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January 30, 2017 /
Surface Area and Time

In this post I’ll be covering how to increase or decrease the overall extraction of a brew from a theoretical standpoint.

(Angela Grieve, a Hustle subscriber asked for this via email. So Angela, this one’s kind of for you!)

There are a few variables that you can use when manipulating extraction. The two main contenders are Time and Surface Area. These two variables are intimately intertwined in every single type of brewing environment; you can’t change one without changing the other.

Time

If you give water more time in contact time with the coffee, it will extract more of the flavours. If given enough time, extraction will continue until there’s nothing left to dissolve. So naturally we want to find a time where the extraction has reached a point that’s delicious.
Within the variable of time, there’s actually two different things happening that we need to separate and consider. The first relates to what the water extracts and when, which is purely based on contact time. The second relates to how easy it is for the water to do that, which is decided upon by a mixture of both time and surface area so I’ll tackle that later on.

What the Water Extracts When

Every soluble component of coffee has a slightly different solubility (that sounds weird, but stick with me). Salts, sugars, acids, phenols, fats and lipids etc. all take different amounts of time to be dissolved by water. Some will be dissolved straight away, and others will need more time. We need to think about this when brewing coffee, because changing the contact time will also change which components of the coffee will be dissolved by the water. The first and most soluble parts of coffee are fruit acids and organic salts (light, bright, fruity flavours), closely followed by light aromatics created from the Maillard reactions and sugar browning during roasting (nuts, caramel, vanilla, chocolate, butter etc.) and lastly heavier organic matter (wood, ash, malt, tobacco etc.).
This makes it rather easy to err on the side of a shorter brew, as most coffee drinkers reading this article tend to steer away from ash and overtly bitter flavours. Unfortunately, this will also sacrifice sweetness in a lot of cases. Coffee, as always, is the master of making you compromise.

Surface Area

Increasing the surface area of the coffee makes it far easier for the water to dissolve its flavours. When you grind coffee, you’re increasing the surface area exponentially.
Think of a coffee bean as a cube. This cube is 1cm wide. It has 6 sides, each of which has an area of one square cm. So our little cube has 6cm2 of surface area.
Coffee Grind as a Cube
Cut that cube in half and cut the resulting pieces in half until you have 8 little cubes. Each of them are now 0.5cm wide and have 1.5cm2 of surface area. This means there’s 12cm2 that the water can easily get at. Not much work and we’ve doubled our surface area!
Coffee Grind 8 Cubes
Now lets put that cube through a grinder at a very coarse setting. This time, we get 64 little coffee cubes 0.25cm wide, each contributing 0.375cm2 with a total of 24cm2. Doubled again.
Coffee Grind 64 Cubes
A typical espresso grind is going to break this 1cm cube into many hundreds of little pieces. As you can imagine, the surface area really starts to add up. This drastic increase in surface area allows the water to dissolve the majority of the coffee’s flavours pretty easily. No, I’m not going to draw 1024 cubes.
Inside these little imaginary cubes, there is still quite a large amount of coffee flavour that isn’t immediately accessible to the water. This is why coarser grinds take longer to extract than fine grinds; the water has to travel into the grind, dissolve the flavour, take that flavour back out, and deliver it into the brew. Compare this to a fine grind (where all water has to do is touch the coffee and it’s dissolved everything) and you see why surface area is so important for increasing extraction.
The most important thing to wrap your head around here is that grind size doesn’t really change what is being extracted. It only changes when the things are extracted. All of the flavour is right there, in the coffee bean, ready to be extracted. Grind size just puts more or less of that flavour in front of the water right away.
Here’s another way to think about it: When talking about grind size you could also say ‘how much coffee flavour am I going to hide inside the grinds, away from the water?’ or ‘how much should I delay the extraction of a portion of the total flavour?’. A finer grind will hide less flavour, and reduce that delay. A coarser grind will hide more flavour and increase the delay.
When you want to increase extraction, you need to grind finer. You need to increase the surface area of the coffee and ’show’ the water more of the flavour. Once adjusted, your extraction will increase because the water can ‘see’ more of the flavour and get to work dissolving it more quickly.
Remember: all of the flavour is waiting there in the grinds. Manipulating surface area just puts it in front of the water, or hides it.

