Copper tubes. Stainless steel boilers. Intelligent firmware and hardware. Everything profiled, programmed, and precision engineered. The inside of an espresso machine is a veritable jungle of metal, wires and steam. You may be surprised to learn this, but underneath the bells, whistles and chrome, most espresso machines do the same thing: force hot water through finely ground coffee.
So how does an espresso machine work? And what is the quickest, most painless way to understand how they function without getting lost in the mechanical thick? In this guide, we’ll show you how espresso machines work by following water as it goes from the source to the coffee.
In most machines, water moves through four distinct phases before it hits your cup. It looks like this:
Water Source » Pump » Boiler » Grouphead or Steam Wand
So let’s start our journey through the machine:
The Water Source
Every espresso machine needs water to function. For home espresso machines, water enters from one of two sources: a reservoir located in the machine or a plumbed connection to a consistent water supply.
Often, the water source is determined by how the machine will be used. For instance, machines that are designed to produce only a handful of high-quality espressos and lattes daily are usually fitted with reservoirs. The benefit of a reservoir is that you can easily dial in the water quality and are not beholden to starting with what comes out of the tap. High-quality water filtration pitchers can do a terrific job of conditioning water to meet SCAA standards for quality.
For larger home and light commercial machines meant to produce hundreds of drinks daily, it’s important to have a steady supply of water. These machines are usually plumbed in and will rely on built-in or whole-house softening and filtration systems. The benefit of these machines is the convenience of never having to refill a reservoir.
Then there are hybrid machines that offer the ability to both direct-plumb as well as fill with a reservoir. This feature, found in the higher-end E61 class, is perfect for consumers who are not yet sure if plumbing in is a viable option – but want the flexibility to do so in the future.
The Pump – The Heart of Your Espresso Machine
To give water the strength to push through a tightly packed bed of finely ground coffee, machines need pressure: 9 bars of pressure to be exact, which roughly translates to 130 psi. Some of the first espresso machines used pistons attached to large levers. Baristas would have to manually pull these levers to force the water to pass through the coffee (hence the term pull an espresso).
Most modern espresso machines have ditched manual labor for electric pumps. In home espresso, there are two categories of electric pump: the vibratory pump and the rotary vein pump.
A vibratory pump, or vibe pump, is a small electromagnetic workhorse. A piston attached to a magnet is set inside a metal coil. Electrical current runs through the coil causing the magnet to rapidly move the piston back and forth, pushing water through the machine. Your average vibe pump clocks in at sixty pushes per second.
Unlike a vibratory pump, a rotary pump is mechanical. It is also a complex mechanism. A motor spins a disc that is offset inside a large, round chamber. The spinning disc is segmented into sections by veins. As the disc spins, the veins press against the wall of the outer chamber, diminishing the size of the section, creating pressure. Water enters in during the large phase and is pushed out as the section shrinks.
There are relative advantages to either pump. Vibratory pumps are smaller, inexpensive and tend to be easier to replace. Rotary pumps are quieter, offer more consistent pressure, and generally have longer lifespans. It’s important to note both pumps produce excellent espresso.
The Boiler – Bringing the Heat
Water needs to be both pressurized and heated to turn coffee into a brilliant espresso. The boiler is where a machine adds heat. But what about PIDs and temperature stability? And what’s the difference between a single boiler and a double boiler? Let’s start with the essentials: the heating element.
The basic function of a boiler is to heat and hold the pressurized water coming from the pump. The earliest espresso machines would use wood fire. Modern machines use electric heating elements to bring the water to temperature. By themselves, heating elements are a little dumb. Most function in an on/off binary: when they’re on, electricity runs through the element creating heat; when they’re off, they’re inert.
It’s important to consider the size of the boiler. The bigger the boiler, the more drinks the machine will be able to produce. On the other hand, the bigger the boiler, the more energy and time it will take to heat all of the water.
The Boiler – Taking Control of Your Temperature
For all the intensity of pressure and heat, making espresso is a delicate art; even small changes in water temperature can drastically affect the quality of your shot. Many manufacturers have introduced different tools to promote temperature stability and user control.
More simple heating elements, like those in small, inexpensive espresso machines, are calibrated to a set temperature by a thermostat. The issue with simple heating elements is that they offer the user little control over their water temperature. This makes it more difficult to get consistent shots, time after time.
For more exacting control over their brew temp, a boiler with PID or Digital Temperature Control is used. Both allow the user to change the temperature of the brewer in single degree increments, and will hold that temperature with little variance.
