Can bottom filters improve flavor by reducing crema bitterness?

#1: Post by **Milligan** » August 19th, 2022, 11:20 pm

Moderator note: Split from Puck paper purveyors?

GDM528 wrote:I still see tiger stripes occasionally, perhaps more muted than without the paper on the bottom of the basket. But I don't associate the striping with a good or bad shot, for me it seems to correlate more with the flow rate of the shot.

I was just curious. I don't look for tiger striping as an indicator of much, but was interested to know if the oil/fines were altered enough to reduce the effect. I've had some coffees in the past that had great liquid taste but the crema was a bit bitter. Dialing out the bitter crema caused the liquid to lose character, so I thought perhaps adding papers would give me the best of both worlds in the event that I run across that again. Another tool in the tool chest so to say.

#2: Post by **Jeff** » August 19th, 2022, 11:30 pm

James Hoffman talks about it briefly in his crema video at about 8:20 and following. It's about the visuals mainly. If the filter paper retains fines, then it is possible that it will alter the taste of the cup, especially if there is a lot of tiger flecking (which suggests a lot of solids in the crema).

#3: Post by **GDM528** » August 20th, 2022, 2:11 am

My experience is from using cut-to-size AeroPress filter paper - I point this out because others are using fancy scientifical filter papers with ratings for minimum particle size, like 3um. Whereas I suspect AeroPress filter paper catches particles based solely on the number of customer complaints they get for gritty coffee. Different papers may also absorb chemicals in the espresso differently, and finally, some flavors may be extracted from the paper itself (next stop: LSD-laced filter paper

That said, I still see plenty of oils on the surface of the brew. What do the oils taste like? (I'm imagining a hapless grad student carefully plucking and collecting the oil drops off espresso shots for a research paper)

I have also observed occasional flecking in the crema. Yeah, the paper should stop that, but perhaps my filter paper is a barn door for particles just large enough to suppress a few of the foam bubbles and create the visual effect of a dark spot. And yes, I've observed someone waxing eloquent over the shot after observing the aforementioned faux-flecking as per JH's video. I'm pretty sure I'd stop drinking coffee if I were to lose my eyesight.

So real fines on the crema = flavor? So, more fines = more flavor, is there an upper limit? Is there such a thing as an 'inverted Turkish'?

#4: Post by **jpender** » August 20th, 2022, 8:27 am

Vince Fedele of VST posted about Aeropress filters on coffeegeek a number of years ago. He said they use standard filter paper which is meant for gravity feed. I.E., on the order of 0.01bar. Even with an Aeropress, where the pressure varies from perhaps 0.1 to 1bar, you can observe a decrease in beverage visual clarity as you press harder. Vince said that the fibers in the filter get pushed apart under pressure. So in an espresso machine the same thing will happen, probably to a greater degree.

Even gravity filtering espresso with Aeropress papers fails to produce crystal clarity. I've tried 0.7µm syringe filters and that's not good enough for espresso (or moka pot coffee) either. The syringe filters VST sells for espresso are something like 0.45µm effective pore size.

And if you want to taste coffee oil borrow a centrifuge. I've read that it doesn't taste very good.

#5: Post by **Jeff** » August 20th, 2022, 11:15 am

Johnathan Gagné looked a papers, including Aeropress, one Watman, and varied "coffee" options in https://coffeeadastra.com/2019/08/04/an ... filters-2/

Lots of information there, though I don't know which "numbers" are meaningful.

A conversation with Rohan on another forum suggests that somewhere around 25 µm, at least for the coffee, grinder, water, and filter-ish grind was where percolation flow rates transitioned from grind-dominated to paper-dominated.

#6: Post by **jpender** » August 20th, 2022, 11:22 am

As always with Jonathan's detailed efforts that was an interesting study. But it's mostly theoretical, based primarily on imaging. The one set of flow rate measurements he performed did not fit his predictions. And they were all done with gravity feed and room temperature water devoid of any solids. So what does it mean in terms of brewing coffee at pressure?

#7: Post by **LBIespresso** » August 20th, 2022, 12:20 pm

Just another data point here.

I use a Strietman with cut Aeropress filters top and bottom. Top for better water distribution and a cleaner group and bottom since I read that it may filter out some of the oils that affect cholesterol (I haven't found that it impacts taste so I stick with this).

The point I find interesting is this: I re-use the filters for the 2 or 3 shots that I pull each morning. I just rinse the filters and switch the top and bottom filters. Why do I do this you ask? Well, after rinsing them, the bottom filter is brown with coffee residue/oils and the top is pristine white. So I take the white one and put it on the bottom and the brown one and put it on top for the next shot. What do you know? After the next shot the top one is now white and the bottom is brown!

So clearly the bottom filter is catching something. Above my pay grade, so I will just enjoy my coffee while the more scientifically inclined amongst you may discuss the whys and hows. Maybe I can learn something.

#8: Post by **GDM528** » August 20th, 2022, 2:47 pm

LBIespresso wrote:The point I find interesting is this: I re-use the filters for the 2 or 3 shots that I pull each morning. I just rinse the filters and switch the top and bottom filters. Why do I do this you ask? Well, after rinsing them, the bottom filter is brown with coffee residue/oils and the top is pristine white. So I take the white one and put it on the bottom and the brown one and put it on top for the next shot. What do you know? After the next shot the top one is now white and the bottom is brown!

THAT is interesting! I think that implies that whatever goes in the filter paper doesn't stay in the filter paper. Do you invert the bottom paper when putting it on top?

I'm thinking that coffee-grade filter paper (e.g. French press, pour-over) has little to no effect on espresso chemistry. Lab-grade paper could be another story, as those papers can influence the pressure and flow rate - which will affect the chemistry. Question is: is this just another variable in an underdetermined system; could I duplicate the effect of the filter paper by making adjustments in grind, pressure, flow-rate?

#9: Post by **LBIespresso** » August 20th, 2022, 3:08 pm

GDM528 wrote:THAT is interesting! I think that implies that whatever goes in the filter paper doesn't stay in the filter paper. Do you invert the bottom paper when putting it on top?

I don't keep track when i rinse them. Plus I dry them in a towel to keep them flat otherwise they curl at the edges when they dry.

But I doubt it would make a difference. And the point about coffee oils and cholesterol was in an article that i read comparing pour over vs espresso with one using a paper filter and the other not. Clearly overlooking the pressure component. So who knows if the bottom filter has a positive impact. At least it's not negatively impacting things for me.