The science behind How a Stove-Top Espresso Maker Works 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.
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.
It’s important to note that some stove-top espresso makers come with a built-in safety valve that will automatically release excess pressure if you don’t do it manually. Check your manual before turning on your machine!