# Why does grind size drive taste?



## tripleshot (Jun 3, 2020)

This might be a bonkers question in its "duh is it not obvious" nature but I don't feel I have a good handle on the answer. If we take finer grind, for example, my understanding is that smaller particles means water dissolves more/different flavour compounds. But if that's the case. doesn't hotter water which has better solvent abilities do the same? Why don't we also say "brew temp drives taste"? What is it that grinding finer, for example, do that increasing water temp does not?

I think I'm missing something obvious. Would some more experienced coffee nerds please explain (will be useful for other noobs too, I'm sure!)

Thanks


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## Rob1 (Apr 9, 2015)

Probably because brewing slightly hotter doesn't have the same potential to increase extraction as grinding finer. e.g. increasing temp by 1c is not going to have a comparable impact on extraction as increasing surface area by 10%, it also won't really effect contact time or pressure.


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## sjm85 (Sep 9, 2016)

I wonder whether it is to do with which variable causes the greatest difference with the smallest change.

For example, temperature (measured in kelvins) is 367 (94 degrees celcius), so 1 up or down is around 0.3% change.

Whereas, even tiny increments on the grind setting I imagine will have a huge effect on surface area in terms of percentage.

Same probably goes with water quality. The ability of water to extract organic compounds is mainly based on pH and presence of ions/surfactants. I imagine also that a small change in water composition could dramatically change the amount of coffee extracted.

So that's might be why you say grind drives taste.


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## tripleshot (Jun 3, 2020)

Right, this is starting to make more sense.

So in many ways, if you have a good grinder you should just grind finer/coarser rather than fiddle with the temp as you're likely to observe the same, if not a bigger, difference.

And am I right in thinking that changing temp is just for small tweaks when for example you don't want to waste any coffee (normally required for grind change)?


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## Bagpu55 (Dec 23, 2019)

Is a good question, I dont know the answer but would like to know also


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## MWJB (Feb 28, 2012)

It's easier to extract the solubles from a finer grind, as each particle has proportionally less volume, compared to a coarser grind.

The range of temp is somewhat limited if you want to brew quickly, but I seem to recall (off the top of my head) going hotter than needed in espresso can lead to higher incidences of non-dissolved solids, which can be bittering.


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## Rob1 (Apr 9, 2015)

It gets a bit more complicated when you consider the flow rates involved and how the water loses temperature to the puck and how that might influence extraction. With a low flow rate, especially pulled to quite a low ratio, there will be a greater temperature loss compared to a faster running shot. If you grind a light roast really fine and increase your brew temp you might effectively be brewing at the same temperature as a darker roast with a lower temp setting after the temp of the puck has equalised with the water.


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## earthflattener (Jul 27, 2015)

Each of the chemicals dissolve/change phase at different temperatures. Some of the plant like bitter ones dissolve at the highest temperatures. Sugars are in the middle.

When the water is in contact with a grain it sets up a temperature gradient and hence dissolves from the edge. The grains are porous, so water will imbibe into the pores, encouraging dissolution in the interior. The bigger the grain, the longer it takes ... but the temperature gradient will depend on things like density of the grain (e.g dark roasts extract quicker).

For perfectly even extraction, the grains should all be the same size. If they are not, then some grains will only have mobilized some of the relevant chemicals. Big grains will deliver less of the bittering compounds, while small grains deliver a lot. Hence, a good grinder is one that delivers a good distribution of grain sizes (meaning fairly narrow range, but I'm not sure that a perfectly regular size would be ideal).

Pressure matters in two ways. High pressure means high water velocity, rushing it water past the grain, hence lower percentage of dissolved solids and less time available for the temperature gradient to do its work. meaning less bittering. However, it can also change wettability of the grain, so the result can be complex.

Lots of variables. Getting the physics right is hard.


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## earthflattener (Jul 27, 2015)

woops, just read Rob1's post after I sent mine. Some duplication.


