In his 2021 World Barista Championship (WBC) routine, three-time Australian Barista Champion Hugh Kelly introduced to the world stage a concept known as “extract chilling”. 

During his performance, Hugh extracted his espresso over a frozen metal cube before allowing the shots to cool down. In his routine, he explained that this extraction method helps to preserve the flavour compounds in the coffee and heighten sweetness in particular.

Hugh was introduced to extract chilling by researchers at the Zurich University of Applied Sciences (ZHAW), but how exactly does it work? 

To find out more, I spoke to Hugh, one of his WBC coaches Pete Williams, and Professor Chahan Yeretzian. Read on to learn more about extract chilling and its effects on coffee flavour.

You may also like our article on cold-pressed espresso.

What is extract chilling?

As part of his 2021 WBC routine, Hugh extracted espresso over a frozen metal block attached to the side of his cups. As the espresso extracted, it flowed across this block, which quickly cooled down the liquid as it entered the cup. 

While Hugh was the first WBC competitor to use this technique, Berg Wu, a Taiwanese competitor in 2016, used a technique that was very similar.

In Berg’s WBC-winning performance, he immersed his portafilters in ice before extracting his espresso shots. He explained that by cooling down the portafilters, he preserved more acidity and floral notes in the coffee.

As the ZHAW team conducted research between 2017 and 2020 on the impact of the water temperature in espresso extraction, they recalled his performance, and started to develop the foundation of the extract chilling technique.

Similar techniques have long been used by chefs and culinary professionals to preserve flavours and aromas in different foods – particularly vegetables, fruits, and nuts. One of the most common methods is known as “blanching”. 

Blanching is when food is briefly submerged in boiling water, before being immersed in ice water to “shock” the food and prevent it from cooking further. By doing so, it’s likely that more flavours and aromas will be maintained. 

During his WBC performance, Hugh explained that the first 12g of espresso were rapidly chilled. This is the point at which the extract is heavily loaded with volatile compounds, and thus the point where the cooling has the most important impact.

In essence, by quickly cooling the espresso down, more volatile compounds can be retained, thereby preserving more flavours and aromas in the coffee.

Similar brewing methods

When we talk about cold coffee, we generally think of iced coffee or cold brew. The latter is made by brewing coffee at room or cold temperatures for hours. The lower temperatures and longer brew time help to enhance sweetness and minimise acidity. 

Cold brew is widely popular in the coffee industry, but it can often reduce the vibrancy and complexity of certain coffees. As such, other brewing methods have been developed which minimise flavour and aroma loss. 

First created in Japan in the 1960s, flash brew coffee is traditionally made as a pour over. Similar to brewing hot coffee, the same water to coffee ratio is used, but a proportion of the water weight is frozen as ice. 

The ice is then placed in the brewing receptacle, before the remaining water is heated to target brewing temperature and poured over the coffee grounds. The coffee is extracted hot, but then rapidly cooled to preserve more of the volatile compounds.

How was extract chilling developed?

The origins of this technique come from research conducted by people at ZHAW, exploring the impact of brew temperature on the volatile aroma compounds in coffee. As part of this research, Chahan discovered a link between heat and volatile aroma compound loss in coffee.

In 2019, Sasa and Chahan formally conducted further research on extract chilling. Sasa brought Sanremo Coffee Machines to ZHAW for research on espresso machines, which is where Hugh became involved.

Hugh then saw that there was huge potential for extract chilling to improve the coffee he was using for his World Barista Championships espresso: Coffea eugenioides, a “rediscovered” parent species of arabica. 

Other 2021 World Coffee Championship competitors also used eugenioides, including 2021 WBC winner Diego Campos and 2021 World Brewers Cup winner Matt Winton. 

Eugenioides is believed to be native to East Africa, but is now grown on a very small scale in other countries along the Bean Belt. Hugh first encountered eugenioides in 2015 during a trip to Colombia. 

