Espresso extraction might be quick, but it’s not simple. Both chemically and physically, there’s a lot going on inside that coffee puck when you pull a shot. Complicating it even more, how the water hits the dry grounds at the start of the extraction will be very different from how it behaves just five seconds later – and this impacts the flavor of that espresso.
To find out more, I spoke with Lauro Fioretti, Chief Engineer at the Italian espresso machine and grinder manufacturer Simonelli Group, which includes Nuova Simonelli and Victoria Arduino. Read on to discover the three phases of espresso extraction and what you can do with this knowledge.
Lee este artículo en español Presión, Pulsación y Las 3 Fases de la Extracción del Espresso
Espresso shots. Credit: Fernando Pocasangre
The pre-infusion phase is when the water first makes contact with the coffee. At this point, the aim isn’t to extract it but rather prepare the puck.
Lauro tells me, “In filter coffee, generally what you do is you apply some water at the beginning on the coffee bed.” This is the start of the bloom: as the water reacts with the coffee, the CO2 inside the grounds is released as air bubbles.
These air bubbles disturb the coffee bed and also block water from reaching some of the grounds. They then dissipate and the coffee bed resettles. As such, allowing the bloom to happen prior to extraction ensures a more even and consistent wetting of the coffee grounds.
The same principle applies to espresso. This is why it’s important to allow espresso beans to degas before brewing them. It means that less CO2 is released during brewing.
It’s also important to remember that different espresso machine manufacturers have different pre-infusion systems. Lauro tells me that the team at Simonelli Group separate pre-infusion into two different stages: pre-wetting and pre-infusion. “Pre-wetting is generally a function that we activate when we have a very fresh coffee,” he says. It allows the machine to release the CO2 that is inside the coffee puck by opening the solenoid valve.
He adds, “If your coffee has been already degassed properly, you don’t need to remove the excess CO2.”
As for pre-infusion, he tells me that the puck “will become wet, will absorb the water, will release CO2, and will expand.”
He stresses the importance of a stabilized coffee bed during pre-infusion and pre-wetting. “It’s very important,” he says, “[to use] the right amount of time.” If this happens too quickly, the CO2 will still be being released as infusion begins.
Another point to consider is the disturbance of the coffee grounds caused by the impact of the water hitting the puck. Espresso owes its intensity and short extraction time to pressure, with many machines extracting at 9 bars. However, this can displace grounds.
Lauro says, “It is very important to reduce, as much as possible, any risk of channeling.” If the water hits the coffee at a full 9 bars of pressure during the pre-infusion phase, it could disturb the puck and make certain areas easier for the water to channel through. In turn, this could lead to uneven extraction.
For this reason, Lauro stresses the need to allow the hot water to flow without any mechanical pressure during pre-infusion. The aim is that it smoothly wets the puck.
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Changing the settings on the Aurelia Wave. Credit: Miguel Regalado
The next stage is infusion. Lauro tells me that this should ideally happen when all of the coffee puck is already wet, “creating a stable structure with a consistent resistance to the passage of the water.” When this happens after pre-infusion, he explains, the coffee puck “has the highest resistance to the water.”
The pressure, he continues, can be increased to the maximum level desired (which is generally around nine bars). “During the infusion phase, you can see that the flow rate is increasing,” he adds. This is because of the changing resistance as the puck becomes wetter.
This is also when most of the chemical reactions take place, according to Lauro. The coffee’s aroma, acidity, sweetness, and bitterness: they’re all being extracted.
Playing with infusion time and levels can affect the flavour of the espresso. The fruity acids tend to be the first thing that is extracted, followed by sweetness and body, and finally bitterness. A well-balanced espresso would tend to have elements of all these attributes. However, depending on the origin, processing method, roast profile, and more, different coffees will suit extraction times.
In particular, it’s worth bearing in mind that darker roasts will be more soluble as their structure will have degraded. Lighter roasts will take longer to produce the same profile as a darker coffee.
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Pulling a shot of espresso. Credit: Fernando Pocasangre
Post-infusion is the last few moments of pulling the espresso shot. Lauro says, “Most of the desirable compounds are already in the cup and we start to [extract] some other compounds that are less desirable, but we need more liquid in the cup to balance the elements.” Without this, the shot might be too intense.
A ristretto. Credit: Neil Soque
Different Phases, Different Flow Rates
Lauro tells me that, traditionally, “during the extraction phase, the coffee cake is pushed by high pressure for the whole time. So, all the coffee grounds are compressed and squeezed one against each other for all this time.”
Working with International Hub for Coffee Research and Innovation and the University of Camerino, Lauro explored ways to “create waves [of water] that can relax the coffee bed to give the possibility to the water to penetrate more, so as to increase the permeability into the coffee grounds.”
This became the foundation of Simonelli Group’s Pulse Jet technology, used in the Aurelia Wave espresso machine. For each of the three stages of espresso extraction, baristas can choose different flow rates that are pulsed through the puck.
Lauro stresses, “We cannot say [that one] profile is better than another, because, of course, that is very personal and depends on what you are looking for.” He gives the example of two coffees, a natural processed Peruvian and a fully washed Uganda, that the team ran tests on. Each coffee was extracted traditionally and also with three different profiles with varying flow rates levels during the different phases, before being blind-tasted by six Q graders.
For the washed coffee, he explains, “applying the Pulse Jet, we noticed that the acidity was coming much more mild. We were able… to control the acidity and make the coffee more balanced in terms of taste. So, we could feel the sweetness, and the acidity in a balanced taste.
“In the natural coffee,” he continues, “we had a great body, with all the extraction systems, that what you expect from a natural coffee, great sweetness.”
“So what we suggest,” Lauro tells me, “is that you put your coffee in, you dial in your coffee – so you choose your brew ratio, you choose your temperature – and then you start to play with the Pulse Jet and you taste the coffee… [It] gives you the possibility to practically create a new profile of coffee, create a new balance.”
The results of different Pulse Jet programmes on the cup profile of two coffees. Data source: Simonelli Group. Artwork: Carlos Santana
From pre-wetting to post-extraction, every stage of espresso extraction is different – and knowledge of this has the potential to help baristas serve better coffee. Understanding that coffee needs to degas, that different flow rates and pressure profiles may suit different coffees, and the importance of a well-balanced extraction: this can contribute to more delicious espresso shots and more satisfied customers.
Written by Ivan Petrich. Feature photo: a barista pulls a double espresso shot.
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