Have you ever wondered why your espresso shots taste so off? Are they extracting at an excessive rate, spraying, or flowing out of one side?
If you’ve encountered any of these issues while pulling a shot, the most likely cause is something called “channelling.”
To learn more about channelling, how it affects extraction, and how to avoid it, I spoke with Jill Hoff, Monogram Coffee’s Director of Coffee and Education, 2020 Canadian Barista Champion, and 2021 World Barista Championship semifinalist. Continue reading to learn more about her observations.
When pulling an espresso shot, you want water to flow evenly and at a consistent rate through the entirety of your coffee puck. Regrettably, this is not always the case.
“Channeling occurs when your coffee bed has weak points,” Jill explains. “Water can quickly pass through these areas, impeding an even flow through the puck.”
This results in a combination of under- and over-extraction, producing an espresso shot that is both weak and sour, as well as excessively bitter.
However, channelling is caused by a variety of factors. Consider some of the most prevalent.
Too fine a grind
While we understand the importance of a fine grind for espresso, going too fine can result in channelling.
The optimal grind size will vary depending on a variety of factors, including the machine you’re using and the coffee you’re using. However, if you go too fine, the puck will become extremely dense and nearly impermeable after tamping.
This makes it more difficult for water to pass through. Water will then follow the “path of least resistance” (or paths) through the puck, resulting in the formation of “channels.”
Inadequate grind distribution
It is not uncommon for the portafilter basket to fill unevenly when filled with grounds. If this is not corrected prior to tamping, you will end up with a puck with an uneven grind density.
This means that certain areas of the puck will be denser than others, creating paths of greater and lesser resistance.
According to Jill, unequal distribution can introduce “boulders or clumps of grinds into your dose, resulting in increased density in those areas of the bed.”
When you fire your shot, the areas with less dense vegetation become the path of least resistance. This is the location of channel formation. Overextraction occurs within these channels; at the same time, they cause the puck’s denser parts to be undersaturated with water, implying that they underextract.
This is why many baristas use a tool to evenly distribute coffee into the portafilter after grinding and dosing.
Jill tells me that Duomo the Eight is one of the best tools she’s ever used.
“Using distribution tools, such as those offered by Duomo, is an excellent way to distribute precisely and consistently while avoiding the creation of channels.”
Tamping and dosing inconsistencies
As mentioned previously, distributing your grounds is critical for properly preparing the puck for extraction. Taming an unevenly distributed puck will exacerbate the channelling effect.
Jill explains why channelling occurs as a result of uneven tamping. She explains that by compressing the grounds with the tamper, the density of the puck increases, increasing the amount of pressure required to move water through it.