How Easy is it for the Water

If you have lots of coarse grinds, it’s hard for the water to get inside them and dissolve the flavour. It has to work its way through the convoluted maze of structural cellulose, be in contact with the flavour for enough time to get what you want (see above) and then has to move those flavours back out again. Coarse grinds require a significantly longer contact time because it takes so long get these three steps done. If extraction takes 1 unit of time (1), getting into the majority of the flavour of the bean takes one unit of time (2), and getting that flavour back out takes one unit of time (3), then every time you hide flavour inside a large coffee ground, it’s going to take 3x longer to get it. Meanwhile, the external surface area continues to extract, most possibly farther than you’d like it to.
Coffee Grind Extraction Stages

What does this mean for espresso?

In espresso, you only have ~30 seconds to extract everything. This means that any coarse grinds are practically useless ‘fillers’ and will never be extracted completely or evenly. Espresso grind sizes are generally extremely small to give the water every chance to extract enough flavour in that short time period. You may have seen my 2013 World Barista Championship Routine. In this routine I helped to popularise (but not invent or discover) the use of a filter or ’shop’ grinder for espresso brewing. The grinder I used was Mahlkonig’s EK43. It’s crucial to use a grinder like this, or similar with flat burrs, that has a more even particle size for espresso brewing. It will help to ensure most (or all) of the flavour is accessible by the water within the short brewing time. Don’t worry, there will be much much more on this in future Hustles.

What does it mean for filter/brewed coffee?

In a filter coffee (eg. french press or pour over) we use much coarser grind settings. This is pretty much for one main reason. The brewing time of filter coffee is much longer than espresso because we don’t have the aid of a pump to push water through the coffee grinds and filter. Instead, we’re usually using gravity which is much slower. Because of this necessity for longer brewing times, we ‘hide’ more of the coffee’s flavours inside larger grinds to reduce the chances of over-extraction.
The most important thing to take from this: while you’re waiting for the larger grind’s interiors to be extracted, the finest grinds are still extracting. The clock doesn’t stop for you. You have to decide whether that extra time will benefit the brew by giving you some more extraction (and yes, sweetness) from the larger grinds, or if it results in the smaller grinds being over extracted (and bitter/dry). Choose wisely!
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Is it Better to Sieve out Particles Smaller than 400μm? - Barista Hustle

[…] grinder produces a range of particle sizes. These extract at different speeds, because of their different surface areas relative to their size. The reason the surface area is so important is that diffusion is very slow […]

Susan
Guest
Susan

So , could you tell me why they suggest grinding it fine for a single cup and more coarse for more than one cup in a pour over / filter brew method ?

Daniel
Guest
Daniel

Because you use more water and more coffee it will take longer for gravity to pull the water through. If you use the exact same values but add water, you basically add time as well, which means more extraction. The extraction time needs to be the same for any amount of cups you want to brew, the water and grounds go up the more cups you want to brew. So if you grind coarser, the water will flow through faster (flow rate). With more coffee and more water on a faster flow rate you aim for the same extraction time… Read more »

Alief Kusumaningtyas
Guest
Alief Kusumaningtyas

Does the coffee filter have specific rules for use?

trackback
Should You Grind Finer For Better French Press Coffee? – Coffee Creativity

[…] highlights that “if you give water more contact time with the coffee, it will extract more of the […]

Crood
Guest
Crood

Thanks. Coffee wizard. X

Marshall Hance
Guest
Marshall Hance

So all grinders are bi-modal, that is fines are inevitable. As we grind finer, do the modes remain relatively distant, or is it possible to bring them much closer together?

Going down this road, I can’t but help think about lower temperature extractions to give us a wider window of time to complete the extraction.

Matt Perger
Guest
Matt Perger

The most important thing to aim for with modality is the tightest overall spread. So even though there are two peaks, the closer they are the better off you’ll be. The best way to quantify this is with a standard deviation from the mean.

Lower temps is a good idea, but it will just do exactly the same thing as hot water except in slow mo. You’ll still get unevenness.

