Coffee chemistry
Why water composition matters in coffee brewing
When you make a coffee the "strength" in terms of dissolves solids vs total liquid is only in the range of 1-3%.
So that coffee you are drinking is 97-99% water. So the composition of the water probably matters, right? Yep!
What do we mean by composition?
By composition we mean dissolved charged species like magnesium, calcium, chloride, carbonate, and so on – often called dissolved minerals.
The water itself is pretty standard - the H2O is not changing much. When you extract coffee, much of this process is physical. There are a lot of compounds in the coffee grind that slowly dissolve into the liquid water, with effects from temperature, surface area of the grind, how soluble those compounds are, and so on.
This process - extraction - involves many physical concepts/parameters.
We know that the water composition affects the taste of our coffee, so this is involved here too - right?
No, not exactly. The trace concentrations of dissolved ions like magnesium, calcium, etc. are not going to have a big impact on the physical extraction processes.
So, why does it matter?
When we look closely at the role of magnesium and other dissolved ions, it's clear the role is sensory.
How do we know it's sensory?
This is made very clear by experiments where you modify the water mineral content pre-brew and post-brew and do blind tests. Judges can't tell the difference, thus the minerals are not affecting extraction to change the flavour.
The ions dissolved in water (and thus the coffee) affect how our tongue perceives the tastes of the coffee. In some cases we can taste the ions themselves (e.g. sodium may add distinct saltiness) and in other cases they mysteriously alter our perception of the flavours coming from the hundreds of extracted coffee compounds.
Think like adding MSG to savoury food. You know the MSG alone doesn't taste great, but when mixed with other flavours it enhances their presence on your tongue.
The dissolved minerals may also provide acid buffering (e.g. carbonate), which oppose the natural acids extracted from coffee. This will influence the acidity of the resulting brew.
Knowing this, we can change water composition to influence how coffee tastes, as well as the presence of acid buffers to affect acidity.
You can check the other sections for more specific information on hardness, alkalinity/pH, TDS and specific ions.