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Mash PH is Very Low with BIAB - Numbers just don't make any sense


Jul 24, 2009
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Corpus Christi TX
I do full volume BIAB.  I start with RO water.  I have measured the TDS of the RO and it is typically less than 3PPM, so I am pretty sure it is truly RO water.  I use Bru'n Water spreadsheet and the new BeerSmith water tool to calculate my salt additions.  Both are usually close to each other.  I add the salts to the RO water before adding the grains bring it to mash temp and wait 20 minutes before pulling a sample to measure.  I let the sample cool to room temp and measure with my digital meter and the cheap strips.  Both measurements are similar between 4.6 and 4.8 pH.  I have also read that lower pH typically increases the OG and FG, but my OG is usually spot on but my FG is always way below the predicted.  That could be caused by other factors.  But I just don't know why my pH is so low in the mash.  Below are the last 3 brews I have done.

8.83 gallons of RO
2.6g Gypsum, 3.9g CaCl, 2.6g Epsom
7lbs 2 Row, 1lb white wheat, 0.5lb dextrin malt, 0.5lb acid malt
Predicted ph 5.4, measured ph 4.6

9.07 gallons of RO
2.4g Gypsum, 4.2g CaCl, 3.1g Epsom
6lbs 2 Row, 5.3lbs wheat malt, 0.5lbs Carapils, 0.5lbs Munich Malt
Predicted ph 5.6, measured ph 5.1

9.16 gallons of RO
2.5g Gypsum, 4.7g CaCl, 1.4g Epsom, 0.8g Baking Soda
6.8lbs Munich Malt, 5lbs 2 Row, 1.8lbs Vienna
Predicted ph 5.4, measured ph 4.8


Grandmaster Brewer
Mar 16, 2013
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New Hampshire, US
Your findings is not too dissimilar to mine.  I am averaging about 0.3 to 0.4 pH units lower than the predicted.  Some of this I am pretty sure comes from the Rahr 2-row that is my base malt for most American styles.  I do not see the same degree of error with Crisp Maris Otter or Weyermann Floor Malted Pilsner which comprise my other two base malts.  With the latter two malts, I am only off by about 0.2 pH units.

I tackle this by shifting my target pH to around 5.6 to 5.8 depending upon the base malts I am using.  Baking soda addition helps with raising the pH slightly so that I hit my actual target of 5.4 to 5.6. 

I would also recommend checking the pH at the end of the mash as well.  IME, the pH continues to change throughout the mash as the starches are converted to sugars.