by the same amount so that the curve simply shifts to the right (higher pH) at room temperature relative to where it was measured at near 50 °C. The curves on Slide 28 illustrate this.

The other factor is time. It takes quite a lot of time for pH to settle. This is why I suggest taking readings every 5 minutes for at least half an hour for each titration mash. Even then the system is probably not at equilibrium but it nearly there. Were this method to ever be standardized as a way of specifying malts one of the questions that would have to be answered is how long we would wait before taking the pH measurements that go into the titration curves.

Weyermann Pils measured pHDI = 5.483 after 25 minutes (the third digit to the right of the decimal place is a consequence of he fact that several pH readings are averaged) at 45.4 °C. Translating to room temperature ww get 5.622. Why it says 5.65 on Slide 25 I do not know. A typo I probably. The first coefficient in the curve fit was -40.69.

-41 + -0.22 = -41.22 but -41 * 0.22 = 9.02
 
Weyermann Pils measured pHDI = 5.483 after 25 minutes (the third digit to the right of the decimal place is a consequence of he fact that several pH readings are averaged) at 45.4 °C. Translating to room temperature we get 5.622. Why it says 5.65 on Slide 25 I do not know. A typo I probably. The first coefficient in the curve fit was -40.69.

-41 + -0.22 = -41.22 but -41 * 0.22 = 9.02

Thank you A.J. for providing me with the information that you felt was missing in our earlier posts. While focusing on the details of your buffering calculations, it seems I allowed myself to be tripped up by some simple math.

If I am correct, when using the example Weyermann Pils malt with a pHdi of 5.62, and targeting a mash pH of 5.4 the following formula can used.

Formula: acid = a1*(pH - pHdi)
-41*(5.4 - 5.62) = -9.02 mEq (acid)
-41*(0.22) = -9.02 mEq (acid)

With that said, I am still at a loss as to how you arrive at the a1 coefficient. I am looking at the curve plotted on the graph on slide 27, there I see the horizontal line plotting 0 'acid added' intersecting with the vertical line at pH5.48. The a1 coefficient shown is -37.5 but looking at the chart I am unable to understand where/how you are able to assign -37.5 to the a1 coefficient. Any help that you can provide is greatly appreciated.
 
How's your calculus? a1 is the slope of the curve at pHdi. Draw the line tangent to the curve at pHdi and extend it till it hits the axes (if you can). The change in mEq per unit change in pH is a1.

The numbers are obtained by doing a least squares fit to the data points which in effect finds the values for a1, a2 and a3 which gets all the measured points as close to the curve a1*(pH - pHdi) + a2*(pH - pHdi)^2 + a3*(pH - pHdi)^3 as possible. If you have had some math this will be easy to grasp. If you haven't it's going to be tough to explain.
 
How's your calculus? a1 is the slope of the curve at pHdi. Draw the line tangent to the curve at pHdi and extend it till it hits the axes (if you can). The change in mEq per unit change in pH is a1.

The numbers are obtained by doing a least squares fit to the data points which in effect finds the values for a1, a2 and a3 which gets all the measured points as close to the curve a1*(pH - pHdi) + a2*(pH - pHdi)^2 + a3*(pH - pHdi)^3 as possible. If you have had some math this will be easy to grasp. If you haven't it's going to be tough to explain.

My calculus skills are best described as being limited to non-existent. I have worked with graphs having an x and y axis. I struggled to understand your latest post which explains the steps taken to assign -37.5 to the a1 coefficient, as shown on slide 27.

'Draw a line tangent to the curve at pHdi and extend it till it hits the axes (if you can)'.

Maybe assigning -43 to the a1 coefficient, as you had mentioned very early on in this thread, will be the closest my calculations will ever get me to being completely accurate. Either way I have thoroughly enjoyed talking DI pH with you A.J. I rather obviously have benefited far more than you from this thread and for that I thank you.
 
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