It started with playing around with the AeroPress coffee system, since I want good coffee while I'm traveling. I varied the grind and dose I used for making AeroPress coffee--unbelievably good cups of coffee emerged. This is one great gizmo!

I'm also thinking about PF baskets--they are very porous and so are the paper filters used for AeroPress coffee. I'm still trying to understand why filter baskets affect espresso even though they present little resistance to water flow--I did some simple tests of just measuring water flows through empty baskets on my espresso machine.

This morning, I noticed that the AeroPress filters nicely tuck into the bottom of my 58 mm baskets--VST, or Breville 920xl baskets sport a cylindrical, straight walled profile. I thought that having a paper filter at the bottom of my basket would not really attenuate my extractions, since the puck is where the vast bulk of the resistance to flow resides, not the basket. I also wanted to see how the presence of a paper filter would affect the basket/espresso particle interface.

The simple test:
I wetted a AeroPress filter and then placed it at the bottom of my basket; followed it with pumping hot water through the empty, filter-lined basket. Water passed quite easily.....no pressure noted on the gauge....

I prepped my basket with a wetted filter and ran it through extraction on my espresso machine without espresso grounds. The filter looked firmly attached to the bottom of the basket. Into this basket, I then packed/tamped my usual dose of espresso for a triple (23 g) and extracted (30 g of beverage). It actually extracted a bit faster--~10%. I repeated this 2 more times and they all ran faster. The coffee tasted like a delicious cross between traditional espresso and an AeroPress brew. Inspection of the filters after extractions showed distinct patterns of dark spots laid out by the holes in the basket.
I then checked through HB files.....not surprising, others have also made "filtered espresso;" check out this excellent thread:
How to Make Decent Paper-Filtered Espresso

Paper filters may offer a simple way to tune extractions for espresso, which was discussed on the mentioned the thread.

What I think is going on:
Interfaces are very complex environments that offer surprising phenomena. The interface between an espresso basket and ground espresso, flooded with pressurized hot water is an example of a complex interface (viscosity, wetting, surface tension, jamming, etc...). I can only offer wild speculation as to what is going on: ??? The presence an intimate fibrous membrane (paper coffee filter) may serve to increase the apparently porosity of the espresso puck/filter basket through modification of the espresso particles/ basket interface, by ensuring a persistently porous layer. (The effective pores in the paper filter are much smaller than the basket holes.) This fibrous layer may ammeliorate the attenuation of water flow by coffee particles that can jam into the holes of a basket. The paper filter allows water to freely flow around coffee particles intimate to the paper filter and this water is then freely passed by the now, relatively unobstructed basket holes.???

Folks, give it a try....I'm going to try more tests.....these are just very preliminary observations....

I hope you do continue to enjoy your testing, I've played around with paper filter for making longer brewed drinks as in my machine the resistance is increased.
Did you use a naked portafilter?
If not, it's less obvious how the water might try to bypass the filter via edge channeling - might have an influence from basket shape.

Neat! Thanks! I did use a naked PF--did not see any evidence of channeling-- even extractions from witnessing espresso passing nicely through basket holes and the used puck looked firm and solid. For laughs, I also did a shot with a filter on top and bottom and another one with just the filter on top. Saw no diminution of flow, but did see a slight increase. The resistance to flow is much greater in the puck than that presented by the basket and the filter.

Interesting, I am curious as to how this thread evolves!

you sound like your pioneering here

Wild speculation. Without the filter paper, a grain of coffee can partially fit into a hole in the basket. The paper prevents that.

I put a piece of filter paper on top to keep the incoming water from eroding divots in the puck and to help reduce cleanup. I am not good enough to know if it does anything for taste though. Sure makes the spent puck look nice.

Have you tried grinding finer or up doser to get the same flow rate and compared taste? Please keep us posted. Thanks.

wkmok1 wrote:Wild speculation. Without the filter paper, a grain of coffee can partially fit into a hole in the basket. The paper prevents that.

I assumed this was the jamming phenomena he wrote about...?

wkmok1 wrote:Wild speculation. Without the filter paper, a grain of coffee can partially fit into a hole in the basket. The paper prevents that.

I put a piece of filter paper on top to keep the incoming water from eroding divots in the puck and to help reduce cleanup. I am not good enough to know if it does anything for taste though. Sure makes the spent puck look nice.

Have you tried grinding finer or up doser to get the same flow rate and compared taste? Please keep us posted. Thanks.

You definitely got the idea. Coffee particles may "jam" in to the holes and thereby attenuate flow. Fliter paper MAY prevent this.
Have not systematically varied grind yet--another weekend is needed for this...a great suggestion. Yet another tact for decreasing coffee particle size and maintaining desired flows.....

aecletec wrote:I assumed this was the jamming phenomena he wrote about...?

Yep-- this is a type of jamming...it's a cute name for an exciting new field of physics:

https://jamming.research.yale.edu/soft.htm

The neat thing here is how jamming might work at the basket/puck interface. Coffee particles stuck in the basket pores may greatly affect flow??
All speculation at this point....I'm going to try image those holes filled with coffee........thanks for asking.....

I feel that perhaps nutation increases jamming, but haven't done enough testing to confirm.