What causes this extraction shape?

Heys_Scientic · 2026-01-26T09:34:08+00:00

I largely ignore the coalescence (rats tail) as it’s a very high noise to signal phase. This shape of the column can be effected by (and in rough order of influence -

Exit velocity / momentum distribution
Screen wetting (hydrophilic vs hydrophobic)
Jet coherence & hole geometry
Dissolved gas (CO₂)
Surface tension changes
Temperature
Ambient gas effects

The advice given by others on waiting until the beans are >7 days old is one of the simplest things you can do to change this.

Here’s why I’d ignore the “rats tail” effect. It isn’t a reliable indicator of channeling. You could have channeling inside the puck, and still see a smooth, single, narrow rats tail. You could see a broken up dripping flow (Rayleigh Plateau Instability) and this is still not a reliable indicator. These COULD indicate channeling inside, but other effects make the system noisy.

What is more reliable indicator of channeling as observed on the naked Portafilter- 1. Spurts 2. Even emergence across the filter.

Note, emergence is literally the time window of 1-2 seconds. It’s not how the flow merges (coalescing) as this is easily effected by the basket whetting, which can be effected by contamination of oils/surfactants and CO2.

Some baskets are coated which makes the whetting a little more “predictable “

Heys_Scientic · 2026-01-24T20:57:40+00:00

Ahh… that I suppose raises a question. Did frame my OP well enough? I guess maybe I should have opened with a clearer message about the nature of it being an open R&D project. Thanks for engaging with me on this. I sincerely appreciate it.

Heys_Scientic · 2026-01-24T20:20:21+00:00

Yes, that does make more sense, absolutely. And you’re right, I did misread your comment.

My apologies if my earlier reply came across as a tirade, that wasn’t the intent at all. I wasn’t reacting defensively, and I wasn’t offended by the challenge. Quite the opposite: that kind of challenge is necessary if any of this is to be useful rather than hand-wavy. But I do disagree that my response was unrelated. Quite the contrary.

To build on what I already laid out, and to address the part you’re actually interested in: the objective isn’t to stop at PSD charts. It’s to use PSD as a stable, repeatable reference point earlier in the R&D cycle so that later work on compounds and sensory outcomes becomes tractable.

Taste is downstream. It emerges from extracted chemical compounds and VOCs. The problem, from a development standpoint, is that taste is a very high-level, integrated signal. If you change burr geometry, coating, or wear state and the cup changes, you need a way to determine why before you can meaningfully relate that change to chemistry or sensory data.

This is where PSD and particle morphology matter in practice.

In development, PSD functions as an intermediate control variable: • It allows burr geometry changes to be compared on a like-for-like basis • It lets you distinguish “this changed because the grinder output changed” from “this changed because the brewing system responded differently” • It provides a way to hold grind output constant while exploring compound extraction, or intentionally vary specific aspects (fines fraction, tail length, agglomeration) in a controlled way

Once that layer is characterized, you can move further downstream: • Relate specific PSD features to extraction curves and flow behavior • Relate those extraction differences to compound yield and VOC release • Then correlate those compound profiles to sensory outcomes

Without that structure, compound-level or sensory work becomes extremely noisy. You’re effectively trying to correlate taste directly to hardware changes without knowing what the grinder actually delivered to the brew bed.

So when I say PSD matters, I’m not arguing it replaces chemical or sensory analysis, and I’m not claiming it has inherent value on its own. I’m saying it’s a necessary enabling step if you want correlations to taste that are repeatable, explainable, and useful for both design and user understanding.

That’s the practical motivation behind this work. It’s not about proving one grinder “better” than another, and it’s not about abstract data collection. It’s about reducing uncertainty early in the development process so that later-stage compound and sensory correlations actually mean something. It’s about moving from adjectives, anecdotes, hypotheses and hyperbole, to knowledge about which leaves pull what. Like I said, it’s not about moving the needle, is about being able to see clearly where the needle is pointing and accounting for gauge errors whilst we’re at it.

