vit wrote:... I suppose the difference is at least bigger towards the bottom (which gets least extracted anyway). ....

Hi Vit, I'm curious what makes you say this? I could see the upper puck coffee giving up solids first and if saturated fluids progress downwards they skip grabbing lower puck solids until the higher coffee gives less solids and more are picked up in the fluid in lower parts of the puck. How this balances out in the end as far as extraction level by puck depth, I have no idea. So what is your basis for thinking this way since all coffee delivered to the cup goes through the whole depth of the puck?

Tonefish wrote:So what is your basis for thinking this way since all coffee delivered to the cup goes through the whole depth of the puck?

The uneven extraction Vit references is partially due to the fact that the coffee fragments on the top of the puck are brewing with a purer solvent. Water will become diluted with solubles as it travels down the puck, meaning the solubles on the bottom will essentially be brewing with coffee water. I've also read that the espresso puck sees different pressures throughout the puck, according to Greg Scace: http://subscribe.baristahustle.com/issu ... -map-13341.

vit wrote:It was for wet environment - I assumed slurry density 1300 kg/m3, specific heat around 2000 J/kgK and conductivity similar to conductivity of water (0.5 W/mK). Conductivity of the wet mixture is several times higher than of dry coffee, but density and specific heat also higher, so heat spreads through it probably only 2-3 times faster than through dry coffee. So there is no enough time for the significant amount of heat to reach the coffee from the portafilter and basket walls, before mixture becomes hotter than basket walls

I wonder how differently the environment of a slower flowrate shot would be affected by this. 60-second long Slayer shots come to mind, where prebrew allows a slow preinfusion but also leaves coffee fragments more prone to varying gradients from environmental factors like burr temperature, basket temperature, etc.

andrewbettis wrote:The uneven extraction Vit references is partially due to the fact that the coffee fragments on the top of the puck are brewing with a purer solvent. Water will become diluted with solubles as it travels down the puck, meaning the solubles on the bottom will essentially be brewing with coffee water. I've also read that the espresso puck sees different pressures throughout the puck, according to Greg Scace: http://subscribe.baristahustle.com/issu ... -map-13341.

I think you basically said the same thing I did, and of course, the pressure must vary through the puck since the top pressure is the brew pressure and the bottom is atmospheric pressure. What I'm getting at is that once the upper coffee gives up the easy to get solids to the water (so cleaner water progresses) and grabs solids from lower level coffee, how do you know that the process hasn't extracted the same easy to get solids from the whole puck? Isn't that what causes blonding, when the easy to get solids are gone from the full path of the water?

Put another way, aren't we ideally trying to stop the extraction just before blonding when the whole puck has been evenly extracted?

So what makes Vit, or anyone else who claims this knowledge, say the bottom is less extracted?

I thought it was well accepted fact ... anyway, measurements seem to confirm it

http://www.coffeecuppers.com/Espresso.htm

That's a great link ... with data! Thanks a bunch Vit. So the lower levels are being "charged" with solubles from the upper levels.

This graph from older thread is also worth reposting

Can hot portafilter burn the coffee even before brewing?