# How an Espresso Machine Works Look inside an E61 heat\-exchanger espresso machine: follow the water from the tank through the pump and boiler, lock in the portafilter and pull a shot By Yan Kurniawan Topic: Machines [Open the interactive Jig](https://ask.jig.com/j/9s1zbMVklgnT) ## Guide ### 1. Meet the espresso machine This home espresso machine has the famous E61 brewing head\. Espresso is made by pushing hot water through a tight bed of finely ground coffee at about 9 bar, nine times the pressure of the air around us\. Follow the water from the tank to the cup\. Drag anywhere around the machine to turn it\. Faema introduced the E61 group in 1961, and machines still copy it today\. ### 2. Cold water and a pump Inside, a tank holds cold water\. A vibration pump draws it out: an electromagnet shakes a small piston back and forth about fifty times a second, and one\-way valves push the water forward\. The pump can reach around 15 bar, more than coffee needs, so an over\-pressure valve sends the extra water back to the tank\. The 15 bar printed on a pump is not the brewing pressure\. The valve trims it to about 9 bar at the coffee\. ### 3. Heating without boiling The copper boiler holds water at about 120 degrees Celsius, hot enough to make steam for milk\. Brewing water must not boil, so it never mixes with the boiler water\. Instead, fresh water from the pump runs through a tube inside the boiler, called a heat exchanger, and warms to about 93 degrees on its way through\. Typical values: the steam boiler runs at about 1 to 1\.5 bar, and every machine is tuned a little differently\. ### 4. A heavy brass group head The chrome mushroom on the front is the group head, more than four kilograms of brass\. Hot water from the heat exchanger rises into it, and cooler water sinks back to the boiler\. This steady loop, called a thermosiphon, keeps the brass at brewing temperature without any pump\. Hot water is lighter than cold water, so the loop circulates by itself with no moving parts\. ### 5. Dose, tamp and lock in The portafilter has a 58 millimetre basket holding about 18 grams of finely ground coffee, pressed flat with the tamper beside the machine\. Lift it into the group and twist it about a quarter turn\. Two lugs slide into the ring and press the basket tight against the seal\. Press the button, or click the portafilter\. A level, even puck matters\. Water takes the easiest path, and a crack lets it rush past the coffee\. ### 6. Pull the shot Lifting the lever starts the pump and opens a valve in the group\. First the water arrives gently and soaks the coffee, which is called pre\-infusion\. Then the pressure climbs to about 9 bar, and dark espresso topped with golden crema runs from both spouts\. Press the button, or lift the lever\. A real double shot takes 25 to 30 seconds and turns 18 grams of coffee into about 36 grams of espresso\. This one runs faster\. ### 7. The coffee makes the pressure The pump cannot build pressure on its own; the coffee does\. Fine grounds packed tight resist the water, and that resistance is what the gauge shows\. Too coarse, and the water gushes through, thin and weak\. Too fine, and it barely drips\. Pick a grind and watch the gauge and the streams\. When the lever goes down, a valve lets the leftover pressure out into the drip tray, so the puck comes out drier\. ### 8. Steam for the milk Turn the steam knob, and pressure in the boiler drives dry steam up the pipe and out of the wand\. Dipped in a jug of milk, the steam heats it and folds in tiny bubbles for a smooth foam\. The same boiler also supplies hot water for tea or an Americano\. Steam only heats the milk\. It never pushes the coffee: that is the pump's job\. ### 9. From the tank to the cup Tank, pump, heat exchanger, brass group, coffee puck and cup: that is the whole journey\. The pump supplies the force, the boiler the heat, the brass a steady temperature and the coffee the resistance\. Turn the machine to see it from every side\. Crema is not milk foam\. It is oils and carbon dioxide from freshly roasted coffee, whipped up by the pressure\.