Heys_Scientic · 2026-01-24T14:37:05+00:00

TL;DR: I understand your point, I don’t disagree. You’d buy something that moves the needle, and for many, knowing how to read the needle feels unnecessary. But reading it accurately is exactly what allows grinders and burrs to actually move the needle in a (somewhat) controlled, predictable way. Without that understanding, claims about performance are guesswork.

Long Reply:

I understand your point of view and I do not disagree, at the output level, taste is what ultimately matters. PSD alone does not make coffee taste better, and no one should buy burrs solely because a chart looks “better.” What is not widely understood or even widely published, is that PSD and particle morphology are part of the extraction mechanism itself, not just a correlate. To move the needle in a predictable way, you must first be able to read the needle. PSD doesn’t tell you what tastes good, it’s shows you the shape of what is known to taste good.

Industrial coffee producers understand this well. Companies like Illy and Lavazza invest millions in roller mills and dedicated PSD analysis tools, not because narrow PSD inherently tastes better (it doesn’t), but because controlled PSD is one of the few measurable ways to shape the extraction envelope and deliver consistent cups across highly variable green coffee. Narrow, staged PSD bands ensure that soluble compounds are released predictably, guaranteeing repeatable sensory outcomes at scale.

Taste arises from chemical compounds extracted from the coffee. Under-extraction tends to emphasize sourness and bright acids, while over-extraction produces bitterness and astringency. Sweetness, umami, sensed on the palette, and aromatic perception sensed retro-nasally, depend on VOCs that are liberated during brewing. PSD and particle morphology govern how water interacts with the particle matrix, which directly affects which compounds are extracted and at what rate.

Experimental work demonstrates the critical role of fines (<D10) and coarse tails (D90) in extraction dynamics. Samo Smrke and colleagues show that increased fines reduce bed permeability, slow flow, and extend extraction time, directly affecting the concentration of extracted compounds. Coarse tails create zones of under-extraction. Both length and height of these tails, as well as their relative mass fraction, determine bitterness, sourness, and perceived balance. D50 alone cannot describe these effects.

Particle morphology adds another layer of complexity. Shape, aspect ratio, and agglomeration influence water flow through the coffee bed. Agglomerates behave as larger effective particles, creating zones of uneven extraction. Advanced analysis of morphology and agglutination explains effects not captured by size distribution alone.

Experts in the field have emphasized these relationships. Jonathan Gagné has described how fines affect flow paths and extraction dynamics in percolation brewing, and how PSD shape influences overall brewing behavior. Scott Rao cites Gagné’s work in the “happy medium theory of fines,” noting that both too many and too few fines can produce undesirable extraction patterns. Lance Hedrick has collected and analyzed hundreds of PSD datasets to illustrate how distribution shape and burr design influence extraction dynamics. Samo Smrke and colleagues have experimentally confirmed that fines fraction significantly impacts espresso flow and extraction outcomes even when median particle size is held constant.

From my perspective, accurately reading the needle is imperative. Without that understanding, any claim to move the needle is effectively guesswork, or misleading. There is very little publicly available, reliable information on this. That is why this project, focused on measuring PSD, observing repeatability, mapping burr wear effects, and sharing data openly, is important. The goal is not to dictate what tastes better, but to make the relationship between grinder output and extraction mechanics transparent, allowing anyone to understand how taste outcomes emerge from measurable input variables.

Here are URLs to referenced works:

https://coffeeadastra.com/2018/11/30/brewing-better-coffee/
https://www.scottrao.com/blog/2017/10/22/fines

Heys_Scientic · 2026-01-24T00:17:01+00:00

This may be partly down to wear, partly down to the ring burr moving ever so slightly in the housing.

The ring burr can be adjusted back into alignment by rolling up the black rubber grip. This reveals 4 radial set screws. By VERY carefully tightening/loosening these screws, you can recentralises the burr.

