Lesson 2 - Brewing Water Sources and Treatment

Water alkalinity is affected by the carbonate and bicarbonate concentration; high levels increase mash pH.

The total alkalinity is defined as the amount of strong acid (such as HCl) required to convert the carbonate and bicarbonate in the sample to CO₂ at a pH of 4.4.

It can be estimated by titrating 100 mL of water with 0.1 M HCl to pH 4.4, using bromocresol green (if the initial water pH is above 8.3), or methyl orange indicator (if the initial water pH is less than 8.3).

Alkalinity is expressed as mg of CaCO₃ – where 1 mL of 0.1 M HCl is equivalent to 5 mg of CaCO₃.

Residual alkalinity

Paul Kolbach is responsible for the concept of residual alkalinity. He performed experiments that demonstrated that calcium and magnesium reacted with phosphate compounds derived from the malt in the mash. Calcium and magnesium in the water react with phosphates releasing H⁺ ions that change bicarbonate ions to carbonic acid. Carbonic acid dissociates into carbon dioxide and water and the carbon dioxide leaves the water. Once all the bicarbonate is converted into carbon dioxide there are no bicarbonate ions in solution to prevent an increase in H⁺ ions, so the pH falls as more are released.  

Kolbach determined that 3.5 equivalents of calcium react with malt phosphate to neutralise 1 equivalent of water alkalinity. 7 equivalents of magnesium have the same effect. Any alkalinity (bicarbonate) that will remain after this reaction is called residual alkalinity (RA).

An equivalent is the atomic or molecular weight of an ion divided by the valance of the ion. For example, the equivalent for calcium ions, which have a charge of +2 and an atomic weight of 40, is 20

Based on this, RA can be calculated as follows:

 or simply

Total alkalinity is typically reported as mg/L as CaCO₃. To use mg/L concentrations rather than mEq we need to convert Kolbach’s factors (3.5 and 7). To do this we divide them by the equivalent of calcium carbonate and multiply them by their own equivalents.

The equivalents for calcium, magnesium and calcium carbonate are as follows:

• Calcium (Ca²⁺):20
• Magnesium (Mg²⁺): 12.1
• Calcium carbonate: 50

So, for calcium we need to divide the factor of 3.5 by 50 (as an equivalent of CaCO₃) and then multiply by the equivalent of Ca (20). If we do this, we get 3.5/50 x 20 = 1.4 Similarly, with magnesium, we get 7/50 x 12.1 = 1.7

So, the above equation can becomes:

Some water analyses report bicarbonate concentration rather than total alkalinity. Residual alkalinity can also be calculated using the bicarbonate concentration (HCO₃^1-). For this, we need to multiply the bicarbonate concentration by 0.83

This makes our equation for RA using bicarbonate:

Let’s attempt a calculation to reinforce what we have learnt:

Your brewing water comes from two different sources. You ask for a water report from both sources and they provide you with the following information. Calculate the residual alkalinity. Which water source is better at reducing the mash pH?

Source 1

• Calcium – 7 mg/L
• Magnesium – 2 mg/L
• Bicarbonate – 16 mg/L
• Sodium – 2 mg/L
• Chloride – 6 mg/L
• Sulphate – 8 mg/L

Source 2

• Calcium – 70 mg/L
• Magnesium – 15 mg/L
• Total alkalinity as CaCO₃ – 25 mg/L
• Sodium – 35 mg/L
• Chloride – 55 mg/L
• Sulphate – 110 mg/L

Answer:

The RA of Source 1 is calculated as follows:

RA = 13.28 – (5 +1.18)

RA = 7.1 mg/L

The RA of Source 2 is calculated as follows:

Source 2 will reduce the mash pH more because the RA is negative.

The residual alkalinity of water increases in line with carbonate and bicarbonate ion concentration and decreases in line with calcium and magnesium ion concentration.

A positive residual alkalinity will increase mash pH, while a negative residual alkalinity will lower the mash pH. Darker malts tend to be more acidic. If we use a large proportion of coloured malts in our grist, we will need water with a higher RA to prevent the mash pH dropping too low.

We will now review what we have covered so far:

Raw water comes from surface and ground waters. It is sent through a water treatment plant that, via several process steps, removes organic and particulate matter and disinfects the water to make it potable. This potable water still has a variety of minerals in it, which we have seen can significantly affect the mash and wort pH.

Is this the only effect that the minerals in water have? Can we adjust water hardness and alkalinity? Is there only one brewing water specification, or do different beers benefit from different mineral levels?

The next lesson will endeavour to answer these questions.