Matt Perger
Guest
Matt Perger

Realised as I was walking down the road. Thanks for that!

Mark Burness
Guest
Mark Burness

Hi Matt., You say in French press & filter coffee we grind coarser because gravity takes longer to push the water through the bed, but with French press there is no flow through the bed as such. The grinds are infusing in the water, the longer the coffee & water are in contact the more water is absorbed into the grinds (3g/g of coffee or more), doesn’t this suggest that there is more/something else going on beyond washing solubles from the exterior, progressively towards the interior of the particle? Even with fine grinds I struggle to overextract French press in… Read more »

Rashel Winn
Guest
Rashel Winn

“You may have seen my 2013 World Barista Championship Routine (here).” – did you mean to insert a hyperlink at “(here)”..? 🙂

Corey Autobee
Guest
Corey Autobee

Matt, This is great in its linearity; very teachable and understandable. Thank you! On the other hand, how much less ‘black and white’ are these grind principles when brought into context with water quality (mineral content, hardness/softness, tds, etc. , and especially roast/development profiles)! I look forward to hearing your thoughts on these topics. Within this particular topic of surface and time, I’m finding that I prefer the taste of less and less (and less!) agitation, turbulence, for my filter coffee extractions. With advances in roasting, grinding, and brewing (even roast development, even particle distribution, and even flat bottom brewing… Read more »

Dean Mercer
Guest
Dean Mercer

Top explanation Matt! Although I think that if I start to speak about your article with anyone who is not a barista, I’ll be banished from all further conversation! #coffeenerdsunite

Mark Burness
Guest
Mark Burness

Hi Matt, Sorry, I’m not talking about a specific coffee/roast level, I mean regarding French press generally. I’m getting nominal extraction, but not overextraction. I don’t think this is atypical.

Matt Perger
Guest
Matt Perger

Hi Mark

If you’re not getting higher extractions after that long, I’d look to roast development as the issue.

bjeck14
Guest
bjeck14

Really nice post Matt. Looking forward to further discussions about grind size/consistency

Gera Davidson
Guest
Gera Davidson

Hi Matt Thanks for all the great work. I am still having issues with relating longer brewing times with higher extraction from my own personal experience, and also from reading comments by Ben Kaminsky about longer espresso brewing (over 35sec) which showed lower overall extraction and James Hoffman who did similar experiments with cupping and found longer brews to show very little rise in extraction if any. I also remember reading an article by Jim Schulman who found espresso to extract almost as far as it would go within the first 20sec (not sure if he changed his mind later… Read more »

Mat North
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Mat North

When you say even particle size, are you referring to an even distribution or normalised grind size, ie a very tight distribution peak?

Mat North
Guest
Mat North

Pressure is an interesting variable to add in to the mix. I’ve had excelent espresso shots from the Ek43 at a 6bar extraction and much longer lungo style shots at 3-4bar.

But pressure and flow rate may be something for a future hustle I feel.

Matt Perger
Guest
Matt Perger

It’s a bit of a misnomer as I’m trying to use a phrase most will understand without explanation. What I really mean is the total spread from smallest to largest grind is smaller.

Matt Perger
Guest
Matt Perger

Hey Corey

I’ve got a few things to write about most of those so I’ll keep it brief here. Agitation and stirring are great and, in my opinion, crucial. Low pressure brewing is amazing with a grinder like the EK43 and I regularly turn the pump down to 4-6bar with great results!

Marshall Hance
Guest
Marshall Hance

Roasters ~should~ be cupping their roasts at ~20% extraction. That’s with low turbulence and moderate grind size in an immersion brew; basically a french press.. If you can’t get significantly more out with a finer grind, something is up with either the roast, the grinder, or the water.

