Espresso is a strong coffee prepared by forcing boiling water through ground dark-roast coffee. The first espresso machines appeared in Italy prior to 1900. More than a century later, espresso machines are available on the consumer and service-industry markets in various sizes and models, ranging to compact and light machines for use at home to computerized professional pieces of equipment for use at social events and in restaurants and coffee shops. The main parts of an espresso machine are the filter or "porta-filter," the heat exchanger or boiler, the reservoir and the pump.
Also called the filter holder, the porta-filter is responsible for holding the ground coffee. It looks like a small metal basket, and it is totally detachable from the machine. Ground coffee is placed into the basket, which is attached to a big metal handle. The espresso flows out from two spouts located underneath the porta-filter. When you're putting ground coffee into the porta-filter's basket, it is important to apply pressure to it, packing it down a bit. This helps to enhance the coffee flavors.
Reservoir or Tank
The reservoir is the cold water depository inside the espresso machine. It is necessary to check the water levels of the reservoir before making coffee. In small espresso machines, the reservoir is often detachable. It is not a pressure-tight component of the machine. Professional machines generally have a water-softener component or filter inside the reservoir to avoid the formation of limescale.
The pump takes the water from the reservoir to the brew head or filter, passing through the heating exchanger. The pump is electricity-powered and it is the noise-making element of a espresso machine. It helps to create the pressure that's crucial to the espresso-making process. The Italian word espresso, in fact, means in essence "pressure," "pressed" or "pressed out."
Heat Exchanger or Boiler
The heat exchanger is a filament, a metal tube connected to electricity, responsible for heating the water coming from the pump to produce the espresso. Heat exchangers are often common parts of bigger and professional espresso machines. Home espresso machines generally have a small boiler, where water is poured in before making the coffee. The advantage of a boiler model is that it does not reheat water, like the heat exchange machines. In both models, a thermostat regulates the temperature necessary to produce the espresso.