When you take the filter out of your espresso machine you might notice some tiny holes in the coffee bed. What’s the reason for that and is it actually a problem? Let’s find out.
Channeling is the forming of little tunnels in the espresso coffee puck during brewing, causing the espresso to be weak and bitter. There are five reasons it happens:
- Ratio and basket
- Coffee Distribution
- Grind size too fine
- Machine pressure is too high
Let’s dive a little deeper into these reasons and why you should care.
What is channeling in an espresso puck
Before we get to channeling, I should explain what the word puck means in this context. The coffee puck are the coffee grounds in the filter basket after using them. Coffee expands a little when you add water so it squishes itself into a relatively hard disc of used coffee grounds. The shape is similar to a hockey puck which is where the word comes from.
So what is channeling? After you brew your espresso you take the filter out of the machine, you can inspect the puck. If you can see tiny holes that go through the puck, that’s channeling.
What’s the problem with your espresso puck channeling? An espresso machine pushes water through the coffee under high pressure. Sometimes even the high pressure isn’t enough to push the water through the bed evenly. The water always flows through the path of least resistance. So if it can’t flow through the bed uniformly, it’ll look for the ‘weak spot’. Now all the water flows through that weak spot and erodes the coffee bed in that spot creating a little tunnel also called a channel.
The problem with channeling is that it means most of the water doesn’t touch the coffee grounds and therefore doesn’t extract anything. Also, the coffee grounds around the channels will be over extracted. So your espresso will be too weak and bitter at the same time. It will be weak because only a tiny bit of the coffee is extracted but at the same time the grounds that are extracted are really over extracted.
What are the reasons your espresso puck is channeling
Channeling is not something you want to happen in your espresso machine. But what are the reasons it happens? Here is the top 5:
#1 Ratio and basket size
The first thing to check is if you’re using the right amount of coffee grounds. You want to get a ratio of about 1:2 which means one part coffee grounds to two parts of coffee in your cup. Since a single shot of espresso is 28 ml, you want about 14 grams of grounds in the basket. The ratio can change slightly depending on your wants but for most people it’s close to 1:2.
If you’re trying to push too much water through the puck, at some point the water extracts so much that the actual volume of the grounds starts shrinking. Brewing coffee is extracting the soluble materials from coffee beans but while they are water soluble they still take up space. By the time you start extracting so much this becomes a problem, you espresso won’t taste very good anyways.
Also, make sure the basket you use is appropriate for the amount of grounds you want to use. Filter baskets have different sizes so make sure the one you use is the right size. If you overfill the basket, you have a much higher chance of channeling around the edges of the basket.
That’s because there is a shower screen in the middle. This is where the water comes from. However, usually that shower screen doesn’t extend all the way to the edges of the basket. That means the shower screen pushes down on the grounds in the middle but not around the edges. That results in the edges being less dense and therefore give an easier way out for the water.
The shower screen shouldn’t push down on the coffee grounds when the filter is in the machine.
When you fill the basket, make sure the coffee grounds are distributed evenly in the basket. While you should tamp the grounds (more on that later), before you do this, make sure to distribute the grounds evenly in the filter basket.
A coffee bed might look flat after tamping but tamping alone doesn’t necessarily distribute the coffee grounds evenly. If you have less density of grounds in one spot, that becomes the path of least resistance and it’s much more likely you’ll get channeling.
Properly distributing the grounds is pretty easy. There are some tools that can help you but even without any tools it’s not difficult. After filling the basket, you’ll probably have a bit mountain of coffee. Just a single tap on the basket will already help a lot. Just tap the basket on the counter top once (don’t hit the spout). Not too hard, just a little tap will decrease the size of the mountain quite a bit.
After that you can just distribute the coffee by using your finger. Just place your finger on the rim of the basket and spread out the coffee until it’s distributed and flat. Alternatively, a few taps with the palm of your hand on the side of the basket is also an effective way although it’ll look a little less flat.
After distributing the grounds, it’s time to tamp. This is basically compressing the grounds in the basket. This increases the density by squeezing out the air from between the grounds. This increases the required pressure for the water to move through the puck. You need this pressure to extract the grounds quickly enough. If you don’t tamp the grounds, the water will come through too quickly because air doesn’t provide any resistance.
If you don’t tamp your coffee grounds at all, it will be too loose and the density will be different in different places in the puck therefore making it very easy to cause channeling.
Most people agree you should tamp your grounds for espresso but, not many agree on how hard. Some people say about 30 lbs. of pressure is the sweet spot while others say you can’t do it too hard but that tamping harder than 30 lbs. doesn’t change anything. So it seems 30 lbs. of tamping pressure seems like a good place to shoot for.