Under Pressure Espresso
Guest
Under Pressure Espresso

In my experience longer brew times allow for more development time of various parts of the coffee, not necessarily extraction %, although that must be a factor to an extent. By this I mean that we are allowing more time to development the fruit acids and allowing the sugars and maillards to be fully extracted to create much more balance in the cup. In pure figures, this could show up with similar TDS % or if you’re calculating the overall extraction yield, maybe it will be similar. But there’s no denying that the flavours extracted will be very different. Of… Read more »

Ian Carl Picco
Guest
Ian Carl Picco

I can relate to this Mark. I’ve had trouble over the past year getting proper ext % yeild with a french press. My eventual solution was grinding finer (same as cupping grind), extending brew time (5 min), and added turbulence (stirring at beginning and end). You’d think with would produce a muddier cup but it doesn’t, as the amount of fines remain relatively the same as grinding at a coarser setting. Many of my restaurant customers brew by French press, and it’s often difficult to get their staff to want to take these steps while brewing (waiters are annoyed by… Read more »

Matt Perger
Guest
Matt Perger

Hey Gera

What you’re seeing is a result of the ‘law of limiting returns’ with espresso grind. As you keep grinding finer you’ll eventually be limiting extraction rather than promoting it because the grinds will be blocking the flow of water.

David Baillie
Guest
David Baillie

Hi Matt do you think that light mixing is a way to counter the effects of concentration gradients slowing down the rate of extraction in an uneven manner within a given brew. That is, solutes move from areas of high to low concentrations of solute and the rate of movement slows as the surrounding solvent becomes more concentrated. if a solvent (water in this case) has high heterogeneity with respect to areas of high and low concentration then extraction rates will likewise be highly heterogeneous/ uneven. Mixing at some point, (midway??) promotes homogeneous solute concentration in the solvent.

Marshall Hance
Guest
Marshall Hance

So I’m hearing find the grinders sweet spot, then adjust other parameters accordingly.

Generally, I see baristas treating brewing as either an over or arbitrarily constrained solution. This is one constraint that holds up to reason.

Beyond eyeballing, how can this concept be applied in practice at home or on the bar?

Chris Grainger
Guest
Chris Grainger

How much affect does the brew temperature have on the overall extraction?

Kook Won Jung
Guest
Kook Won Jung

Hi Matt
good time at the South Korea Cafe show
In one description may have questions.
organic salts… What do they talk about chemical

Matt Perger
Guest
Matt Perger

Full immersion with boiling hot water and a reasonable grind should always give you a decent extraction. Are you using cooler water/not stirring/not covering the brew?

Gera Davidson
Guest
Gera Davidson

Tnx for your reply Matt, I can see what your saying but how does that work if the same brew ratio is kept, after all you’ll be getting the same amount of water to flow through the coffee but slower. could it have something to do with a shift in particle distribution at such a fine grind?

Gera Davidson
Guest
Gera Davidson

Hi tnx for your great comment, how do you differentiate development from extraction on your example though? seems to me they would be the same thing no? don’t get me wrong I’ve had the same experience my self – a 25 sec shot might show similar extraction to a 35 sec shot (if brew ration is same), but would taste and feel very different. I always thought it might have something to do with differences in water temp and pressure profiles and the change in exposure time.

Matt Perger
Guest
Matt Perger

That’s precisely what stirring is and does. Spot on!

Matt Perger
Guest
Matt Perger

20 is a bit limiting. Maybe better to say ‘cupping at a consistent extraction that mirrors service’.

Matt Perger
Guest
Matt Perger

I wish it was that easy to find a sweet spot. What I try to do is achieve the highest possible extraction that doesn’t taste dry with the coarsest possible grind. That is almost always the best combination.

Matt Perger
Guest
Matt Perger

In the case of the super fine grind, the water isn’t actually passing through all of the coffee. Because it’s blocked, the water will find some paths through the coffee bed, but won’t be able to ‘flow’ through the entire mass of coffee. It’s essentially channeling, except through the middle instead of the edges of the basket.

Matt Perger
Guest
Matt Perger

Hey! It means that the salt is an ion with a carbon ( C ) atom, like Acetic Acid C2H3O2− , Carbonic Acid CO32− or Citric Acid HOC(COO−)(CH2COO−)2.

Shaun Spruch
Guest
Shaun Spruch

Great hustle Matt! Thanks so much for starting this awesome initiative! So much to learn…

Surely we are looking for the balance between the over extracted ‘fines’ and relatively well extracted ‘coarse grinds’. It would be impossible to only extract the coarse grinds… And this is where the ‘sweet spot’ comes in? In this elusive balance?

