We have written at length on how extraction works and the science behind what is happening in the coffee bed. This is all really important to truly understand how to optimize your espresso at home or in the cafe. However, there is one stone that we have left unturned to date. How to achieve even extraction.
What is "Even Extraction"
An even extraction is the result of a three-step process for 1) dosing your coffee correctly from your grinder, 2) distributing your coffee in the portafilter, and 3) even, consistent tamping. To start, we'll re-emphasize some classic fluid dynamics wisdom.
Water will always choose the path of least resistance.
So, we start with a basket of freshly ground coffee, that has been tamped and inserted into the group head. If it has been evenly distributed and tamped, the density of the coffee grounds will be identical throughout the basket, meaning that all of the grounds will have the exact same interaction with the water (both time and pressure). Keep in mind that pressure is a measure of resistance, so an uneven distribution of coffee will also lead to varying pressure across the puck. But let's keep this simple.
If the distribution of particles is uneven, water will flow at different rates through the bed of coffee. In this example, the right side of the portafilter is more densely distributed than the left. Therefore, the coffee will be under-extracted from the left and possibly over-extracted on the right. Either way, it's not good and usually leads to a sour espresso.
This type of uneven extraction is fairly common amongst new baristas. Something as simple as roughly handling your portafilter will cause the puck to unevenly distribute the grounds. The biggest challenge with a defect in particle distribution is that it is a hidden problem. The same volume of espresso may extract in the same time as an evenly distributed puck, but it will taste very different. That is why we recommend bottomless portafilters. Bottomless portafilters will allow you to see the different rates of extraction and the telltale signs of channeling. If your espresso machine is a splattered mess after pulling a shot with a bottomless portafilter, you have some uneven extraction occurring. The water is finding a path of less resistance and spraying out of it.
How do I evenly distribute my coffee?
There are more opinions about coffee distribution than there are about wealth distribution, and those that argue the virtues of each are just as passionate! Our opinions are highly informed by teaching individuals how to achieve consistent results as well as by work done by Matt Perger at Barista Hustle. He has done some excellent analysis of distribution techniques here, but we will try to summarize them for you.
First, we need to set ourselves up for success at the grinder level. Distributing the coffee evenly in the portafilter from the grinder is one of the easiest ways to get great results. If you grind into one side of the portafilter - it's going to be very hard to evenly distribute that coffee - even if you follow all of the tips below. Below is a shot of Victoria evenly distributing from a Eureka Zenith grinder into a triple basket 58mm portafilter.
The Distribution Technique
Matt did research on the following techniques: Side to side and vertical tapping, the Stockfleth method or just tamping right out of the grinder. He found that the side to side and vertical tap yield a more consistent distribution. We certainly would encourage you to try the tap method as well as the Stockfleth. Both take quite a bit of practice but will yield consistent results if done correctly. The key here is to ensure that the entire basket is filled evenly with grounds, all the way to the sides. Those are the most common problem areas of under-extraction.
How to Tamp
How many opinions can there be about as simple an action as pushing coffee grounds? At Clive, we prefer to keep it simple. Hold the tamper like you would hold a doorknob and evenly tamp using enough pressure that you have to put a bit of shoulder into it. In the end, it should feel the same as pushing strongly against a flat counter. Consistency is more important than having a specific amount of pressure, so don't hurt your wrists. The real key, however, is making sure your tamp is perfectly level. If it is tamped even at a slight angle, it will create an area of uneven distribution and a path of less resistance. Keep in mind, the technique he recommends is for professional baristas who have to do this hundreds of times a day. For the home user - just focus on straight.
If you need more tamping help, check out how to Perfect Your Tamp Skills on our blog. Once you nail it all - you can start to work on replicating the cafe at home. If you'd like to take a look at some of ours, check out Espresso Drink Recipes on our blog post.