Much has been made about mash pH, and I won't get into it all over again. I think that John Palmer's How To Brew gives as fine a rundown on mash pH as anything out there and it's already up on the web. If you want to understand how residual alkalinity works, how different minerals affect mash pH, and how to dial your mash pH into the correct range, read up. I'll wait...
I think that the most frustrating thing about the discussion of mash pH is that nobody tells you how to measure it. I'll attempt to address that in this very space.
You'll often hear that your mash pH should be 5.2. It's a common statement that you'll hear from brewers. And there's truth to it. Palmer says that mash pH should be in the 5.2-5.6 range and this is for a number of reasons; optimal enzymatic activity in the mash, wort and beer clarity, healthy fermentation, a more pleasant hop bitterness, finished beer flavor, and more.
However, when it comes to measuring mash pH, there's a critical point that should not be overlooked. Palmer makes it himself, but for the best and most succinct explanation, I'll refer to page 75 of Dr. Charles Bamforth's Standards of Brewing:
"Remember that pH changes with temperature, and so wort at 149F will have a pH about .35 lower than that measured at 68F."
With that in mind, let's think about a couple of things:
- Most pH meters that we homebrewers can afford are not rated to take measurements at standard mash temps (145 to 158F).
- When you take a measurement of your hot mash pH with a pH strip, it almost immediately comes to room temperature once you're looking at it out in the air, up against the guide on the package, or against a white background of your choosing. You don't read the strip while it's immersed in the hot mash.
So, even though your mash at mash temperature should be 5.2 to 5.6, you're actually going to look to read 5.55 to 5.95 at room temperature (68F). In the case of my most recent brew day, I took a 3 oz sample from my mash and cooled it to room temperature. I took pH measurements with five separate instruments and here are the results with some pictures:
Martini meter with ATC: 6.04 (+/- 0.1 pH)
The first thing that I learned from these readings was that my mash pH was higher than what I wanted. I made an addition of a 10% phosphoric acid solution to my mash to lower the pH and came up with the following readings:
Martini meter with ATC: 5.52 (+/- 0.1 pH)
This brought me to a mash pH of about 5.2 (correcting the Martini meter from 5.53 - 0.35 = 5.18). What I learned from these two sets of data was the following:
- The meters are simply much more accurate than the strips, both in their stated levels of accuracy and in the elimination of operator error. Which leads me to...
- The strips can be difficult to read. The difference between 5.5 and 5.8 on the economy (4.6-6.2) strips and 5.3 and 5.5 on the ColorpHast strips is extremely subtle. I honestly don't know that my eyes can tell the difference between 5.3 and 5.5 on the ColorpHast strip.
- The ColorpHast strips seem to measure consistently low. There's a fella who's been doing a study of the shift on the ColorpHast strips and he's determined that they're pretty consistently off by about -0.3 from actual pH.
- Having ATC (automatic temperature correction) on the meter is pretty useful. In the case of this exercise I took a lot of time out of my brew day to take measurements and pictures. But on a normal brew day I absolutely would not want to. With ATC I only really need to get my sample into a reasonable temperature range to get an accurate reading. I don't need to get the sample to a specific temperature, which makes it easier to get an accurate reading on the fly.
Martini meter with ATC: 5.76 (+/- 0.1 pH)
is primarily due to the precipitation of calcium phosphate. Calcium ions in brewing water reacts with phosphates from the malt to form calcium phospate and hydrogen ions, which lower wort pH.
If anything, my wort pH went up (if only slightly)!!! I wonder why? Steve...
This demonstrates the importance of excess calcium ions in the wort after mashing. For this reason, it is sometimes a good idea to add gypsum to the kettle. If your mash pH is fine, but the pH does not drop to at least 5.4 by the end of the boil, add 1/4–1/2 teaspoon of gypsum per five gallons.