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## MWJB (Feb 28, 2012)

earthflattener said:


> Each of the chemicals dissolve/change phase at different temperatures. Some of the plant like bitter ones dissolve at the highest temperatures. Sugars are in the middle.


 None of this is known to be, or ever shown to be true. All the solubles derive from plant like ones and the roasting process.

Sucrose in roasted coffee is almost inconsequential.

The bigger the grain, the longer it takes in the same scenario, but you can have a fine steep grain take longer to extract then a coarse percolation (drip) grain.

Temperature usually just clamps your limit of extraction for a given time, except in cases where you can extract enough, in a comparable time, at a few degrees below limiting factor (e.g. otherwise where you could conceivably over-extract).


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## tripleshot (Jun 3, 2020)

Thank you all for your replies. So does this have any implications on choice of dose? It sounds like it would do. If one were to drop from 18g to 16g dose, if grind stays the same, contact time is shorter as there's less resistance. Grind could be made finer to keep contact time with water the same. But in both cases sounds like there would be a noticeable change in taste as particle size has changed or contact time with coffee has changed.


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## MWJB (Feb 28, 2012)

Dose doesn't greatly affect taste at the same extraction, neither does particle size directly (there is a range that works, it will affect concentration if you go coarser & need to push more water through to normalise extraction), unless you go bonkers coarse. There will be tipping points where you over/underfill your basket and extractions will stay lower than possible, so really, just look at dose changes as small changes, for fine tuning.

Use the dose your basket is rated for, to get into the ball park.


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## earthflattener (Jul 27, 2015)

MWJB said:


> None of this is known to be, or ever shown to be true. All the solubles derive from plant like ones and the roasting process.
> 
> Sucrose in roasted coffee is almost inconsequential.


 We might be off down the technical wormhole here, but it is very interesting for a small few I guess...

I'm not sure what you are saving is not true? That coffee components are multiphase? The following blog says, with a bit more detail, things that I have seen repeated in many places, that "water will always extract the different flavor compounds in this order: fats and acids, then sugars, and finally the plant fibers."

https://clivecoffee.com/blogs/learn/how-coffee-extraction-works

Perhaps you disagree with that? If you have something that contradicts that, could you please post a link. It would be useful to harden up the information a bit. Not being an expert on the specifics of the chemistry here, I'm assuming what was written is right. It certainly fits with our experience of under/over extraction.

We can't assume that the plant extract just waits patiently in a queue, biding it's time for the others to leave before it becomes extracted. There must be a physical process. As I see it, those physical processes are dissolution or, possibly, phase transition, both of course coupled with diffusion into and subsequent viscous flow of the brewing water. Possibly other processes - not sure what though? For dissolution and phase transition, temperature and pressure are controlling variables. Anecdotally, phase transition seems to play a lesser role. Sure, some volatile compounds transition from solid to gas with heat alone (we literally smell the coffee), but roasting doesn't appear to release much liquid - maybe some oils around 2nd crack. So it looks like the primary action is transport of dissolute in a porous media under an applied pressure drop.

Regarding the dissolution of coffee compounds into water as a solvent, the basic ideas are covered in this https://www.pharmpress.com/files/docs/remington-education-physical-pharmacy-sample-chapter-3.pdf. So each compound will have it's own rate of dissolving and for any given temperature of the solute will have a saturation ceiling. The heavier, bittering compounds appear to dissolve more slowly, or need a higher temperature gradient. Apart from the fact that the chemistry is pretty complex, this is more or less enough to begin to get a handle on static extraction methods such as immersion where you can assume that the immersion lasts long enough for an equilibrium to be achieved. My suggestion is that what is different about dynamic methods, such as pour-over and espresso is that equilibrium is not achieved for all compounds. Lighter ones are preferentially extracted by managing the time the compound is exposed to the solvent.

