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Making a delicious french press redux. For nerds.

inspired in most part by today's Gizmodo post by Matt Buchanan http://gizmodo.com/5868602/why-you-shouldnt-buy-anyone-a-french-press

Critical analyses of coffee brewing methods involves taking a close look at what happens to individual coffee particles through the course of the brew. Every brew method involves three notable steps: Wetting, extraction, and separation. For some methods, extraction and separation happen mostly simultaneously (i.e., drip brewing). For other methods like cupping, french press, or Clever dripper (among others), extraction happens in a relatively static environment with separation occurring only (or mostly) at the end. It's important to note: separation always includes extraction, often with an accelerated or more forceful dynamic.

During such methods, the fact that separation of grinds and brewed product (the beverage) happen at the end of the brew also means that the separation is occurring when the risk of over-extraction is high. You could stir the hell out of a brew at the beginning, but the cross-over into over-extraction usually won't happen until later in the brew time.

MOST french press brews out there in the world are woefully underextracted, and not representative of this brew method's potential. Worse, high-sludge content due to sloppy or poor technique, contribute what can only be called "false body," that is, mouthfeel that comes from the super-fine coffee particles in the brew, rather than from the coffee itself. This is even further exacerbated by very dark roasted coffees, for which the roasty-toasty-burnt flavors are so soluble that they dominate an otherwise weak and underextracted brew. Grinding approximately for drip and brewing for under 5 minutes is generally leaving desirable flavors out of your brew.

So to make a delicious french press is to make a properly extracted french press. To do that, there are three key elements to consider:
1) proper wetting
2) the static environment
3) the separation dynamic

1) Proper wetting
In order for extraction to happen efficiently, the coffee particles must be free of gas, and surrounded by water on all sides. If coffee grounds are floating, or intermixed with gaseous bubbles, one or both of these requirements aren't met. Gentle stirring, a prolonged pour of brew water, or a light plunge of the filter screen will help fully wet the coffee.


2) The static environment
A static extraction is both good and bad for the quality of the beverage. It's good because certain brew methods can inflict too much kinetic energy (read: turbulence) on the coffee grounds which causes the surfaces of the coffee grounds to extract too quickly. A more static environment allows the solubles to diffuse more naturally into the brew water. Conversely, a static environment can cause the water around the coffee grounds to become over-saturated with solubles, which slows down the osmosis effect necessary to get the solubles out of the coffee particles.

So the solution is to extend the brew time beyond what's typical for a drip-style brew, and grind significantly coarser. If an ideal drip-grind peak particle size is approximately 800μm (0.8mm) in diameter, something around 1200μm (1.2mm) is great for french press. Why?

The longer brew time is necessary to correspond with the static brew environment. However, since a longer brew time would normally lead to over-extraction, reducing the overall surface-area to coffee-mass ratio helps reduce the proportion of over-extraction in the brew. Remember, because coffee grinds are not truly uniform in particle size (unless we physically separated them and removed outlying particle sizes), a coffee brew is always a mixture of "good" brew, overextraction, and underextraction. The overall better-quality brews are simply higher-proportions of "good' brew than not. The smaller-than-peak grinds will overextract. The larger-than-peak grinds will underextract.

So if a drip brew with 800μm grind sizes is a 4-minute brew, and a french press is a 6-minute brew, and we wish to reduce the surface-area-to-mass ratio of the coffee grinds accordingly, we come to a 1200μm grind size.

Obviously a coffee particle is not a sphere, but the results are the same if you use any shape as a model:
Volume of a sphere = 4/3 * π * radius-cubed
If diameter = 0.8mm, volume = 0.268 cubic mm
If diameter = 1.2mm, volume = 0.905 cubic mm

Surface area of a sphere = 4π * radius-squared
If diameter = 0.8mm, surface area = 2.01 square mm
If diameter = 1.2mm, surface area = 4.52 square mm

Volume / Surface area
@0.80mm, 0.268/2.01 = 0.133
@1.20mm, 0.905/4.52 = 0.200

∴ 0.133 : 0.200 :: 4 minutes : 6 minutes

So grind coarse (about 1.2-1.5 mm diameter peak grind), and 6-8 minutes. Yes, really. Not only does this coarseness work in consort with the brew method, grinding this coarsely results in a smaller proportion of extra-fine particles.

It may help to very-gently stir-up the coffee grounds at the end before you plunge, so that the flavor compounds that diffused out of the grinds sitting at the bottom of the glass (which you'll have if you were successful in your wetting) will be added to the mixture, rather than just sitting at the bottom of the glass.


3) The separation dynamic

The separation between grinds and resulting beverage should always be as gentle and low-kinetic-energy as possible, especially for brewing methods with a primarily static brew dynamic like french press. The particles at the end of a brew have already released the desirable flavor compounds. If you forcefully stir or squeeze the coffee particles at the end of the brew, you're much more likely to add overextraction-flavors to the brew than otherwise.

So plunge gently. If you feel even the slightest resistance due to a layer of coffee grinds building under the plunger, back off gently and press on. If you forcefully press grounds downward in the glass with the plunger, you're forcefully extracting those grounds. So don't do it. You're also forcing more super-fine coffee particles (a.k.a. fines or sludge) through the mesh if you plunge hard.


Otherwise, you know what to do. 60-70 grams of coffee per liter of water, and ~200°F (±5°F) brew water to start. Final point: technique (or method) is important, but quality of coffee, quality of grinder (uniform as possible), and quality (clean) water round-out the four pillars of great coffee brewing.

