There are several types of electric
home espresso machines on the market, and they can be
divided into three kinds: the electric steam, the electric
piston, and the electric pump/boiler. The last kind,
the electric pump/boiler, will, for the purpose of this
article, be further split into three categories: semi-automatic,
automatic, and super-automatic.
Steam-driven
A steam-driven unit operates by forcing water through
the coffee by using steam or steam pressure. The first
espresso machines were steam types, produced when a
common boiler was piped to four group heads so that
multiple types of coffee could be made at the same time.
The design is still used today in low-cost consumer
machines, as it does not need to contain moving parts.
Piston-driven
The piston, or lever, driven machine was developed in
1945 by Achille Gaggia in Italy. The design generically
uses lever, pumped by the operator, to pressurize hot
water and send it through the coffee grinds. The act
of producing a shot of espresso is colloquially termed
pulling a shot, because these lever-style espresso machines
required pulling a long handle to produce a shot.
There are two types of lever machines; manual piston
and spring piston design. With the manual piston, the
operator directly pushes the water through the grounds.
In the spring piston design, the operator works to tension
a spring, which then delivers the pressure for the espresso
(usually 8 to 10 bar).
The piston-driven machine is the origin of the crema,
which was originally thought to be an undesirable waste
product but is now considered characteristic of a properly
made shot of espresso.
Pump-driven
A refinement of the piston machine is the pump-driven
machine, which has become the most popular design in
commercial espresso bars. Instead of using manual force,
a motor-driven pump provides the force necessary for
espresso brewing. Commercial or some high-end home machines
are often attached directly to the plumbing of the site;
lower-end home machines have built-in water reservoirs.
Home (consumer-grade) pump espresso machines typically
use a single chamber both for heating water to brewing
temperature, and to boil water for steaming milk. Since
the optimum temperature for brewing coffee is much less
than the temperature for creating steam, the machine
requires time to make the transition from one mode to
the other. Commercial-grade and "semi-commercial"
high end home espresso machines use the boiler chamber
only for making steam. Water for brewing most commonly
passes through a heat exchanger (taking some heat from
the steam, without rising to the same temperature).
In a few commercial espresso machines (notably La Marzocco),
water for brewing is heated in a separate chamber.
