How to Tamp Correctly
After your portafilter has been on your machine long enough to be nice and hot, take the portafilter off the machine and make sure your filter basket is clean and dry.
Grind coffee into the portafilter and fill so that the coffee grounds are mounded somewhat above the edge of a filter basket.
Level the coffee with the top of the filter basket by gently smoothing the mounded area, into the low spots around the edge of the basket. Wipe off any excess that is above the edge of the basket with your finger.
Place the portafilter on your countertop or tamping mat and hold it so that the top of the basket is level and parallel to the countertop.
Take your tamper and make sure the base is level and parallel to the countertop as well.
Hold the tamper as shown, then apply downward pressure while keeping the tamper base parallel with the counter. Apply about thirty pounds of pressure. If you have a standard tamper, you can get an idea of how this feels by pushing on a bathroom scale. If you have a calibrated tamper, push until the tamper handle clicks with the tamper base. Please note that thirty pounds of pressure is just a rule of thumb. The number of pounds isn’t that important, but it is important to keep it consistent every time.
Release downward pressure gently, then gently and slowly pull straight up.
After the tamper has been removed, take a look at the compressed coffee in the filter basket. There should be a nice even surface, with the walls of the filter basket being even on all sides.
Put the portafilter into your espresso machine grouphead and start extracting. If you have a bottomless portafilter, observe how the shot is coming out of the bottom of the filter basket. It should look something like in this video if the tamping is correct and if the grind size has been dialed in correctly.
After extraction has completed, take a look at the extracted puck. It should look something like this if the tamping and grind size are correct.
That’s all there is to tamping. There are a lot of people that use other techniques, but there isn’t really a need to complicate it more than that.
Here are some common issues that happen during tamping.
Failing to keep the tamper level with the top of the filter basket will create uneven extraction. Water takes the path of least resistance, so rather than flowing evenly through the compressed coffee, the water will flow towards the side that is least compressed.
Filling the filter basket with too much coffee creates uneven extraction and makes your espresso machine grouphead dirtier. Filling the filter basket with too little coffee also creates uneven extraction. If the shower screen touches the compressed coffee when the portfilter is put on the machine, there is too much coffee in the basket. You can check if this is the case by putting the portafilter on the machine, and immediately take it off and see if there is an indentation on the coffee. Another indicator of this is if the extracted puck sticks to the shower screen after extraction. You can tell if the basket is filled to little if there is a puddle on the top of the coffee after extraction has completed.
Pushing down on the surface of the coffee grounds prior to tamping creates uneven density when compressed with the tamper. Water will flow through the less dense areas and avoid the dense areas, creating uneven extraction.
Lifting the tamper quickly after applying tamping pressure can suction the compressed coffee puck and lift it slightly, creating a slight air gap around the edge of the basket. Water will take the path of least resistance through this air gap and not flow through the compressed coffee evenly enough to provide consistent extraction across the coffee puck.
Spinning the tamper after applying full downward pressure, especially when pressure is still being applied can crack the puck and create channeling. Spinning is something a lot of people do, but it is not necessary and can create more issues than benefits. Water will find cracks in the puck and flow through them and avoid the denser areas of coffee, making extraction uneven.
Many people knock the edge of the portafilter with their tamper to distribute coffee in the basket or to knock off grains of coffee from the edge of the basket. Many people also knock the portafilter on the countertop top prior to tamping. This creates uneven density or can crack the puck if done after tamping.
Some people don’t level the surface prior to tamping. If you do this, the surface of the compressed puck will look fine, but extraction will be uneven due to some areas being denser than others. The area where the mound was will be denser and around the edges it will be less dense, causing water to flow away from the dense areas to the less dense areas.