Coffee grounds are inherently slightly sticky. We rely on this fact during tamping to hold a puck together, and to create a good ‘seal’ between the edge of the puck and the basket. Unfortunately, this also means that coffee grinds can stick to tampers and tools, in extreme cases creating small divots in the puck. This tendency increases each time the tamp or Puqpress is used until the tamping surface is wiped clean and dry.
Three factors make grinds stick to surfaces: static electricity, moisture, and oils. It’s easy to see static at work, around the grinder chute for example, but it’s unlikely to be the major factor here – the static forces created by grinding aren’t strong enough to pull whole clumps of coffee out of the puck. Also, if static was the culprit the effect wouldn’t get worse over time — the tamping surface of a Puqpress is grounded, and even if a tamper was picking up charge from the coffee the effect would be to equalise with the coffee, reducing the static effect.
On the other hand, every barista has experienced the effect of a wet tamper. Ground coffee is highly absorbent to moisture, so even if there isn’t surface water around, humidity in the atmosphere will be attracted to the grinds, causing them to stick to each other and to surfaces. A hydrophobic coating does seem to help reduce the amount of coffee sticking to tampers, but this isn’t the whole story — coffee will still stick to the surface of a Teflon-coated tamper.
The last and most likely culprit is the oils in coffee. Grinding releases oils from coffee, as cells that were previously intact are broken open, which makes the particles clump together. “The oil acts as an adhesive on the particles, forming… stable aggregates.” (M Petracco, 2005).
This effect will become even worse as a grinder heats up. Coffee oils are very viscous, but above 40°C, the oil in coffee becomes more fluid. “Oil can easily flow through micro cracks and cover the outer surface of the particles with a layer that regains its sticky, semi-solid state at room temperature,” Petracco writes.
Keeping a grinder below 40°C isn’t a practical option for most of us, so how else can we prevent this build up?
A well-polished tamper surface might help but doesn’t solve the problem. Matt observes that mirror-polished tampers can pick up just as much coffee. That said, a rough or dented surface will have more surface area to collect oils and very fine particles, so might start to pick up coffee more quickly, and will and be harder to keep clean.
A coated tamping surface can definitely improve matters, by reducing the interaction between the tamping base and the oils or moisture in the grounds, and the technology has improved enormously in recent years with newer nanocoatings. However, we haven’t yet seen a tamper or tool with a coating good enough to eliminate sticking.
For now, the only remedy we can suggest is to clean more often — after every shot where possible. Even a clean-looking tamper can harbour a thin film of coffee oils, and coffee can start to stick to a Puqpress or distribution tool after just 3 or 4 uses.
The implications of this are very slight from a sensory perspective. The mass of grinds residing on a tamper base is only a few milligrams at most — too slight to even register on a scale with a resolution of 0.1 of a gram like most of us have on our bars. So the behaviour of espresso shots are not appreciably affected by the loss of this tiny film of coffee. The trouble is more noticeable over time. If unchecked across a whole shift, the buildup can accumulate a bit like one of those arcade machines with the coins gradually accumulating at the edge of a precipice. So in the short term, we recommend a quick dry-wipe of the tamper or PuqPress base at least every ten shots.
M Petracco, 2005. Grinding. In: A Illy and R Viana (Eds), Espresso Coffee: The Science of Quality, second edition (pp 215-229)