The science behind espresso making is concerned with applying a greater amount of pressure, or force on the coffee grounds then normal gravity, in order to extract more of the coffee’s flavour. For an espresso, by definition of the term, the coffee grounds need to be subjected to between 7-9 bar of pressure.
Although Moka pots operate under a much lower pressure, typically 1 bar, they still generate enough pressure to make great-tasting espresso style coffee when combined with heat.
Trapping of air – When you prepare your stove-top espresso maker for use, air gets trapped inside the water tank (A)
Pressure build-up – When heat is applied to the stove-top espresso maker, the air and water inside expand. As they are contained within the water tank, this leads to a build up of pressure in the tank. Water is pushed through from (A) to the filter basket where the coffee is contained (B)
Pressure release – Eventually, the pressure builds up to the point where the hot water is forced up from the filter into the upper chamber of the stove-top espresso maker (C). This is where the end result is stored ready to pour and drink