What is not accounted for but is mentioned in that chapter is agitation of the solvent. For us, that means the dynamic act of passing water through the grain mass. While this doesn't appear to have been much studied in the coffee industry, it has much in common with the extraction of oil from subsurface reservoirs which has been very extensively studied over the last 80 years or so. One paper that I have found on it for Coffee combines the Noyes-Whitney equation in the previous link with the transport equation to cover the flow of the water though the porous medium. These are equations 10-11 for the dissolution and equation 4 for transport. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6668809/. This paper has the objective of showing that grain size matters for extraction. It doesn't tackle the real world case. For example it uses an unrealistic model of permeability for the coffee bed allowing for solution of the equations in 1d - rather similar to what was done in oil&gas back in the 70s....As such it can't handle real world phenomena such as channeling - the fact the the full bed does not get uniformly wetted. Once water has 'broken through' then the majority of subsequent water will follow that path and the extraction rate dwindles. Moreover, it makes an assumption that I can't quite get my head around, which is that the extractable compound sits between spherical cores, and the difference between coarse and fine ground is also reflected in the coffee concentration between cores. This might be a result, but doesn't sound like a good assumption. However, scruples aside, the paper still accomplishes what it sets out to do and the final result do match empirical data showing the importance of grain size and the effects of having heterogeneity in grain size distribution.

Heuristically, the problem is availability of the solvent to the coffee compounds. The dissolution happens at the grain boundaries, so grain size matters. All the questions are about getting water into the grains, wetting the grains (imbibition) and then having enough water moving through to keep potential high (and this is where the differential for each of the compounds can be 'managed' separately, allowing us to hold back some heavier compounds and preferentially extract lighter ones.

I'd be the last person to say that this is an easy problem to manage - I've spend most of my working life on a 'fluid flow in porous media' problem where huge resources have been spend to understand and model the physics with some success, enough to have make the global effort technically and therefore economically very successful, but the results are still statistical (i.e understanding any individual medium is still very difficult). But I'm not sure that I'd agree that the vague gernalities that I spoke of in my last post were 'not known to be true'. They are just physics. Controlling the complexities of the interactions is quite a different matter though.


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## MWJB (Feb 28, 2012)

Fats and plant fibres are solids and remain as solids in suspension, they are not dissolved & not extracted. Sure they are washed into the cup, but flushed. Coffee beverage is still coffee beverage irrespective of the proportion of non-dissolved solids. The Clive coffee statement is flat wrong...a widely held misconception, but still wrong.

Yes, you can detect the non-dissolved solids, but they tend to be generic & not part of the unique profile resulting from growing, process & roast.

Yes, the fats tend to get into the cup quickly, but as stated earlier, they are not "extracted".

Caffeine extracts in a fairly linear fashion, so most cups have a reasonable proportion of the caffeine available.

Extraction for CGAs, trigonelline & tannins drops exponentially, but is usually proportional to extraction yield (dissolved solids in the cup). Sucrose is simply not a significant compound in beverage coffee (other sugars are, but they're not particularly sweet, thus hard for humans to detect as such).

As it says in the paper you linked to coffee is enjoyed at various strengths, but EY overlaps greatly between filter, lungo, espresso & ristretto. Grind size plays a part in reaching comparable levels of extraction, but not (as far as we know what is extracted). Finer grinds lead to higher proportions of non-dissolved material. Shorter ratios lead to the fats making up a larger proportion of the beverage in the cup.

If you could change the compounds extracted with grind size, then it should be easy enough to take a blackcurranty, washed Kenyan extract to a nominal level, change grind size and extract to the same level & have it taste like a hazelnut/caramel Brazil. But you can't, because that is alchemy. What happens in the real world is that the flavour profile of a coffee as espresso is very similar to that as brewed, allowing for difference in intensity based on ratio & mouthfeel based on the efficiency of filtration.

In other words, the compounds are put into the beans as they are grown, processed & roasted, you can extract what is in there...but you can't change essentially what is in there after the fact. You can extract more, or less & shift the balance of acidity/balance/dryness a little for a given coffee (nominal brews, lacking distinct malfunctions). almost all roasters QC their espresso by cupping, how would this be possible if different things were being extracted?


