How Does Machine Espresso Work?
Machine espresso makes use of precise pressure and incredible filter technology to produce the perfect cup of coffee. How exactly does it work?
To make an espresso hot water is forced under high pressure through finely ground coffee. The process is similar to making drip coffee but the key difference is in the pressure.
The Group Head
The group head is the portafilter you insert when brewing espresso. It is responsible for dispersing water into the portafilter and controlling the pressure that results from the extraction. There are various kinds of group heads each with its distinct advantages and disadvantages. Some are designed to provide stability in temperature, while others are built to handle pre-infusion. Some are designed to control the lever. Some come with a combination of both, like the E61. This is a preferred choice for baristas as it provides a variety of advantages in one package.
As you can see from the photo above, the group head is fitted with several notches. You place your portafilter inside these notches, and then twist the head to secure it. This Resource site is located in the notches to create a seal as you insert your portafilter. The notches on the head allow for an exact positioning of the portafilter which is vital to ensure an efficient extraction.
The group head not only lets the portafilter to be inserted easily however, it also keeps a constant temperature. It does this by cycling hot water through the brew container and around the portafilter, making sure that it is always at the correct temperature to extract. It is crucial to keep in mind that even a few degree differences can make the distinction between a good and great espresso.
The Pump
Contrary to manual piston machines which use a lever to pressurize water, rotary espresso machines rely on motorized pumps to provide the nine atmospheric bars of pressure that are required to extract espresso. The pressure builds by drawing tap water from a reservoir, and pumping it through a heat exchanger prior it is shot through the ground coffee in the group head.
Pumps are typically less expensive and last longer than piston-driven machines. However, both types of machines are susceptible to degrading because of excessive use and inadequate cleaning. They also add mechanical complexity, which can result in the cost of even the simplest models.
Some espresso machines remove the pump completely and utilize steam pressure to make espresso. The disadvantage of this is that the boiler that produces steam also raises the temperature of the water until it reaches boiling which can cause over-extraction. These machines also have to constantly rebuild their pressure between cups. This requires energy and time.
Many espresso machines make use of a vibration or rotary pump, which has a vibration model using a vibrating disc to create pressure, and rotating models that push hot water through the grounds under high speed. Both types of machine can produce an excellent espresso, however Rotary machines are quieter and more durable than vibration pumps.
The Boiler
The boiler is the one that will heat the water to the ideal temperature for extraction. The resulting steam is then transferred to the portafilter which holds the ground espresso and is then pumped down into the cup. The steam is able to create enough pressure to push the grounds of the coffee through. This results in a crema on top of the espresso. This is one of the hallmarks of a great espresso.
There are three distinct types of espresso machines, each with different types of pumps and the temperature of the brew. There are different ways to control the amount of brew, as well as the size of the cup that can be made by the machine.
The first espresso machines were steam types. They employed one boiler to create both steam and brew however, the pressure they could produce was very low - just two bars of atmospheric pressure at the most. This resulted in the coffee tasting bitter and burnt. This is the reason Milanese makers Luigi Bezzerra and Desiderio Pavoni developed the modern espresso machine.
The most popular espresso machine is a semiautomatic model with an electric pump. These are the things people think of when they imagine an espresso machine. Semi-automatic machines require you to grind and tamp the beans by yourself however the pump controls the water flow and pressure. This is an excellent compromise between human touch and the mechanized accuracy.
The Filter
Espresso machines usually use a filter that separates the grounds of the coffee from hot water. The filter is also an important component of the temperature control system as it keeps the machine from overheating.
A filter can also help with flavor since it allows for a longer flowering period. This lets the beans release their nuances, and allows for better extraction.
However it is crucial to keep in mind that even a good filter can make a terrible cup of coffee, as the quality of the beans and extraction are vital.
This is where the magic happens, it's what makes an espresso really taste good. The grouphead (also called the brewhead) is where you put the portafilter, the thingy used to place the coffee grounds into, when making espresso.
Steam-driven espresso machines make use of hot water that is heated in an airtight container to create steam. The steam then pushes hot water through grounds of coffee at high pressure. They are less expensive and are easier for the user to maintain than pumps-driven models. They are however limited in their ability to create the ideal conditions for brewing as they only operate with 1-1.5 bar of pressure. The perfect shot requires 9-10 bars.
In recent years, compressed air pump-driven espresso machines have been gaining popularity. They utilize an air compressor to force hot water through grounds, and are far more portable than electric steam-driven machines.