PID is shorthand for proportional-integral-derivative controller. A PID is a simple computer that controls the heating element to keep the water at a set temperature. To do this, a PID is connected to the heating element and a temperature probe inside the boiler. The PID continuously reads the input from the temperature probe and cycles the heating element on and off based on a preprogrammed algorithm.
Digital Temperature Control essentially performs the same way. There are only two major differences between PID and Digital Temperature Control. One: the digital display on a PID offers more control and information than the panel on a Digital Temperature Control. Two: a user can adjust the algorithm on a PID, but cannot with Digital Temperature Control.
The Boiler – Getting Steamy
Espresso is only one part of the overall machine – especially machines designed for the American market. Equally as important is richly textured milk, used to make cappuccinos or lattes. To get milk textured and hot, machines need steam. In order to produce steam, water needs to be boiled – however coffees brew best at temperatures well below boiling (195°-205°F). The following are the four ways different machines solve the problem of keeping some water boiling, and other water at a lower temperature:
In Single Boiler machines, the boiler has one heating element with two thermostats. One thermostat is set for a temperature range that is ideal for brewing coffee. The other is set at a temperature meant to boil water and produce steam.
Since this is a pretty simple build, machines in this category tend to be relatively low cost. There are two big drawbacks. The first: you cannot pull shots and steam milk at the same time. The second: you’ll always have to wait for the water to come to the correct temperature when changing from espresso to steaming and vice versa.
In order to pull shots and steam milk at the same time, a machine needs to be able to have a way of heating water to two different temperatures.
One solution is a Heat Exchanger machine. Instead of using a boiler to heat water for both brewing and steaming, the water in the boiler is only used to make steam. An additional water line is run from the pump, which then connects to a copper tube, or heat exchanger, that pases through the body of the boiler. The steaming water in the steam boiler heats the water in the heat exchanger without bringing it to a boil. In this style of system, the brew water will never come in direct contact with the boiler water.
A Heat Exchanger offers some benefits over a Single Boiler. Because the brew water is separate from the steam boiler, the machine is able to steam milk and pull shots simultaneously. Unfortunately, the temperature of the brew water is harder to control with a heat exchanger. The brew water in the exchange coil can overheat if left too long. This is why it’s important to bleed or purge a small amount of water immediately before brewing an espresso with a heat exchanger, a practice called temperature surfing.
Another option is a Thermoblock machine. A thermoblock is a thick piece of metal with a small coil machined into it; the whole block is heated by a built-in heating element. Small bursts of water are sent from the pump through the coil; these bursts are rapidly heated to past boiling, turning them into steam. There is still a separate brew boiler, but since the steam and brew water are heated separately, the machine is able to both at the same time.
For machines aimed to make high volumes of drinks with the requirement of steaming and brewing at the same time, the solution is a Double Boiler. In this style of machine, the pump sends water to two separate boilers. One heats water to boiling; the other heats water to brew temperature. Most double boiler machines feature a PID or Digital Temperature Controller, so temperatures for both boilers can be carefully controlled.
Because of the extra boiler and heating elements, these machines will have a higher price point. The advantage is the ability to produce plenty of drinks with significantly higher control and consistency of espresso and steam.
The grouphead is the final stop for water as it moves through an espresso machine. All groupheads involve four basic parts: a portafilter, a place for the portafilter to lock in, a way to activate the pump, and a pathway for the water to move from the boiler to the portafilter.
There are three primary styles of groupheads: an E61 Grouphead, used in the Profitec Pro 700; a Saturated Grouphead, used in the La Marzocco GS3; and a Semi-saturated Grouphead, like the one used in the La Spaziale LUCCA A53.
The Grouphead – E61
Originally patented in 1961 by La Faema, the E61 Grouphead is a classic component of espresso machine design. You’ll see this kind of grouphead on wide variety of machines at many different price points.
The E61 has a few distinct features. The whole grouphead is machined out of brass and weighs a hefty nine pounds. While it will take about 15 minutes for the whole group to come to temperature, it will easily hold that temperature once it’s ready. Also, for the mechanically inclined, it is relatively easy to get into an E61 to do regular maintenance and repairs. It is important to note that these groupheads require more active participation from the user. You won’t be able to start a shot, walk away, and have the machine stop it for you; instead, you’ll have full control of the length and volume of the shot.
At its most basic, an E61 is a large, mechanically operated three-way valve: one valve to let the water into the grouphead from the boiler, one valve to move water from the group to the portafilter, and a third valve to relieve back pressure from the portafilter.
Heres how it works: water enters the grouphead from the boiler through a large opening on the back of the grouphead: the water inlet. The water fills the outer chamber at the top of the grouphead and moves up to the very top chamber, called the filter chamber. From here, the water passes through a gicleur, or water intake nozzle, into the brewing channel. When the brew lever is down, the brew valve seals the top chamber, and no water runs to the rest of the group.