If you don’t want to do any of this, get a super-automatic machine. That type of machine does everything for you and you only have to put water and coffee in it. Learn more about types of espresso machines here.
#4 Grind Size
The first three points are the basics you should get right before looking at any of the other causes. If you got everything above right and it’s still happening, it’s time to look at your grind size. It’s the biggest cause of channeling if the other basics are good.
Espresso machines require quite fine coffee grounds. How fine exactly depends on the machine, type of coffee and even the age of your coffee. If you get channeling, the grind is likely too fine. If the coffee is ground too fine, the resistance gets too high and the water starts looking for those weak spots again.
By using pre-ground coffee, you get a grind size that the coffee producer thinks works best in most situations. And it will be close in most cases. However, because espresso is ground so finely, small differences have a big impact. If you use pre-ground coffee: A. Make sure it’s meant for espresso machines B. Try a different bag next time and see if that works better.
If you use pre-ground coffee, it’s really worth upgrading to grinding your own fresh beans. The freshly ground coffee will taste better and be more aromatic and you’ll have control over the grind size. However, to get a grinder that can grind finely enough for espresso and have good size control that can make fine enough adjustments, it’s not very cheap.
Already grinding your own coffee? Adjust your grinder to a slightly coarser setting. With espresso a small adjustment goes a long way so take small steps.
Tried this already end can’t adjust your grinder finely enough? It might be time to upgrade your grinder. If you like really good espresso and the brewing process, a good grinder is worth it.
#5 Machine pressure
Espresso machines are all a little different. They might look similar and aim to brew the same drink, but they are all a little different. Just slight differences in construction can have large effects on the end results.
Some espresso machines have really high pressure pumps. Some advertise they have pumps that can create up to 15 bars of pressure and some even more. Usually this is just at the pump and not the actual brew pressure since there are losses in between. However, some machines brew with pressures that are much higher than the ideal 8-9 bar.
The higher pressure means the puck is under more stress and the water will start looking for the easy way out quicker. And when the water does find a weak spot, the higher pressure digs out the channel much faster.
There are two reasons espresso machines have pressure that is too high at the group head:
- The machine is set up wrong from the factory. The excess pressure is released through an overpressure valve. This valve is set to open at a certain pressure. Sometimes this is just not set right. Weird but true. It will open too late and cause the pressure to rise too high.
- The machine is set up to be used with a pressurized portafilter and coarser grinds. Most people don’t have high-end grinder at home. That means they use pre-ground coffee or grounds from less than ideal grinder. If coffee is ground too coarse, it doesn’t provide enough resistance to the pressure and lets the water through too easily. Manufacturers compensate for this by adding a basket with pressure valve to increase the resistance. However, because the grounds are bigger, they extract slower so to still be able to pull a shot in about 30 seconds, manufacturers compensate by increasing the pressure which increases extraction.
In both cases you’ll have to adjust the machine by opening it up and changing the overpressure valve. This is quite easy in some machines and nearly impossible in others. It will also void your warranty. For some espresso machines like the Rancilio Silvia (Amazon) or the Gaggia Classic (Amazon) there are large communities online that can help you change the machine to your liking. However, it’s a DIY project and most normal consumers are not comfortable with this.
Favorite Espresso Tools
Besides an espresso machine, there are a few other tools that can make your espresso better. Here are my favorites:
- Tamper: A nice tamper helps you tamp your grounds in the filter for the best result. Any correctly sized tamper can do the job but a nice heavy one just feels so much better in your hand than a plastic model. This Luxhaus one (Amazon) has a nice trick up it’s sleeve to make tamping very consistent.
- Beans: Good espresso starts with good beans. Using fresh beans is a big improvement over pre-ground coffee.
- Scales: Getting consistently good espresso means you have to know how much grounds is going into the machine and how much is coming out and how long this takes. A coffee scale is going to make your espresso much more consistent and also makes adjustments a lot easier. The Apexstone coffee scale (Amazon) is cheap and doesn’t look too sleek but is just as accurate as more expensive scales. The TimeMore scales (Amazon) look and feel a lot nicer but cost a bit more.
- Grinder: To make the best of your fresh beans, a good grinder is necessary. Espresso requires a good grinder to get the best results. The Baratza Sette 30 (Amazon) is a good espresso grinder that can also be used for other brewing methods and while not cheap, is good value for money. If you prefer hand grinders, the 1ZPresso JX-PRO is one of the best options (Amazon)
- Distribution tool: After grinding you can get some clumps in the coffee grounds. Those clumps should be broken up so the water can extract all the coffee grounds equally. Distribution tools are very simple things but this one (Amazon) is beautifully made and will look good in your kitchen.