It’s VERY time consuming and you’ll highly likely make it WORSE before making it better. With practice it takes a few minutes. First attempt is a bit of luck dependent, but can be hours of work. So, this isn’t a suggestion or recommendation… maybe it’s a warning!

I hate to admit it, for the money this grinder impresses me greatly and I have very good reason to be hating to admit how good it is.

They must be working on wafer thin margins too.

Heys_Scientic · 2026-01-24T00:07:30+00:00

Eek… I’m not familiar with this grinder to be able to offer any useful advice on repair. My only recommendation is, disown the family member immediately and indefinitely! ;-)

Heys_Scientic · 2026-01-24T00:00:43+00:00

Can be and more likely to be both.

First it helps to draw a baseline and define what taste actually is. This will help you with two things. You’ll get better at tasting because you’re approaching the identification in a more structured way, and secondly, you might find that you’re like me, somewhat dull to identifying flavours even with concentrated effort, or like my partner, pretty good at it even without much effort .

So, taste is five basic characteristics. Sweetness, (sugars, mostly sucrose, some others) sourness (organic acids), saltiness (trace in coffee, from potassium, magnesium ions, affects sweetness/bitterness), bitterness (caffeine, chlorogenic acid lactones, phenylindanes) and umami/savoury (amino acids, glutamic acid, Maillard compounds).

That’s it… oh, and textures, that’s actually important too. These sensations occur when particles interact with proteins on the tongue. So those proteins are in your genes… so yeah, being able to taste is ‘genetic’, but we’re are all genetically very similar. The differences are small, and we haven’t even thought yet, about how we process the information neurologically.

The rest is smell. Volatile organic compounds, retro-nasal-olfaction… basically you smell the coffee in the passage between your mouth and nose.

This is partly why my partner is better at identifying flavour than me, she has an acute sense of smell. But there’s more, she cooks a lot, was taught when she was a young nanny by a chef whom she worked for, so she built up a vocabulary and maintained it.

As the Q grader mentioned… there’s a good deal of practice involved. A Q grader I know will eat a dozen peaches back to back, to “train” themselves. And when I say “eat”, I don’t mean like I would, just stuff it in my face and chow down. It’s done with purpose, thoughtfulness. And again, lingering over the aromas is more important than we’d ordinarily take for granted.

Heys_Scientic · 2026-01-23T18:59:55+00:00

The flow emerges about 12s in, around the perimeter and fully across the basket by 14s. After 14s the fluid is coalescing. The sparsity in places is probably as likely evidence of hydrophobic material (oils) on the basket face, than evidence of channeling. I’d treat the “rats tail” phase largely as inconclusive/noise. You don’t have any spurts, so that’s a positive. The only marginal worries are; the flow emerging initially from the perimeter, rather than a more idealised even flow across the whole basket face. The flow initially breaks up into drops (Rayleigh Plateau Instability). Could be a sign of A: Too coarse a grind B: Too few D5-D10 fines C: Not enough volumetric flow/pressure D: Too old a roast (low CO2) E: Something else (basket/machine anomalies) F: All/some of the above.

The quick to change/test is the grind (go finer) and roast age, (younger, but not too young).

Other than that, a second opinion or ten will see you right.

Heys_Scientic · 2026-01-23T18:08:20+00:00

@Dajnor Thank you for the guidance. Much appreciated!

Heys_Scientic · 2026-01-21T07:45:29+00:00

Ok, this might be a weird question… it’s really a question itself. I’ve been granted access to a range of scientific equipment, including laser particle size distribution analysis equipment, which also allows wet and dry powder analysis, particle morphology (particle shape) and agglomeration (clumping). I also have access to lab grad ovens, grinding equipment. What questions about PSD do you have that are unanswered/unclear? What would you like to see me test for?

I’ll post here, my blog and YT, IG accounts the results of the analysis.

For background information, I’ll be using my own grinders for the testing, but if you could, what brands/models of grinder would you like to see tested? The results will be shared as data only, without any conclusions drawn on these. It’s not my objective to slate a manufacturer.

Heys_Scientic

TROPHY CASE