“I scored eugenioides 95 points during my first pass around the cupping table,” he tells me. “It was interesting, it had no acidity, but it had these floral and raw sugar tasting notes. 

“Half of the people at the cupping hated the flavours, while the other half didn’t know what to think,” he adds. 

Four years later, Hugh came across eugenioides again when German barista champion Wojtek Biaczak used the species in his WBC performance. 

“There was something really interesting about it,” Hugh says. “No one had tasted anything like it.” 

Pete Williams is the 2014 Irish Barista Champion. 

He also helped train Hugh for the 2021 WBC. He explains how eugenioides extracts differently to arabica coffees. 

“We struggled at first to get it to taste the way we wanted, mainly because we were using traditional arabica-focused extraction techniques,” he says. “You can apply these techniques, but they won’t bring out the best in eugenioides.” 

Hugh explains that this rare coffee species has an “unusual profile that leans towards green tea, beefy, umami, and savoury notes”. 

He adds: “It inherently has lots of sweetness, but almost no acidity. It also has a marshmallow-like texture.” 

Hugh wanted to highlight the fruiter notes of eugenioides to brighten the overall flavour, however, there were some challenges in achieving this. 

“As a species, it’s one of the best-feeling espressos you can drink,” Pete says. “But you lose this tactile aspect when you try to highlight the fruity notes in extraction. 

“However, extract chilling resolves this issue,” he adds. 

Partnership with ZHAW 

Hugh, Sasa, and Chahan began experimenting with these techniques in more detail to understand more of the science behind chilling espresso.

“We carried out some testing and blind tastings and found that 12g extracted over the frozen blocks resulted in a great espresso,” Hugh explains. “There were some tropical notes and a more pronounced fruit-forward aftertaste. 

“It was like the coffee had woken up,” he adds.

Although sensory research about both the species and the method is still in its early stages, extracting espresso using extract chilling helped to brighten the flavours of eugenioides. 

This is because volatile compounds have higher vapour pressures than non-volatile compounds when at room temperature. So as temperature increases, volatile compounds absorb more kinetic energy, which means they evaporate more quickly. 

By rapidly cooling the eugenioides extract, more of the volatile compounds which contribute to these fruitier flavours were retained in the liquid. 

Chahan tells me that extract chilling can preserve an average of up to 10% more volatile compounds than traditional espresso extraction – meaning more of the delicate flavours can be retained. 

“In some cases, up to 40% more volatile compounds can be preserved,” he adds.

Wider applications in the coffee industry

Despite the fact that more research is needed on extract chilling, the results appear promising. And as the WBC is known to influence the wider specialty coffee sector, could we see this technique become more prominent in the future? 

Well, while the concept was first explored by Hugh for eugenioides, it has also been used for arabica coffee, and it seems to work well in both cases. 

“However, keeping portafilters in the freezer and bringing them out moments before pulling each shot is too impractical for many coffee shops,” Pete says. 

Coffee shops would also need the facilities to store a large number of frozen metal blocks, requiring investment and space that many may not have access to. 

Furthermore, customer reaction is also important to consider. It can already be challenging for baristas to effectively and clearly communicate with customers on a number of factors, such as origin, varieties, processing methods, and roast profiles. Baristas could struggle to break down the concept for some entry-level specialty coffee consumers. 

Hugh agrees that coffee shops using extract chilling on a smaller scale is more likely until technologies and methods are introduced to make this more accessible to the café setting. “At ONA in Melbourne, we’re already using extract chilling for some of our reserve arabica coffees,” he says.

The World Barista Championships are known for driving innovation in the coffee industry, and extract chilling is no exception. 

However, whether or not this experimental method of rapidly chilling coffee is accessible or scalable remains to be seen. But as more research is carried out on extract chilling and its effects on coffee extraction, we could see this unique brewing technique used more frequently across the world.

Enjoyed this? Then read our article on flash brew coffee.

Photo credits: Jordan Montgomery

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