Espresso channelling

Is espresso channelling really that bad?

Jordan Montgomery speaks to coffee researcher Dr Robert McKeon Aloe about whether it is time to challenge long-held beliefs about the impact of espresso channelling on the final cup.

For many baristas, channelling is both feared and hated in equal measure. Defined as the localised extraction of certain points of an espresso puck, it typically occurs when pressurised water finds pathways of least resistance in the ground coffee.

Rather than flowing evenly through the entire puck during the course of the shot, it extracts some areas more than others. This can lead to a range of issues, such as killing clarity and producing muddy flavours.

It is often notable by the spray it causes from the bottom of a naked portafilter – or an obvious, blond streak coming from the spouts of a portafilter. However, to add insult to injury, there is more than one type of espresso channelling:

  • Side channelling, in which water unevenly flows down the sides of an espresso portafilter rather than evenly through the coffee
  • Spot channelling, where variations in particle size distribution force water through specific points in the puck

Dr Robert McKeon Aloe is the author of Engineering Better Espresso: Data-Driven Coffee. He explains that there is rarely an extraction without some form of channelling – but some forms may have a larger impact on the final cup than others.

“Even with the best puck preparation, micro channelling occurs,” he says. “Major channelling is usually caused by an uneven tamp, uneven water input, or uneven filter baskets.

“When talking about channelling, side channelling has the biggest impact. Side channelling is also influenced by uneven water and heat flow.”

Robert adds that heat flow is a factor that’s often overlooked. “When the sides of the basket heat faster due to water flow and heat from the basket connected to the group head, the sides extract faster,” he says.

Is espresso channelling as bad as we think it is?

Espresso channelling can be incredibly frustrating. And, often, the first instinct when it occurs is to halt extraction, discard the coffee, and simply try again. However, there is a growing body of research that suggests channelling may not have the negative impact on extraction many believe.

During a presentation in London in 2019, Professor Stephen Abbott suggested that since major “channels” pass through such a small portion of the coffee puck, they may not affect the flavour of the espresso as significantly as once thought.

To test this theory, Robert intentionally caused channelling during espresso preparation by using toothpicks to artificially create channels in the coffee puck. He found that despite having a visible effect during extraction, the intentionally channelled shots don’t have as much of a decreased extraction yield as expected.

“The experiment showed the channel closed up and the shot wasn’t completely ruined by the channel in both taste and extraction yield,” he explains. “To me, the result means that channelling due to bad puck preparation is more recoverable than something in the system like a group head issue.”

Robert’s experiments indicate that despite there being points of the coffee puck that are over-extracted, the higher water flow through these points results in the rest of the coffee puck being under-extracted. Ultimately, this creates a relatively balanced extraction – at least in terms of extraction yield.

Furthermore, the total dissolved solids (TDS) are similar to that of a properly extracted and non-channelled espresso. However, uneven extraction of the coffee can still cause unfavourable results when it comes to taste.

“From an extraction perspective, the coffee grounds around the channel are over-extracted and the rest of the coffee is under-extracted relative to the channel,” Robert says.

“Usually, this gives the worst of over-extraction and under-extraction combined. In terms of flavours, more bitterness comes from the over-extracted coffee and more sourness from the under-extracted.”

How can espresso channelling be avoided?

As the science around espresso continues to evolve, avoiding the risk of channelling is no longer the number one priority of every barista. The impact of factors such as flow rate, temperature, and grind size have all become equally important variables that technology with the means to manipulate them develops.

That said, if steps can be taken to reduce espresso channelling then they should. And, fortunately, there are more techniques and tools than ever before that can be used to mitigate it.

Distribution tools enable baristas to evenly distribute ground coffee throughout portafilter baskets so that water will evenly pass through the coffee.

Having gained immense popularity since the release of World Barista Champion, Saša Šestić’s OCD tool in 2015, a wide variety of distribution tools are now used by home baristas, in coffee shops, and in coffee competitions.

Another development in recent years is improved technology and a focus on tamping, including the development of automated tamping machines such as the Slingshot Kilo. As uneven or angled tamping pressure can cause espresso channelling, automated tampers allow for speedier and more consistent preparation.

For professional baristas working in coffee shops, Robert believes that combining basic skills, knowledge, and experimentation is key to creating more consistent and less channelled espresso.

“This goes back to the fundamentals of cleaning your group head and basket, consistent puck preparation, and even tamping,” he says. “You can’t do much once everything is busy, but when things are slow, use a bottomless portafilter and visually look for unevenness. Try some experiments to see what channelling looks like.”

He also highlights a rising interest in the composition of filter baskets, including precision spacing and even the diameter of holes in the filters.

“This year is turning into the year of filter baskets,” he says. “I hope that more attention will be paid to water input and making sure there is even water distribution. Even when using a bottomless portafilter, often a cone forms in the coffee extractions – it’s hard to see whether the centre of the part is actually extracting or not.”

In his experiments, Robert found that the centre of the puck simply doesn’t extract well at all. This, he concludes, could hold the key to improving extraction for everyone. “If we pay a bit more attention to understanding how much coffee extracts from the middle versus the side and work to improve that from the top, I think we can reach higher efficiency for coffee extraction across the industry.”

Is espresso channelling really that bad?

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