Thanks again!
Shaun

Matt Perger
Guest
Matt Perger

Brew temperature affects extraction a lot. I’m saving it for another post though 🙂 In a nutshell, high temp = quicker dissolving action, and cooler = slower dissolving action.

Mark Burness
Guest
Mark Burness

Thanks Matt (& other responders), I don’t think I have made myself clear. I can easily achieve nominal extractions, what I am finding difficult is to overextract the coffee (pretty much any coffee, given reasonable composition brew water). Of course, I can make ropey brews but usually this is down to not extracting enough/overagitation/high incidence of non-dissolved solids, not measurable extraction, which will be comparable to great tasting brews. Typical target is 23%, in many cases that is all you will see (maybe even <21% at typical grinds)…even if you leave the press until cold. Tasty brews have measured up… Read more »

Daniel Smith
Guest
Daniel Smith

Hey Mat, awesome article. I’m not sure if you will cover this in a later issue. I was under the impression that a conical burr would produce more consistent grind size. Could you please shed some light on this? Thanks mate.

Matt Perger
Guest
Matt Perger

The elusive balance indeed! I have a lot of tips for finding that little spot. Coming soon 🙂

Steven
Guest
Steven

Heyyy fellas! I’ve played around a lot with trying to get good results out of higher-than-20% extraction brews but haven’t been satisfied and always go back to around 20. I assume it’s just that the roasters I’m working with aren’t roasting coffee to be brewed that way. Have I just not had coffee roasted/calibrated to be tasty there? It can’t be a simple light/dark thing as we’re often using very light profiles, so… what’s going on development-wise that is keeping me from enjoying coffees brewed at higher extractions? Is it just me??? Thoughts?

Marshall Hance
Guest
Marshall Hance

WOW! Grinding all the way coarse on the EK and stirring like a champ, I’m getting mind bending sweetness. I’m looking forward to where this goes. THANK YOU, over and over.

bwen
Guest
bwen

Besides grind size, are there any other ways to control the flow rate/time for pour over? My burr grinder seems to be a bit faulty and can’t grind as fine as it should. I don’t want to toss it and get a new grinder, so I can control the water some other way, that would be amazing. I was thinking in my head some kind of suction/vacuum to cover the V60 that would retain water until it was removed.

Tim C.
Guest
Tim C.

I’m assuming the reason you recommend the coarsest possible grind along with the highest extraction is to avoid over extracting the fines while getting the most out of the rest of the coffee, however, won’t the particle shapes in general be less uniform with a coarser grind? Will this make a more even extraction more difficult? And wouldn’t the coarser grind inevitably increase the distance between your two peaks since one of them is going to be finer particles?

Marshall Hance
Guest
Marshall Hance

yep. felt that one coming.

Eric Palumbo
Guest
Eric Palumbo

Are you finding that the same grind works for all of your different coffees – Africa vs. Americas for example? Because they have different positive attributes, in my mind, I find that slight alterations in grind, to accentuate those positives, results in an improved cup.

Marshall Hance
Guest
Marshall Hance

I’ve had some major breakthroughs with regards to roasting in just the past few days in no small thanks to Rao’s wisdom sinking in and the wonderfully fresh revisit to brewing basics as presented by Perger here. There’s a certain fractal quality to the formula where one revelation reverberates through the whole. I’m having good luck with well stirred brews from coarse (1mm) grounds. The fines get over extracted, but there’s fewer of them so their overall contribution is less. Super light but fully developed roasts are tricky to say the least, but more simple sugar and complimenting acidity is… Read more »

Learning from the Best: The Barista Hustle | Nordic Coffee Culture
Guest
Learning from the Best: The Barista Hustle | Nordic Coffee Culture

[…] of these hustles have already made it on the web: Coffee Extraction and How to Taste It and Surface Area and Time. If the titles seem technical, don’t worry—the articles make their points without delving […]

Nicholas Cho
Guest
Nicholas Cho

Nice one, Matt! Lots of great stuff here!

I’ll say that I teach a lot of the same stuff, but with an overall bias towards extraction of the innards of each particle. Probably because you’re spending more time on espresso while I’m focused on filter. 🙂

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