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## earthflattener (Jul 27, 2015)

Thanks, I appreciate your perspective on this....and btw, on the work you've done on water chemistry which has been very useful. Your comment does run against the work of the paper on flow too, as they explicitly make the same assumption as CliveCoffee and use it (in equation form) to prove their result, but I take you point that fats are not particularly soluble. Actually, it doesn't change much from the flow physics perspective if the fats and plant dissolve or not, so long as they are in suspension, but it probably does invalidate use of the Noyes-Whitney equation as potential. However, replace it with another exchange mechanism and the transport component remains the same. Similar to 'produced sand' in oil reservoirs.

Regarding grind changing Kenyan to Brazilian, that is not possible because the raw materials are different - as can be seen by smelling or chewing on a bean. I didn't imply anything different and I didn't see it raised in the thread, although I might have missed it. You simply change emphasis in preferentially extracting lighter compounds, but that is still a huge difference. What we are trying to understand is why different grind size drives taste. That coupled with the relevant set of extraction parameters is the case, whatever the bean and it is also obvious that the quality of the grind matters. What geologists call sorting, roughly our equivalent of grain size distribution, plays a major factor in multiphase flow recovery through the concept of relative permeability. I would very surprised if this did not carry over to the the coffee porous medium


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## MWJB (Feb 28, 2012)

earthflattener said:


> Regarding grind changing Kenyan to Brazilian, that is not possible because the raw materials are different - as can be seen by smelling or chewing on a bean. I didn't imply anything different and I didn't see it raised in the thread, although I might have missed it. You simply change emphasis in preferentially extracting lighter compounds, but that is still a huge difference. What we are trying to understand is why different grind size drives taste. That coupled with the relevant set of extraction parameters is the case, whatever the bean and it is also obvious that the quality of the grind matters. What geologists call sorting, roughly our equivalent of grain size distribution, plays a major factor in multiphase flow recovery through the concept of relative permeability. I would very surprised if this did not carry over to the the coffee porous medium


 OK, I stretched the point with the Brazil/Kenya scenario, But, if Clive coffee (I appreciate they're not alone in their unsubstantiated theory) suggest that acids come out first, then sugar, it implies that you can pick where you get sweet & acidity, changing the profile of a nominal cup, willy nilly...and to some relatively small degree you can, but only in a fairly subtle way *at an equivalent extraction* (same grinder). You can wildly under-extract and achieve high sweetness before getting into objectionable acidic/sour range...so what's going on here? Sugars coming out first, then hiding until they reappear later?

Check out this video. It seems that the components in coffee largely all extract together (some differences in the rate, sure), but none suddenly appear from nowhere, none stop extracting before the others. This is similar in findings as a late 50's filter coffee analysis by MIT, "Extraction Rates for Certain Components in Coffee Brew", Merrit & Proctor, 1958).






Quality of grind. What exactly are you referring to here? What is nominal/typical grind quality, what is poor/inevitable malfunction region, what is exceptional?

To date, I am not aware of any study that has linked grind size distribution with sensory preference. It is true that tighter distributions can lead to higher EYs, but then the lower limit of tasty EY moves upwards too. You shift the tasty box somewhat, but most people (well, just about everybody) have no idea what their grind distribution is, nor which grinder to get to move in the direction that they think they want. Grinders with greatly varying distributions are still popular & widely used, producing cups representative of that coffee.


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## MWJB (Feb 28, 2012)

earthflattener said:


> Your comment does run against the work of the paper on flow too, as they explicitly make the same assumption as CliveCoffee and use it (in equation form) to prove their result, but I take you point that fats are not particularly soluble. Actually, it doesn't change much from the flow physics perspective if the fats and plant dissolve or not, so long as they are in suspension


 Extraction yield measures only that which is dissolved. If it can be filtered out, it's not dissolved. You can increase total solids yield by grinding fine & have more pass through the basket, but this isn't the same as extracting/targeting soluble compounds. It's just a function of grind size.

