Espresso 201

It only took us four years, but we’re finally sharing the second part of our espresso-making blog! In our Espresso 101 post, we mentioned that when making modern espresso with lightly roasted coffee (Turbo shots excluded), it’s best to have outrageously long shot times (40, 50, 60 seconds or even longer) in order to get shots that are sweet, fruity, and full of character and complexity. Now, we’re focusing on the required pressure and flow rate to actually make that happen.

Channeling

People may assume that in order to slow the shots down, you need to grind finer. But if you try this, you’ll most likely get terrible results! That’s because very fine grinds with the traditional nine bar pump pressure setup are a surefire guarantee to cause channeling–and nothing good comes from channeling (both with espresso and pourover/auto-drip). It happens when some of the water that we want to have flowing evenly and uniformly through all the coffee grounds instead flows faster and more turbulently through small pathways in the grounds. This high energy flow extracts tannins, or tannin-like flavor compounds, that create astringency, which is that mouth-drying sensation you sometimes get in the aftertaste when drinking coffee (or in red wine). Since that’s generally undesirable, we’ve got some solutions for you.

So What’s The Solution?

First, make sure that the grounds in the puck get saturated with brew water very thoroughly and evenly before hitting them with pump pressure, which is also known as preinfusion. You’ll want low pressures and low flow rates here. Once the grounds are fully and evenly saturated, they will allow the higher pressure water to flow more evenly through them, and thus, avoid any channeling that ruins the espresso. However, despite a nearly perfectly even distribution of grounds in the basket and an almost flawless preinfusion, nine bars of pressure is still likely to cause some channeling, especially with lighter roasts. So turn down that pressure!

Pressure & Flow Rate

Speaking of pressure, let’s talk about the roles that pressure and flow rate play here. It’s important to note that pressure and flow rate are not the same thing, but we need to understand their relationship in order to avoid channeling and ruined espresso. Your grind size is what determines the exact relationship between pressure and flow rate. Pressure is best described as the force that is causing the water to move (really, it’s the force per unit area, but that’s not relevant here), whereas flow rate is how fast the water moves–you could measure it in miles per hour or meters per second. If you kept everything else the same but decreased the water pressure, that would give you a lower flow rate. In terms of channeling, it isn’t the pressure itself that’s the problem, it’s the flow rate. Fast flow means the water is carrying more energy, and that energy can break the bonds that hold the tannins and tannin-like compounds in the coffee grounds. Turn down the pressure in order to decrease the flow rate at a given grind size and save your espresso!

Let’s Put This In Practice

Now, let’s put this all into practice. If your espresso machine allows it, the lower pressure and the slower your preinfusion is, the better. One bar is a wonderful preinfusion pressure, but three bars will do nicely too. It’s best not to turn on your pump until there are lots of drops falling into your cup, as it's much safer to “over-preinfuse”. There aren’t really any negative flavors caused by this, although you will get a very slight reduction in the amount of sweetness/fruit flavors/etc. in your espresso. Next, turn your pump pressure down to around seven bars. Some machines won’t allow you to directly adjust pump pressure so you’ll have to “fake it” by adjusting your over pressure valve (OPV) to open at a lower pressure.

Finally, if you can, either slowly decrease the pump pressure as your shot gradually starts speeding up, or turn the pump off and let the line pressure/preinfusion pressure (“post-infusion”) finish the shot. This is helpful as shots always run faster and faster as time progresses because some soluble compounds have been removed and put into your cup, so there is less resistance to flow. That fast flow can cause channeling near the end of the shot.

If you put any of our tips into practice, let us know how it goes!