Phosphoric acid & pH change during mash?

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btbnl

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I just brewed a three of my regular BIAB batches, using a freshly-calibrated pH meter and allowing the samples to cool before measuring.

Normally I unwrap the mash after 30 minutes and take the pH, gravity and temperature, and invariably the pH is within 0.1 of the Bru'n water calculations (targeting pH 5.4 in all cases).

On this occasion I waited till 60 minutes to take the readings. Both batches that included 3ml 85% phosphoric acid registered pH 4.9, whereas the batch that had no phosphoric acid registered the expected pH 5.4. Again, previously when I've measured the pH at 30 minutes with exactly the same recipes and water additions they've all been at pH 5.4 +- 0.1.

Any ideas what might be going on? Does the pH really change that much during the mash with phosphoric acid? Incidentally, I didn't see any impact on the efficiency.

Details:

Oakland CA water: Ca 9, Mg 3, Na 11, Cl 6, SO4 9, HCO3 34 mg/L, pH 8.9

Batch 1: 20lb 2-row, 4lb Vienna, 1lb Carapils, 1lb Crystal 30 in 8.25 gallons with 2 tsp CaSO4, 1 tsp CaCl, 3ml H3PO4 (85%) => 60 min pH = 4.9

Batch 2: 9lb Maris Otter, 1lb Crystal 80 in 8 gallons with 2 tsp CaSO4, 1 tsp CaCl => 60 min pH = 5.4

Batch 3: 7lb 2-row, 3/4lb Carapils, 1/2lb Vienna, 1/2lb Crystal 10 in 7.5 gallons with 3ml H3PO4 (85%) => 60 min pH = 4.9
 

ajdelange

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Any ideas what might be going on?
No. This is very puzzling. Looking at the first listed brew
Oakland CA water: Ca 9, Mg 3, Na 11, Cl 6, SO4 9, HCO3 34 mg/L, pH 8.9

Batch 1: 20lb 2-row, 4lb Vienna, 1lb Carapils, 1lb Crystal 30 in 8.25 gallons with 2 tsp CaSO4, 1 tsp CaCl, 3ml H3PO4 (85%) => 60 min pH = 4.9
the predicted pH is about 5.47 - 5.52 for typical base malts but could go as high as 5.63 with an alkaline base malt.

Does the pH really change that much during the mash with phosphoric acid?
Given enough time to react, acid is acid. Assuming a base malt that would give a mash pH of 5.47 you would need a whopping 32 mL of phosphoric acid (10 times what you used and 6 times more than would be needed to drive the base malt to 5.47) to reach pH 4.9.

Incidentally, I didn't see any impact on the efficiency.
Putting that together with the ridiculous amount of acid required to reach the observed pH leads pretty strongly to the conclusion that the pH observation is bad. The fact that you had two such readings and both when phosphoric acid was involved is, of course, very disturbing. Bad pH readings would explain what you saw. If those pH readings are valid then I have no idea what could have happened to the extent that I think it must have been bad pH readings. Phosphoric acid provides about 1 mEq/mmol at pH's near mash pH. There is no way it can provide 10. Try the stability test on your meter. Also try some test mashes.
 
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btbnl

btbnl

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Thanks AJ.

Had it not been for the middle batch I would simply have dismissed it as bad readings. I agree that something must be off, but it's very odd.
 
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