Estimated reading time: 9 minutes
Key takeaways
- Extract chilling is a brewing technique in which baristas extract espresso or filter coffee over a chilled surface, such as a frozen stainless-steel ball.
- Rapidly cooling espresso can preserve up to 40% more of its volatile aroma and flavour compounds.
- Research on extracting chilled filter coffee shows that room-temperature spheres can outperform frozen ones.
Extract chilling is no longer a new idea in specialty coffee.
First gaining widespread attention through coffee competitions several years ago, the technique has since become increasingly common among baristas looking to maximise flavour clarity and aromatic intensity in espresso. Products such as Nucleus Coffee Tools’ Paragon have helped bring extract chilling into cafés and competitions around the world.
Yet despite its growing adoption, coffee professionals and researchers are still attempting to uncover exactly how and why it works.
“The breakthrough came when we experimented with running hot espresso over a chilling rock directly into a room-temperature cup,” says three-time Australian Barista Champion Hugh Kelly, who helped introduce extract chilling to a global audience during his 2021 World Barista Championship routine.
“We discovered that by cooling the extract instantly but serving it in a standard temperature cup, we achieved the best of both worlds,” he adds. “We captured the volatile aromatics that are usually lost to the air, while simultaneously preserving the body and tactile sensations that a cold cup can sometimes diminish.”
The principle appears straightforward: cool the coffee quickly enough to minimise the loss of volatile aroma compounds. However, recent research on the technique’s applications to filter coffee brewing suggests it may be more complicated than first thought.
“We proved that the phenomenon exists,” says Professor Chahan Yeretzian, founder of the Coffee Excellence Center at Zurich University of Applied Sciences (ZHAW). “But we’re not so sure about the explanation.”
You may also like our article introducing extract chilling.
The origins of extract chilling can be traced back to a series of experiments that had little to do with chilling coffee in the first place. “Originally, the idea wasn’t to chill espresso,” Hugh says. “It was to cool down a beverage without dilution.”
When World Barista Championship rules changed in 2017 to allow competitors greater control over espresso extraction temperature, researchers at ZHAW began investigating whether adjusting brewing temperatures influenced flavour.
“We saw, at a sensory level, that this indeed had an effect,” Chahan says. “But contrary to our expectation, the colder the extraction water was, the stronger the sensory profile.”
The finding surprised researchers. “First, we believed that this was due to an increase in the extraction of non-volatiles,” Chahan explains. “But we never saw an effect on the non-volatiles.”
Instead, attention shifted towards volatile aroma compounds, the molecules responsible for much of coffee’s aroma and flavour.
Around the same time, Chahan and 2015 World Barista Champion Saša Šestić revisited another influential competition routine. During the 2016 World Barista Championship, Taiwan’s Berg Wu used chilled portafilters and discussed their impact on flavour perception, prompting further conversations about temperature and aroma retention.
As their research progressed, Šestić and the ZHAW team began exploring whether cooling coffee after extraction could preserve more volatile compounds in the cup.
“When Saša prepared for Brewers Cup, we realised we needed a practical way to apply this to filter coffee brewing,” Hugh says. “We worked with designers to develop a stand with a moving arm that allowed us to swing the chilling rock in and out of the extract stream with precision.”
These experiments eventually contributed to the development of the Paragon brewer and helped move extract chilling beyond espresso and into a new frontier of filter coffee brewing.
What actually changes in the cup?
While researchers continue to investigate the chemistry behind extract chilling, baristas and competitors tend to focus on a simpler question: what does it taste like?
According to Hugh, the sensory effects can be substantial, particularly with highly expressive coffees. “For heavily fermented or anaerobic coffees, extract chilling shifts the profile from heavier and darker characteristics towards fresher fruits,” he says.
The effect isn’t simply about making coffee taste brighter, however.
“Sensorially, it increases the perception of vibrancy and acidity, but more importantly, it improves the quality of acidity,” Hugh explains.
This distinction has become increasingly important as competitors work with coffees with intense fermentation characteristics. Rather than indiscriminately amplifying acidity, Hugh says that extract chilling can help create greater balance and definition.
“For floral coffees, we’ve found it results in a softer, more elongated expression with a smoother texture and a longer finish,” he explains.
