A Tamper is a tool used to flatten and pack powdered coffee into the portafilter of your espresso coffee maker.
Coffee Tamping ‒ the simple act of compacting the ground coffee into the portafilter so that it forms an even puck ‒ is a simple but crucial technique that you’ll need to learn. A puck is the tablet-shaped mass of compact coffee that is formed after tamping. Why is this important? If you don’t tamp properly with adequate force, then you leave holes and channels in the coffee puck. This allows water to take the path of least resistance and exit through the portafilter without actually extracting the oils in the coffee. If you tamp too hard, you might over extract, resulting in blacker coffee and poorer taste.
So how hard do you tamp? Well most people in the know recommend using about 12‒14 kg of force. But that is hard to perfect unless you practice tamping over a weighing scale. Try to keep it consistent and steady, resulting in a flat and polished coffee surface with no visible cracks or holes. The edges of the portafilter should be sealed with the coffee to prevent the water from seeping around the coffee rather than through it. You’ll need to find a good solid metal tamper that fits the portafilter well and ensures consistent pressure. Most domestic espresso coffee makers come with a “pressurized” portafilter that provides an artificial mechanical reverse pressure to compensate for imperfect tamping. Not what a perfectionist needs!