Let me know in the comments how this works out for you!
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15 comments
 
Excellent Nick and a timely bit of writing. I hate to see the old french press being dismissed because it's kind of hard or requires more care. Somebody forgot to tell Duane, who built his business on pressed coffee, at least in part. Perhaps it is not the best brewing device for introducing someone to hand brewed specialty coffee but it is the method that made me a specialty coffee drinker some 18 years ago. For certain coffees, and when I have the time to give the process the attention it deserves, I still consider french press something of a luxury and a deeply satisfying cup. And next time I'll have this page open to check my moves. Thanks.
 
thanks for including some science up in this piece. It bothers me to no end when someone who is an "expert" dismisses a standard without having some sort of hard proof that what they are promoting has any benefit.Also,new tag line for the SCAA= "coffee: based on science and yummyness.
 
I just tried to replicate this method. I agree that it makes a far superior coffee to the tradition approach. Clearer flavors, less body. Thoroughly enjoyable. But slightly over-extracted. I will try again at a coarser grind soon. 
 
We are going to pull our our micrometers and run this today. We are going to test using Freling presses, so I will be interesting to see how temperature stability affects the over all brew time. If the equipment you used was pictured, how much drop did you have using glass? This is what I love about brewing coffee, the variables of the equipment change the outcome so much. Thanks nick
 
I'm lacking a bit of understanding in the extraction. Although I'm aware it's incorrect, why is it not possible to simply brew everything quickly with a fine grind? Such as French Press.
 
+Dylan Johnson A great question! Basically (and to simplify things a bit), the coffee solubles take one of two routes:
1) the stuff towards the center of the coffee particle dissolve into the brew water by diffusing through the coffee particle and out of it, or...
2) the stuff on the surfaces of the coffee particle dissolve readily into the brew water since they're right there.

The problem when brewing coffee is that you have desirable flavor compounds, and undesirable flavor compounds. Generally speaking, the longer you brew, the more you delve into those undesirable compounds. If you grind fine, you're not only reducing the size of the coffee particles, you're also exposing a lot more surface-area (per unit mass of coffee). On top of that, there are certain desirable compounds that take some time to extract (especially some reducing sugars). When you put it all together, you want your grind size at a point large enough that you can minimize surface area so you have less surface-area-overextraction, small enough so the solubles inside the particle will diffuse out during the course of the brew, and all in a brew time that maximizes extraction of the good stuff, while minimizing the extraction of the not-so-good stuff.

You can brew quickly with a fine grind, but it's not easy (and extra-sludgy), and even if you're successful, you're still going to miss out on some of the desirable flavor compounds that contributes rich sweetness. A lot depends on a lot of variables though, so your results may vary.
 
Thanks so much for that explanation! 
 
So would the reason coffee for a manual drip filter (eg pour over) is ground finer (than french press) be because of the increased turbulence on the outside of the coffee particle, and therefore a need to complete the brew faster? Or, I guess, simply the fact that increased turbulence = faster extraction. (Apologies if this is diverting from the topic).
 
Increased turbulence means you need to (or _can_) brew for a shorter time, lest you over-extract the outer layers of the coffee grounds. Since you brew for a shorter time, you have to grind the particles smaller, lest the solubles towards the center of each particle have too far to travel in that short amount of time.. It's all about balance and managing multiple variables simultaneously. Nobody said it was easy.
 
My french press coffee improved markedly when I stopped plunging entirely. I use a spoon to scoop off the grounds and foam (which seems to contain most of the fines) floating on top of the press and then gently decant the coffee off of the sunken grounds. I think I got this technique from Jim Hoffman's blog post at http://www.jimseven.com/2010/11/04/cupping-vs-french-press/

The one concern I have about increasing the grind size and the necessary increase in brew time is that the temperature drop during brewing would result in underextracted or otherwise poor coffee. Does the brew temp stay in a reasonable range during 6+ minute brew times? Is the best FP coffee made in a 196F, pid-controlled water bath?
 
Mmmmm.... time to brew another cup using the press.  Thanks for the info.
 
 I know i'm not using top of the line equipment, nor coffee, but i recently purched some fresh light roasted ethiopian s. beans and want to get everything down before I start using this stuff.  (especially at 16$ a lb) 

I'm using a bag of charbucks cafe verona whole bean, i have a main brand burr grinder ( i think it's mr coffee or something like that. just a cheapo)   An oxo FP (with the grounds keeper deal)   i use a stove top kettle and thermometer.   

I start the water to boil,  which i let boil then cool to about 203 F
 ( to account for the temp drop when i pour it on the grounds) I ground the beans to be between one and two millimeters, ( just a few click above what would be drip machine setting)  out the really fine particles with a sieve, (put them aside for a quick espresso in a bit) put them into the FP,   got the timer ready for 6 minutes, and hit go as soon as the water hit the grounds.    I stirred the grounds in the water to get out air and gas,  then plunged just a tad.   The resulting coffe did have a layer of crema on it (light, but there) as well as the coffee poured off of it very easily, leaving a cake in the bottom of the deal.  after 6 mins, poured off into my mug.

(previously i would fine-med grind the coffee, and do a 10 second stir, and a one min 20 sec steep, then pour off)   

My results-   extremely different smell,  extremely different taste.   good body,  not too much sediment, (very little actually)  but it wasn't near as sweet as previously.  I wouldn't go as far as bitter,  but more burnt.    The cup was not as satisfying,    is there something i should be doing different?   larger grind?  keep the fine stuff?   smaller steep time?  or is it what I think it is.  the coffee itself  (not a huge fan of real dark coffee as it is,  but the coffee was a gift.     Should i just try a cup of the good stuff?!   i really don't want to ruin the beans extremely delicate flavor. 
 
What is a recommended manual course grinder?  i have the porlex and it sometimes gets a bean stuck making everything fine because it forces the burrs too close.
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