Types of Espresso Machines
Espresso is a concentrated coffee beverage brewed by forcing steam or hot water under pressure through finely ground coffee. The defining characteristics of espresso include a thicker consistency than drip coffee, a higher amount of dissolved solids than drip coffee per relative volume, and a serving size that is usually measured in shots, which are between 25 and 30 ml (around 1 fluid ounce) in size .An ideal double shot of espresso should take 20-25 seconds to arrive, timed from when the machine's pump is first turned on. Varying the fineness of the grind, the amount of pressure used to tamp the grinds, or the pump pressure itself can be used to bring the extraction time into this ideal zone. Most prefer to pull espresso shots directly right into a pre-heated demitasse cup or shot glass, to maintain the ideal temperature of the espresso. As a result of the high-pressure brewing process, all of the flavours and chemicals in a typical cup of coffee are concentrated. For this reason, espresso lends itself to becoming the base for other drinks, such as lattes, cappuccino, macchiato and mochas.
Since their invention in 1901, multiple machine designs have been created to produce espresso. They generally share some common elements. The portafilter (or group handle) contains a metal filter-basket and holds the ground coffee. It is locked under the group head's diffusion block.
An espresso machine may also have a steam wand which is used to steam and froth milk for beverages such as the cappuccino and latte.
Espresso machines can be generally broken down into four categories. Manual, semi-automatic, fully automatic and super-automatic which are explained in more detail below.
Manual or Piston-driven
The piston, or lever, driven machine was developed in Italy in 1945 by Achille Gaggia, founder of espresso machine manufacturer Gaggia. The design generically uses a 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; a reddish-brown foam that floats on the surface and is composed of vegetable oils, proteins and sugars. Crema has elements of both emulsion and foam colloid.
Semi-Automatic or 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. 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).
Fully automatic machines are basically the same as semi-automatic with one exception. Fully-automatic machines usually have one or more buttons that can be programmed to deliver a desired quantity of espresso. The user presses a pre-programmed button (for example for a double shot) and the machine will start the extraction and then stop on its own when the pre-programmed volume is reached.
Super Automatic or "beans to cup" machines as they are commonly refered to add an entirely new convenience to espresso extraction. Super-automatic machines operate by automatically grinding the coffee, tamping it, and extracting; all an operator needs to do is fill the bean hopper, and if the machine is not connected to a water line, add water to a reservoir, and press a button. Some newer super-automatics also feature a milk reservoir and are capable of preparing a cafe latte or cappuccino with the press of a single button.