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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