Tamping for espresso has been riddled with an amazing amount of folk-knowledge. Tamping pressure matters. If you press until the tamper stops moving down, and keep it flat, it’d be awesome. Tamping is fairly simple. You’re compressing the coffee grinds to promote even extraction within a ergonomic and consistent way.
Let’s break the factors down a little further:
1. Compressing the Grinds
When there’s atmosphere between the grinds, water can treat it as a shortcut and rate through. When there’s no (or minimum) atmosphere, the grinds will slow the water down.
Sadly, coffee grinds are not always dense and ‘someplace between compact and fluffy’ isn’t quite precise. So we need to compress them together, eliminating that space. This way, the grind size is the deciding factor for flow rate; not the amount of atmosphere between the grinds.
2. Boost an Even Extraction
Tamping should create a bed of coffee with even density. Density means there’s no shortcuts or ‘easy manners through’ to the water. You truly want the water to flow throughout the entire mass of grinds as evenly as possible. Water is idiotic, it may take the easy route. When there’s an area of reduced density, then the water will travel through it. The grinds in that area will be over extracted and the rest will be below pulled. To achieve density we need a horizontal, horizontal bed of coffee. Horizontal tampers are critical here. Whether you’ve a curved softball, throw it out and purchase a flat one. And definitely your tamping technique should be perfectly horizontal.
The amount of pressure applied should be as small as possible, without needing to sacrifice any one of the above mentioned. Repeating strain injuries are too common among Baristas, so aiming for the lightest pressure that works is crucial.
You would like to control the flow rate of your own espresso by adjusting grind dimensions, not tamping pressure. If tamping pressure is a variable then you should do this it the exact same every time. This is, frankly, a ridiculous ask for a human and must be completely avoided. Luckily, there’s a very straightforward way to practically eliminate pressure as a variable.
5. Maximum Density
Tamping squeezes the air from between the grinds. There’s only so much air in there, and eventually the grinds can’t manage any further. At a specific tamping pressure, you’ll hit maximum density. At this time, the grinds are fully compressed (on the vertical axis). Regardless of how hard you try, they won’t move any further. Coffee beans are tough. You can’t crush or harm them with a tamper since the load is spread across millions of particles.
It’s great for consistency and ergonomics, because:
1) No maximum tamping pressure. You can’t overdo it. The only thing you can do is approach maximum density. Once you reach the maximum density, the job is done.
2) Maximum density doesn’t need much pressure.
To find out how little pressure is required try slowly, softly increasing the pressure you apply with the tamper. At maximum density, the tamper will automatically stop moving. If you keep applying pressure, it won’t move any further.
When the tamper stops moving, that’s when you need to stop pressing.