When the brew lever is up midway, the water inlet cam begins to lift the brewer valve and water moves from the brew channel into the area around the water inlet cam. From here, the water goes two places. It travels up through the infusion channels and out through the dispersion screen onto the ground coffee. It also flows down, past the still open preinfusion valve into the preinfusion chamber. At this stage, the pump isn’t activated; instead water moves through the system from the gentle pressure generated by heating.
This gentle pressure stage is often referred to as preinfusion. With preinfusion, the bed of coffee in the portafilter basket is given extra time to stabilize with the hot water before the adding intense pressure of brewing. This can help reduce the risk of channeling and also compensate for any variations in dosing, distributing, and tamping. As the E61 is operated by mechanical lever, a user can completely control the length of preinfusion.
It should be noted that preinfusion is not necessary to produce great espresso; it only helps make great espresso easier to achieve.
When the brew lever is all the way up, the pump is activated and pressurized water begins to move through the machine. At this stage, the water inlet cam will complete raise the brew valve while simultaneously sealing the preinfusion valve. With the preinfusion valve sealed, the intensely pressurized, hot water is only allowed to travel out through the dispersion block and onto the ground coffee.
At the end of the brewing cycle, the lever is returned to the down position and the pump is deactivated. At this point, the water inlet cam seals the brewer valve, ending the flow of water to the rest of the group. At the same time, the water inlet cam reopens the preinfusion valve and the drain valve; this releases any back pressure in the portafilter basket and dries the spent puck.
The Grouphead – Saturated and Semi-saturated
In Saturated Groupheads, like the La Marzocco GS3, the grouphead is open to the boiler and vice versa. As such, the grouphead acts like an extension of the boiler and is saturated with hot water (hence saturated grouphead). Instead of waiting for large chunks of brass to come to temperature, a saturated grouphead will quickly come to correct temperature with the water in the boiler.
Here’s how it works: A gicleur or water intake nozzle inside the grouphead/boiler is attached to a copper tube. This tube runs to a computer-controlled three-way valve that’s attached just outside the boiler/grouphead by a rubber gasket. There are two more lines attached to the three-way valve: one is a copper tube that runs directly to the dispersion block; the other is a “waste” line that runs to the drip tray.
When the machine is activated, the three-way valve opens the lines running to the boiler and the dispersion block while closing the waste line. If the machine is designed for preinfusion, it will wait to activate the pump; instead, it allows the ambient pressure of the hot water to push through the dispersion block to slowly soak the bed coffee. Once the pump is activated, the full nine bars of pressure are applied to the water and it is pushed through the open lines and out of the dispersion block.
When the shot is done, the pump deactivates. At the same time, the three-way valve closes the line running to the boiler while opening the waste line; this relieves back pressure and removes moisture from the spent puck.
Over the last decade, this has become the most common grouphead design for high-end commercial machines. Generally, this kind of grouphead is more costly to produce, due to the precision welding and thick metal builds. Because of the exposed design, it requires careful maintenance and repair; generally, we recommend most repairs be performed by trained technicians. For most home users making smaller quantities of drinks, we tend to recommend Semi-saturated Groupheads.
Semi-saturated Groupheads, like those in the LUCCA A53 or the Profitec Pro 300, function in essentially the same way. The biggest difference is that instead of having the whole grouphead exposed to the boiler, the area directly above the dispersion block is separated from the boiler. The gicleur line runs from the extended boiler into the dry area above the dispersion block. There, it runs to a similar three-way valve that is connected to the dispersion block and a third “waste” line.
Semi-saturated Groupheads are less expensive to produce and tend to be easier to repair than a fully saturated grouphead. Unlike the mostly manual E61, both saturated and semi-saturated groupheads depend on an internal computer to function. Different machines will offer more robust programming and control, depending on the kind of computer used.
The Grouphead – The Portafilter
A portafilter, shortened from portable filter, is a metal filter basket set inside a handle. Finely ground coffee is put into the filter basket, then is compressed by tamp, before the portafilter is inserted and locked into the machine.
There are many different sizes of portafilter, however the two most widely used are the 53mm and 58mm Portafilter. While the 58mm is the most common size for commercial machines, both sizes will produce excellent espresso. Both can use the same amount of ground coffee when fitted with correct baskets. The most important thing to know when purchasing either a 53mm or 58mm portafilter is to make sure to buy the corresponding size tamper. Even though a 53mm tamper can fit in a 58mm basket, the smaller tamp won’t be able to evenly compress all of the coffee.