THE PROCESS IN MOTION

Where do the ions in water come from, and what do they do to coffee?

Start the scene or choose a stage. The animation simplifies the mechanism; what follows gives the details and the available references.

▶ Play the process01Water breaks salt apart02Limestone gives calcium and bicarbonate03Cations go looking for flavor04Bicarbonate buffers the cup

Water breaks salt apart

The water molecule is a small triangle: two hydrogens bonded to an oxygen that pulls the electrons toward itself. It therefore carries a negative pole on one side and two positive poles on the other; it is said to be polar. When a salt falls into it, the molecules turn around it, negative side toward the sodium, positive side toward the chlorine, until they pull the two apart. Each ion leaves wrapped in a shell of water: it is dissolved.

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1. Water breaks salt apartThe water molecule is a small triangle: two hydrogens bonded to an oxygen that pulls the electrons toward itself. It therefore carries a negative pole on one side and two positive poles on the other; it is said to be polar. When a salt falls into it, the molecules turn around it, negative side toward the sodium, positive side toward the chlorine, until they pull the two apart. Each ion leaves wrapped in a shell of water: it is dissolved.
2. Limestone gives calcium and bicarbonateThis is why almost all water contains ions: it picked them up from the ground it flowed through. By dissolving the calcium carbonate in limestone, it carries off calcium ions and bicarbonate ions together. These are the two properties that matter most for extraction, at concentrations far too low to change the taste of the water itself.
3. Cations go looking for flavorTo dilute an espresso into an americano, the water only needs to be good. To extract, it needs cations, that is, positively charged ions: magnesium and calcium above all, which go looking in the grounds for the compounds of taste. Sodium plays a lesser role and needs higher concentrations to be felt.
4. Bicarbonate buffers the cupBicarbonate does a different job: it is a buffer, capturing the hydrogen ions that acids release. If there is too little, the cup is sour and unstable. If there is too much, it neutralizes the aromatic acids we set out to extract before they can be tasted. These ions cannot be sensed in a glass of water; they are sensed in the cup.

To remember: The ions in water come from the ground it has flowed through: the cations, magnesium and calcium, help extraction, bicarbonate buffers the acids, and their dose is judged in the cup, not in the glass.

Read the full article and its sources ↓

To understand why water extracts coffee, we have to spend a minute on its shape. It is worth the effort, because everything else follows from it.

The water molecule is a triangle: two hydrogens bonded to one oxygen. In these bonds, the oxygen pulls the electrons toward itself. As a result, the molecule carries a negative pole on one side and two positive poles on the other. It is said to be polar.

Because it has a plus side and a minus side, each molecule clings to its neighbors. These are hydrogen bonds, and water behaves like a crowd of small, lopsided magnets.

Now let us see what happens when a solid falls into it, table salt for example. The water molecules turn around the intruder: they present their negative side to the salt's positive charges, and their positive side to its negative charges.

This attraction is enough to separate the sodium from the chlorine. Each ion ends up wrapped in an oriented shell of water. It becomes invisible. It is dissolved.

And this is why almost all water contains ions, picked up from the ground it has crossed. The calcium carbonate of limestones, for example, gives it both calcium ions and bicarbonate ions.

Remember these two: they are the two most important properties for coffee extraction. And they act at concentrations far too low to change the taste of the water itself. You do not sense them in a glass of water, you sense them in the cup.

Gagné, The Physics of Filter Coffee (2021), ch. 2, pp. 19-22

To dilute an espresso into an americano, the water only needs to be good. To extract, it needs cations, magnesium and calcium above all, which go looking in the grounds for the compounds of taste; sodium plays a lesser role and needs higher concentrations to be felt. Bicarbonate, for its part, is a buffer: it captures the hydrogen ions of acids. Too little, and the cup is sour and unstable; too much, and it neutralizes the aromatic acids we set out to extract before they can be tasted.

Gagné, Water for Coffee Extraction, Coffee ad Astra, December 16, 2018, pp. 1-3; Gagné 2021, pp. 22-23