What are the black spots on the bottom side of the puck?

Want to talk espresso but not sure which forum? If so, this is the right one.
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#1: Post by guijan12 »

I have been wondering for some time already: what are the black spots on the puck that I see if I knock it out?
It is there always, no matter what coffee I use.
The pattern differs and I believe I keep the baskets nice and clean (rinse with cold water, directly after use).
Is it some kind of oxydation?

The first picture is from a puck that is one day old, the second of a puck, only a few seconds after having pulled the espresso.
The third picture is of the espresso that came from it.

Kinu M47
LPP pre-millennium, with Tije cooling ring.
Naked pf with 18grams Strietman basket

Guatemala Finca Bedición
Roasted on 4 March 2020
in/out 15gr/30gr (1/2)

Grouphead temperature 92 Celsius
Pull Pressure 10 bar.
Taste: excellent, every time :roll:




Its not a problem, just curious... :lol:


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Team HB

#2: Post by Jake_G »

I believe it has been postulated that those spots are in fact soluble coffee solids that have been washed down from the upper layers, but not extracted from the puck. If this is true (it seems plausible), pulling the shot a little longer should decrease their appearance. A former HB member swore by reading the bottom of the puck that he could determine the EY% within some amount or another.

Like you, I don't worry about such spots, but I do notice them, especially with shorter ristretto pulls, which incidentally are my preferred shots...


- Jake

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#3: Post by guijan12 »

Thanks Jake.
That could be the reason.
Is there a possible explanation why the pattern does not cover the whole puck (like uneven tamping or so)?
I'll try and see if a longer extraction decreases the spots.


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#4: Post by slipchuck » replying to guijan12 »

I would have thought the darker spots are scorching from sitting in the hot basket for a while

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