Coffee chemistry
How do minerals and "hardness" affect brewing?
When looking into "water for coffee" you'll often see information about mineral content and "hardness".
Hardness itself does not directly affect coffee brewing, but does affect taste of coffee. But it's more important to focus on actual species in the water, not a generic term like this.
I don't like using this term for a number of reasons:
- it's quite archaic
- it's most relevant to typical drinking (tap) water
- it does not provide a clear measure of any one thing
- it often has annoying units
- measuring it is also annoying
What does hardness mean?
Hardness essentially translates to the mineral content of water.
Water with a high concentration of dissolved minerals = hard, with a low concentration = soft.
In a typical home situation, "hard" water leads to more limescale in your kettle, showerheads, etc. because of the concentration of dissolved calcium and carbonate ions.
There are two types of hardness: permanent and temporary.
Temporary hardness
If you boil water you can precipitate out solids, removing them from the water.
That component that can be removed as a solid is called the "temporary hardness" of the water, and it comes from dissolved calcium and magnesium bicarbonate.
This is what happens when you get limescale in your kettle – it's this "temporary" component dropping out as a solid.
Permanent hardness
On the other hand there are some things that will not precipitate out as a solid when boiling, such as magnesium chloride, magnesium sulfate, calcium chloride and calcium sulfate.
Because these contributors to "hardness" can't be removed by boiling, we call it the permanent hardness component.
Total hardness
This is just the two components added together, and it's essentially a measure of the calcium + magnesium concentrations in the water. How much corresponding bicarbonate, chloride, sulfate etc. is present is not clear from total hardness.
Why is it annoying to use?
When we are making water for coffee, we start with pure water then add known exact quantities of ingredients.
So why would we want to use some term relevant mainly to tap water, when we already know the exact concentrations of everything we have added to the water?
It would be like having a clear recipe for a cake, baking it, then being asked to describe the ingredients and saying "1/4 sweet stuff, 1/2 carbs and 1/4 liquids".
Additionally, hardness is often expressed in ppm units relative to ppm of calcium carbonate, which makes it even more ridiculous.
So just forget hardness and focus on what's actually in the water.
What to use instead?
Rather than hardness, we can know and exactly describe concentrations of :
- Magnesium
- Calcium
- Sodium
- Chloride
- Sulfate
- Bicarbonate
- Citrate
And we can know all of these exactly in mol/L or mmol/L (mM) units. We can also use mg/L but it's better to focus on amount or mol, not weight per litre.
Ionic strength
Rather than hardness, ionic strength is a much better concept for describing the overall water composition you are using – basically, it's "strength".
Ionic strength is a measure of all the ions present in the water. A summary of the total content of dissolved ions. The higher it is, the more we have dissolved in the water. However, knowing the concentrations of each specific item is more important, as each species matters for different things.
Ionic strength is calculated by taking the concentration of each ion, multiplied by it's charge squared, and summing these all together. In this way, ions with multiple charge like Mg2+, SO42-, etc. add more to the ionic strength than Na+ or Cl-.
While you can't use ionic strength to directly understand coffee brewing effects, it's a useful hand-wave measure of "how much stuff is in this water".