Specific minerals/ions
The impact of magnesium on coffee flavour
Magnesium
Magnesium is probably the most talked-about ion when it comes to coffee flavour.
It's the main ion people want to increase and it has the most pronounced affect on flavour.
What effect does it have on taste?
Generally, it enhances "sweetness" and the general "intensity" of the coffee taste.
It certainly enhances the forward and bright flavours in coffee that you sense early in the tasting process.
In our experience, at least 0.5 mM combined magnesium/calcium is advised to improve flavour. Conversely, >1.5 mM combined tends to get too intense and heavy, which we wouldn't recommend for black coffee. The 1-2 mM range is good for milk coffee.
How does it work?
Well, on their own, the taste properties of divalent salts (those of Mg2+, Ca2+, etc.) are quite complex.
They are generally characterised as bitter and salty, sometimes metallic and to a lesser extent, sour/sweet.
That sounds like everything under the kitchen sink!
The issue here is... how do we taste them? This is not well-understood, but studies have shown magnesium ion stimulates the "Human Taste Receptor TAS2R7".
The TAS2R family of receptors on the tongue is responsible for a lot of bitter taste reception, so it's not surprising that magnesium sulfate (and other salt solutions) primarily taste bitter.
So... how then does magnesium in the water make coffee taste better?
Our suggestion is that the increased stimulation of receptors by magnesium likely causes your brain to sense the flavour more intensely – in the same way that MSG can enhance the sensory perception of umami and other flavours.
Because coffee is a bitter drink, all things considered, it is not surprising that co-stimulation of another bitter receptor might influence our taste perception.
In the end, experimentation shows that adding magnesium often makes us enjoy the coffee taste more!
Calcium
Calcium is the sister to magnesium in the context of coffee water.
What effect does it have on taste?
Typically, it is thought to have less effect on flavour but have additional affects on mouthfeel and smoothness. Magnesium also affects mouthfeel.
In our experience, higher calcium levels are beneficial when mixing with milk and higher levels tend to bring out different, more vegetal flavours (eww!).
High levels of calcium can lead to a chalky mouthfeel and can promote bitterness/astringency – the chalky taste is more likely when bicarbonates are around, less so if citrates dominate.
How does it work?
Calcium, much like magnesium, will activate bitter taste receptors on your tongue.
The stimulation likely differs to magnesium a little, which may explain why calcium doesn't seem to increase flavour as much.
However, calcium is involved more generally in taste sensing – with calcium ions entering taste bud cells during activation (see here).
It is also known that adding calcium to the outside of cells (i.e. mouth) will enhance the amount of calcium entering cells, potentially increasing taste sensation going down our nerves to our brain.
So, it's likely that calcium overlaps with magnesium in some regards, but may also affect taste by different mechanism as well.