Don't Trust Your RO Water!

written in science, water

When brewing light colored beers such as Saisons, Blonde Ales, Pale Ales or IPAs, I like starting off with Reverse Osmosis (RO) water and controlling the water profile for an appropriate mash pH and for flavor using a few additions such as Calcium Chloride and gypsum salts as well as food grade Phosphoric Acid.

RO Water is meant to serve as a great baseline for your additions, beceause it is stripped out of most minerals, providing the brewer with a blank canvas to build the water profile for the beer being brewed.

However, today, I was faced with a disturbing inconsistency that threw off my numbers and assumptions. Here’s the story.

I always measure my starting water pH. It’s become a good process checkpoint along with many others along the brew. My local RO water has consistently been in a pH range of 7.0 to 7.2. I use this information when treating my brewing liquor.

Today, I stuck my pH meter in my liquor tank before heating up the water to get a baseline, and my meter showed an RO water pH of 5.6. While strike water pH has little to do with actual mash pH, a higher pH is definitely required. The phosphates in the grain react with the water in the mash and lower the pH reaching a bit of an equilibrium point, so if we’re targetting somewhere in the 5.2 to 5.4 range for a recipe, and the starting water pH is 5.6, and with roughly zero alkalinity to boot, I’d expect the mash pH to dive way under the 5.0 mark. This would mean that not only is the final beer pH going to be way too low for style, but also now we’re inhibiting enzyme activity and impairing conversion. No good.

Of course, my first reaction is that I’m doing something wrong. Either my pH meter needs calibration, or it’s flat out broken, the HLT was dirty from a prior sanitizer solution, or well, this is just some really weird RO water.

I proceeded to recalibrate my pH meter. It was already pretty much spot on as it read the 4.01 and 7.01 buffering solutions from last week’s calibration, but I thought it’d be good to just recalibrate.

I measured again, same number.

I then poured some of the RO water straight out of the jug and into a measuring cup, to avoid being in contact with the kettle at all, or any other agent that might affect it’s pH for that matter. To my surprise, same reading of 5.6.

At this point I’m thinking my pH meter is broken. I waited another half hour for my LHBS to open and went over there with a sample of the water to test it with their meter as well. I’m lucky enough that they’re just under 10m away. Surprisingly their meter agreed with my meter, so it’s pretty conclusive: This RO Water is weird.

I have no idea what could’ve caused RO water to have such a low pH. Our local city water has consistently had a pH of about 8.0. RO is just a filtration system that removes minerals and nasty chlorine/chloramines from the water, so how would that lower the pH this way? I don’t know, but the store might be hearing from me soon.

I digress though. It’s now time to troubleshoot. I have a few options to work with this water. I could:

  • Add Baking Soda, Bicarbonate (HCO3-1) or Calcium Carbonate (CaCO3). This would have the effect of increasing alkalinity and pH of the water to an appropriate range for my mash, but at the quantities required might also have an effect on flavor that I’d like to avoid.
  • Dump the RO water and go straight to my local tap water. My local tap water has a huge amount of bicarbonates, which makes it suitable for brewing dark beers such as stouts, and that’s what I usually do. But for a pale beer, I’d have to counteract the high alkalinity with a combination of lots of phosphoric acid, acidulated malt, gypsum and calcium chloride. This would leave me with a far too minerally tasting water, unsuitable for most pale brews.
  • Blend the faux-RO water with my tap water to reach a desired pH range.

I ended up going with blending. I slowly added tap water to the base RO water until it reached a pH of 7.0. This also meant that I was introducing chlorine into the mix, so I added a campden tablet to remove it. This brought in some amount of bicarbonates into my brewing liquor, and I adjusted further with calcium chloride and a bit of gypsum.

At the end, the mash pH got right on the target range at 5.4.

But now you know, don’t take anything from granted. Create a habit of checking your process along the way, and adjust as necessary. It’s the best way to make consistently great beer!

The question does remain: What would cause RO water to have such a low pH? If you have any thoughts, please share in a comment.


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