Like with filter coffee (not all coffee is espresso & not all espresso is unfiltered), you get greatly reduced suspended solids, less artifacts from NDS but it is still coffee & tastes like coffee & will do over a range of grind sizes (maybe with/without normalisation of extraction via brew ratio/pour rate) as I said earlier...



MWJB said:


> Yes, you can detect the non-dissolved solids, but they tend to be generic & not part of the unique profile resulting from growing, process & roast.


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## earthflattener (Jul 27, 2015)

MWJB said:


> Check out this video.


 Just seen this now...so will have a look. 1 hour..eek!

There are also some quasi-experimental results here https://www.mpechicago.com/wp-content/uploads/2019/12/espresso_grinding.pdf. He suggests that espresso needs a bimodal distribution of grain size, stating that the larger particles are the bricks which control flow and the smaller are the mortar. It's not a great analogy, as it is the 'mortar' which controls the exposure time. Permeability is not a (direct) function of porosity, rather it is controlled by the pore throat sizes and so it is the fines sitting in the larger pore spaces that slow down movement in a porous medium. In our case, that changes over time as the smaller core bodies become reduced in size by dissolution/extraction of compounds hence increasing permeability. Still, in his theory it is size distribution that controls access of water to the grounds.


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## MWJB (Feb 28, 2012)

Lots of good info & downloads at the MPE site. As far as burr grinders go, bimodal distributions in the espresso range seem to be unavoidable (as referenced in the last paragraph).

It's more the size than the distribution that controls the access in real world/everyday, burr grinder scenarios. Because we can't readily change distribution, only burr gap. We can't analyse espresso distribution in th 30-50micron range either without laser particle size analysers (though with coarser espresso you can roughly show relative coarseness/fineness with the same grinder & sifting).


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## earthflattener (Jul 27, 2015)

That Yeretzian presentation is excellent! We must have been talking a bit at cross purposes, because it makes perfect sense to me. I suspect you were mostly focusing on properly extracted coffee, whereas I was talking (I think?) about coffee that is under or over-extracted and how the compound extraction ratios are very different if you are outside the target zone. Anyhow, a really good presentation.


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## MarkHB (Jun 12, 2020)

MWJB said:


> almost all roasters QC their espresso by cupping


 Sorry to ask an off-topic question here, but does this mean that the tasting notes on the bags are usually gathered from cupping? It's just something I've thought about before. Since a latte is going to taste a lot different from a french press coffee.


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## MWJB (Feb 28, 2012)

MarkHB said:


> Sorry to ask an off-topic question here, but does this mean that the tasting notes on the bags are usually gathered from cupping? It's just something I've thought about before. Since a latte is going to taste a lot different from a french press coffee.


 A latte will taste different to black coffee, however you make it. But, yes, I think cupping is often used to identify notes, maybe best to check with the roaster concerned if they have any insights how things work with milk specifically.


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## Mrboots2u (May 10, 2013)

MarkHB said:


> Sorry to ask an off-topic question here, but does this mean that the tasting notes on the bags are usually gathered from cupping? It's just something I've thought about before. Since a latte is going to taste a lot different from a french press coffee.


 Some roasters will gove you notes with milk , James gourmet for example .

More should as most coffee is consumed with milk


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## Rob1 (Apr 9, 2015)

MarkHB said:


> Sorry to ask an off-topic question here, but does this mean that the tasting notes on the bags are usually gathered from cupping? It's just something I've thought about before. Since a latte is going to taste a lot different from a french press coffee.


 Green coffee is bought with cupping notes. The roaster may provide their own notes in addition to the cuppers' notes or one or the other. The notes can be quite subjective even with standardised water compositions and brewing parameters. With milk/without milk could be hard to do as it complicates things much more. I would perhaps simply err on the side of caution and tell people "works well with milk" or "sours milk" just to give people guidance.


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