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Compressing ground coffee into a portafilter seems like it ought to be pretty straightforward. And no matter what anyone tells you, it is. The goal is consistency... do it the same every time! You want a consistent dose, tamping pressure and an even, flat surface to the compressed coffee.
Traditionally, a single espresso is made using 7 grams of coffee and a double is 14 grams. Using those exact amounts is not as important as using the same amounts every time. Probably the most popular and easiest dosing method is to simply overfill a portafilter and level it off with finger swipes. Alternate methods include timed grinding, measuring with a coffee scoop and actual weight using a scale.
However you get the coffee in, it should be relatively clump free and as level as possible prior to tamping. So if you didn’t finger swipe, give the portafilter a shake or knock with a tamper to level things off. Flow through the puck is determined by grind size and tamping pressure. If tamping pressure is consistent then grind fineness can be adjusted to change the flow rate. The standard tamping pressure is 30 pounds. You can use a bathroom scale to help you get a feel for what 30 pounds feels like. An alternative is using a calibrated tamper. A popular model from Espro is spring loaded and clicks when 30 pounds of force is applied. Simple.
Begin with the portafilter edges parallel to countertop. Angled handle portafilters will naturally rest in this position. With straight handled portafilters, raise the handle to make it parallel. With things lined up, push down using pressure from your palm while your fingers keep the tamper head flat. When you feel you’ve reached 30 pounds of force stop, release some pressure and give the tamper a twist. This polishes the surface of the puck and assures no coffee sticks to the tamper. Remove the tamper and check to see the coffee is level from side to side.
Tamping at 30 pounds is a starting point. Some prefer to grind finer and tamp lighter or go courser and tamp with more pressure. The point is to be consistent! Once you’ve developed a feel for tamping pressure you’ll be able to play with it along with other factors.