Ever pulled an espresso shot that runs too fast, drips painfully slow, or looks perfect—but tastes thin, sour, and bitter all at once?
This is channelling; unfortunately, this occurs when water finds weaker areas in the coffee puck and rushes through those areas instead of flowing as evenly as possible through the whole puck. Espresso is brewed under high pressure (usually 9 bars), and water always seeks the easiest path, otherwise known as the “path of least resistance”. When the puck isn’t uniform, which we’ll dive into shortly, water carves "channels" into the coffee, leading to uneven extraction.
In a channelling espresso, the high-pressure hot water is pulverising the channel with a larger-than-normal volume of water, which is essentially creating a tunnel to the bottom of the coffee puck. The grounds in the channel are being severely over-extracted, creating bitter and astringent flavours. The channel also acts as a bypass with water heavily diluting the espresso shot. In contrast, the other areas of the coffee puck have barely been touched by water. Some water does flow through the non-channelled areas, but it’s severely under-extracted, creating acidic flavours. As a result, a channelled espresso shot is a strange mixture of bitter acidic tastes and weakness due to the dilution.
What should I be looking out for?
A useful way to recognise espresso channelling is to look for colouring issues. Typically, at the start of a well-extracted espresso, the colour is a deep dark brown or, in some cases, even a dark maroon, which is beautiful to look at, especially if you have a naked portafilter without spouts; it’s quite mesmerising.
The colour at the start of the extraction depends on many factors, such as the roast profile, grind size, and even the pressure and temperature of your espresso machine. At the start of the extraction, the high-pressure hot water (90-96°C) is at its busiest – dissolving the smallest solubles fastest.
As more water flows, the espresso passing through the basket gradually becomes weaker as there is less to extract. If you have extracted a few shots beforehand and it all looks relatively similar with the rich brown colour at the start, great! But if suddenly it’s a much lighter shade of brown at the start of the shot, sometimes yellow, or even just water, something is going wrong.
What could be causing this?
There are several potential causes of channelling. Let’s review each of these separately, so you can identify which is the likely cause.
Why your coffee grinder matters the most
If you are preparing a budget for your home setup, then ensure that a significant portion of this budget is used on a coffee grinder. The grinder is the most influential piece of equipment and is arguably more important to flavour than the espresso machine. Imagine baking muffins in the oven, and they’re all very similar in size; the cooking duration and result are likely to be consistent for each muffin. But what if the muffins were all different sizes? Would they cook the same? Of course not. That is the difference between a high-quality grinder and a low-quality grinder. A good quality grinder will give you consistency of grind.
With a lower quality grinder, the coffee grind particle size is less uniform. Think about the different-sized particles in a coffee puck. We know that water under high pressure passes through the areas of least resistance, so there might be an area of the puck that has a slightly higher concentration of larger-sized particles. The water will find it, flow through them, and create a channel. The more uniform your grind size, the more even your extraction will be across the entire puck of coffee.
There are many factors that define a high-quality grinder, but it often comes down to the burrs - the material they’re made from and the geometry that determines how they break beans into particles.
We performed a short test to understand the difference between a great grinder (Niche Zero) and a cheaper alternative (blade grinder). For those who are unfamiliar with a blade grinder, it uses fast rotating blades to chop the beans into coffee grinds. The result is a random selection of different-sized coffee grinds. The only control you have over grind size is how long you grind for; less time is coarser and more time is finer, but this is very difficult to keep consistent from grind to grind.
There was just one problem with testing the blade grinder – it simply couldn’t grind fine enough. Even at its absolute finest, the longest espresso shot I could achieve was just 17 seconds at a 1:2.2 ratio. So rather than force an unfair comparison, I used the same short shot time for both grinders. And that’s where things got interesting.
Even at only 17 seconds, the Niche Zero produced a noticeably richer, more concentrated espresso. The flavour still leaned slightly sour (as you’d expect from a fast shot), but it had a pleasant acidity and a much fuller body. The blade grinder, on the other hand, told a very different story. The shot was thin, harsh, and confusing – both sour and bitter at the same time. Normally, a fast shot points to under-extraction and sharp sourness. But here, the inconsistency of the grind introduced loads of microfines, adding bitterness and a dry, unpleasant finish.
What’s really happening is uneven flow through the coffee puck. With such a wide mix of particle sizes, water finds weak points and rushes through, creating channelling. The result? An espresso that’s completely out of balance and far from enjoyable.
Distribution of coffee grinds
Even uniformly ground coffee particles will contain a small amount of variance, so it’s important to ensure they are evenly distributed when placed into the portafilter. If the grinds are evenly distributed, then when the puck is tamped, there is a level bed of coffee.
The Niche grinders come with a grind cup that measures 58mm in diameter, which fits nicely into most portafilters. After the coffee is ground into the cup a 58mm portafilter can be placed on top and then inverted. This distributes the coffee, ready for puck prep. There are several other distribution tools such as the WDT, while many experienced home baristas are comfortable with a small hand tap to distribute the grinds.
If the coffee isn’t evenly distributed before tamping, then the shot could end up channelling. This diagram below demonstrates a poor shot and a good shot. The left side of the diagram shows a coffee dose on the side of the basket, which would create an uneven tamp – water will naturally pass through the right side of the puck quicker. Even if you were able to achieve a level tamp, there would be a higher density of coffee on the left, which would have the same result. The faster-flowing right would be weak and bitter, whereas the slower-flowing left would be strong & sour, together unbalanced.
The right side of the diagram shows a level puck and, as a result, an even flow of water through the coffee puck. This is simply due to even distribution, which leads to an even tamp with adequate pressure, 10-20kg. With even water flow, the coffee should consistently achieve a similar extraction yield across the entire puck rather than lower and higher depending on flow direction.
Failing at the final stage
Sometimes, channelling can be caused by rushing at the final stage. If everything was perfectly executed, and then you inserted the group handle into the group head a little bit too aggressively, you may dislodge the perfectly prepared coffee puck and create a gap around the outside or even a crack in the actual puck! The pressurised water will easily find these gaps or cracks and create channelling.
It seems like a fairly harmless thing, but you’ll lose up to an entire minute to remake an espresso because it channelled, rather than take the extra 1-2 sec per espresso. So, please be careful inserting it into the group head; it could ruin the entire espresso!
A quick recap on avoiding channelling in your espresso
To avoid channelling, focus on three key areas: grind quality, even distribution, and careful puck preparation. Get these right, and water will flow evenly through the coffee, delivering a balanced, consistent extraction. It’s a small amount of extra care that makes a big difference in the cup.