Residual Alkalinity and pH for All Grain Beer Brewing

This week I take a look at Residual Alkalinity, and what it means for your mash pH for all grain beer brewing. Residual Alkalinity is an important characteristic of your brewing water that you need to understand if you’re going to get the mash chemistry right.

The Importance of Mash pH

I’ve written previously about why mash pH is important for all grain beer brewers as well as the many advantages you get if your mash pH is correct. Getting the mash pH down to the 5.2-5.4 range during conversion gives you better enzyme activity, better yeast health, better hop extraction, improved clarity and better flavor stability.

Most household water is slightly alkaline, which means it has a pH of 7.0 or higher. Fortunately grains are acidic, and then tend to drive the pH down closer to the desired range when you mix them with your water. Darker grains are even more acidic, which is why pH becomes less of a concern when brewing darker beers.

Residual Alkalinity

Residual Alkalinity (RA) is a measure that helps us determine how resistant our water pH is to change. If you have a very high residual alkalinity, then it will take quite a bit of acid, either in the form of malts or additives to get our pH in the desired range. Low RA indicates the pH value is relatively easy to change and in many cases the grain additions alone may be enough to get to our desired mash pH.

Residual alkalinity is determined by your base brewing water’s profile. In particular the Calcium, Magnesium and Bicarbonate ions play the dominant role. You can also use the Alkalinity (ppm as CaCO3) measure in place of the bicarbonate if you don’t have the bicarbonate value. These ion measurements can be found on your local water report or can be measured using a home water test kit or by sending a sample of your water to a lab.

Note – all of the equations are in parts per million (ppm) – first we can determine the effective hardness:

Effective_hardness = Ca_ppm/1.4 + Mg_ppm/1.7

If working from the Alkalinity_as_CaCO3 (ppm) we can calculate the Residual_alkalinity (as CaCO3 in ppm):

Residual_alkalinity = Alkalinity_as_CaCO3 – Effective_hardness

Alternately if your water profile includes the Bicarbonate (ppm) instead you can use:

Residual_alkalinity = (50 * Bicarbonate)/61 – Effective_hardness

Looking at the above equations, we can see that effective hardness from the Calcium and Magnesium ions drive the residual alkalinity down, while alkalinity and bicarbonates drive the RA up. Calcium tends to play a dominating role (as magnesium levels are typically low) in lowering RA, while Bicarbonates (HCO3) drive it up.

Interpreting Residual Alkalinity Without Adjustment

Now that you know your RA, you can look at what it means for brewing beer. John Palmer published a chart in his How to Brew book showing recommended color ranges for a given RA. Though the chart includes a lot of assumptions (gravity, water ratios used, malt acidity), it does give us a rough guide to asses how much malt acidity in the form of dark grains is needed to offset a particular RA. He also provided the equations:

Here’s a rough guideline for RA levels above -128:

low_color_srm = RA*0.082 + 5.2

high_color_srm = (RA + 122.4) / 12.2

So to do a quick example, an RA of 40 would correspond to a color range of roughly 8-13 SRM based on the high/low above. So a beer brewed in that color range would likely provide enough acidity in the malt to offset our residual alkalinity and give us a pH approximately in the 5.2-5.4 range. I would still recommend measuring your mash pH in the mash and making adjustments as needed.

Additives and Adjusting Residual Alkalinity

Once you know your RA and what it means you can consider adjusting your RA ahead of time.

Adding calcium will reduce your RA, so additions like Gypsum (CaSO4) and Calcium Chloride (CaCl) can be added. Magnesium also reduces your RA, so Epsom Salt (MgSO4) will also work. You do need to be cautious, however, as you don’t want to raise the Calcium or Magnesium levels beyond the range recommended for brewing. Calcium has a recommended range of 0-150 ppm, and Magnesium a range of 10-50 ppm, and you don’t want to exceed those.

Similarly adding carbonates will increase your RA. So you can use additions like Baking Soda (NaHCO3) and Slaked Lime (CaOH) to reduce your RA. Some sources also recommend Chalk (CaCO3), though chalk dissolves poorly. Again you want to watch your overall ion levels to keep Ca in the 20-150 range, and bicarbonates (HCO3) generally below 250.

Because of the limited range you want to keep your overall water profile in, most often mineral additives like Gypsum, Baking Soda, and Calcium Chloride simply won’t get you in the mash pH range you want, particularly for lighter beers. As a result most professional brewers measure their actual mash pH and use acid additions to drive the pH down to the desired range. Typical acid additions for a home brewer include acid malt and lactic acid. Some commercial brewers also use phosphoric and hydrochloric acid additions.

Limits of Residual Alkalinity in Predicting Mash pH

I will close by saying that even if you understand your water profile and residual alkalinity in advance, it is not a perfect predictor of mash pH. Residual alkalinity is a tool to help you get into the ballpark. Despite recent advances in understanding water and grain acidity, there is still no substitute for measuring the pH of your mash after mixing in the grains and then adjusting if needed with acid, acid malts or water additives.

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