These effects may be potentially game-changing for filter coffee. Unlike highly concentrated espresso, filter coffee often relies more heavily on aromatic compounds to communicate complexity, especially in more delicate coffees.
“Filter coffee is roasted lighter and brewed at a much lower strength,” Hugh says. “Since the TDS is relatively low, often 1.3% to 1.4%, we cannot afford to lose the aromatics that provide the vibrancy.”
For this reason, Hugh believes extract chilling will become especially relevant to Brewers Cup competitors and baristas working with high-quality filter coffees.
Is chilling actually the important part?
Although extract chilling is often described as a cooling technique, recent research suggests that chilling itself may not be the most important factor.
In a recent master’s thesis conducted at ZHAW, researchers compared filter coffee brewed over metal spheres held at different temperatures. The theory was simple: colder spheres would produce stronger results.
Instead, the findings only raised further questions.
“The temperature of the sphere seemed to have no real significant effect,” Chahan says.
In their experiments, coffee brewed over a room-temperature sphere produced similar or even greater increases in volatile compounds than coffee brewed over a frozen sphere.
“The whole extract flowing over the ball produces good results,” he says. “But the room-temperature ball, with the whole extract flowing over it, is often the best.”
These findings challenge one of the main assumptions of extract chilling. If colder surfaces aren’t always producing better results, then other variables are at play. “The phenomenon is real,” Chahan says. “But we don’t understand it fully.”
His current hypothesis is that when coffee flows across the surface of a sphere, it forms a thin film that changes the balance between heat transfer, evaporation, and volatile retention. But exactly how those processes interact remains unclear.
One size doesn’t fit all
What is becoming increasingly apparent, however, is that the effect is highly dependent on the coffee being brewed. “The sensory effect is real, but not for all coffees,” Chahan says.
“I believe that the better the coffee is, the more high volatiles are in the coffee,” he adds. “For some coffees, particularly good coffees with a high proportion of high volatiles, we see an effect. For more mainstream coffees, it basically results in no effect.”
In other words, extract chilling may not be a universal upgrade for filter coffee brewers everywhere. Instead, it may simply be another tool that delivers tangible results only when paired with the right coffees.
A new competition variable?
As extract chilling continues to evolve, many competitors are beginning to view it less as a novelty and more as another variable to be explored when dialling in high-quality coffees.
Chahan points to one early competition example involving Šestić that highlighted just how dramatic the sensory impact could be.
“The jury believed that the effect was too strong to be real,” he says, recalling a Brewers Cup performance where judges reportedly questioned whether the resulting flavour profile could be produced by coffee alone, resulting in disqualification.
Today, the conversation has shifted considerably. Rather than debating whether extract chilling works, the devil is now in the details: researchers and competitors are increasingly asking when and why it works, and which kinds of coffees will benefit most from its application.
“In competition settings, it could become a way to elevate particular coffees,” Chahan says. “But it’s a little bit too complicated for the mainstream.”
Hugh argues that the industry still needs to move beyond treating extract chilling as a trend or shortcut. “For it to become a universal standard, the industry needs to move beyond seeing it as a hack and treat it as a fundamental brewing variable,” he says. “Much like grind size or water chemistry.”
Whether extract chilling becomes commonplace in Brewers Cup remains to be seen. Researchers still don’t fully understand the science behind its effects, and the evidence suggests it won’t benefit every coffee equally.
But as more competitors explore what happens after extraction, one thing seems increasingly clear: the future of coffee brewing may involve paying just as much attention to temperature after brewing as before it.
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- What is extract chilling in coffee?
Extract chilling involves brewing espresso or filter coffee immediately over a chilling surface, such as a metal sphere or rock. The goal is to cool the liquid rapidly, preserving volatile aroma and flavour compounds that would otherwise escape.
- Does extract chilling work for all coffees?
No. Research from ZHAW suggests the technique delivers the most noticeable results with high-quality coffees that contain a large proportion of volatile compounds. For lower-quality, less complex coffees, the sensory difference is minimal.
- Do you need a frozen surface for filter coffee extract chilling to work?
Surprisingly, no. ZHAW research found that a room-temperature sphere often produced equal or greater increases in volatile compounds than a frozen one, suggesting temperature is not the primary factor at work.
Photo credits: Nucleus Coffee Tools, Jordan Montgomery
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