Moka Pot Basics: Brewing With Steam Pressure
The moka pot is often called stovetop espresso, which is not quite right: espresso runs at around nine bar and a moka pot at one or two. It does produce something far more concentrated than filter coffee, though, and it needs nothing but a hob.
Steam pushes the water upwards
A moka pot has three parts: the boiler at the bottom, the coffee basket in the middle, and the chamber on top where the brewed coffee collects.
As the water in the boiler heats, steam forms, and because the vessel is sealed the pressure rises. That pressure forces water upwards through the coffee basket, and the brewed coffee travels up the column into the top chamber.
The key point is that water moves upward under pressure rather than falling under gravity. Pressure, not gravity, drives the extraction, which is why the result is far more concentrated than filter coffee in a fraction of the time.
Why you start with hot water
This is where moka pots most often go wrong. Fill the boiler with cold water and it takes several minutes to reach the boil, and throughout that time the coffee basket above is being heated as well.
Grounds held at high temperature before brewing begins lose aromatics and develop burnt, bitter notes. The reputation moka pots have for harshness largely comes from this.
The fix is straightforward: fill the boiler with water that has already boiled. Extraction then begins within a minute or two and the coffee spends far less time baking. The body gets hot quickly, so use a cloth on the handle.
Doses by size
The basket size fixes the dose, so in practice the only things you adjust are grind and heat.
Fill the boiler to just below the safety valve. Covering the valve prevents pressure regulating correctly and is unsafe.
Grind and filling the basket
Moka grind sits between espresso and filter: caster sugar is a good reference.
Too fine and water cannot pass, so pressure climbs, steam escapes the safety valve or extraction happens violently and tastes burnt. Too coarse and water runs straight through, leaving it weak.
Level the coffee in the basket but do not compress it. Tamping as you would for espresso raises pressure dangerously and makes the coffee worse. Smoothing the surface with a finger is enough.
Stopping at the right moment, and upkeep
Towards the end you hear a gurgling sound. That means most of the water has passed and steam is now coming through, and steam contributes nothing but bitterness.
Take it off the heat as soon as you hear it, and wrap the base in a damp cloth to stop extraction immediately. This one habit noticeably improves the cup.
The part that needs attention is the gasket. Repeated heating hardens and cracks the rubber, pressure leaks and extraction stops working properly. Check it every few months and replace when needed.
Traditionally these are washed with water alone. Detergent strips the oil layer from the aluminium and can leave a metallic taste. Rinsing straight after use matters, though, because oils left to sit will go rancid.