Coffee Science: How to Make the Best French Press Coffee at Home
A French press is often treated like Jason Segal's character in Forgetting Sarah Marshall. He's actually the one you want, but people tend to flock blindly to the flashy, temperamental types like coffee-siphon-somethings or Russell Brands. The French press is definitely a potential coffee happily-ever-after, but as with all things coffee, it ain't rocket science... but it is science! Let's delve a little deeper into how the French press works, and how you can make the best cup of coffee using this tool.
The French press, also called the cafetiere or coffee press, is a cylinder-shaped beaker (usually glass, but often plastic or steel) with a plunger. The piston of the plunger is made of mesh, allowing liquid to flow through it but not the larger coffee grounds.
With some coffee-brewing methods, the amount of brewed coffee you're trying to make and the grind size of your beans will affect how quickly the water will flow through the coffee—and how long your total brew time will be. This is true for drip brewing, pourover, and even espresso.
But brew yield, grind size, and brew time are not always inextricably linked. You can use a French press to make a lot or a little bit of coffee, you can grind your coffee however you want, and you can stop the brew in 10 seconds or in 10 days. None of these variables affects the others. This doesn't mean that the resulting brew will taste great no matter what, but this bit of freedom means you can approach a French press a bit differently. In fact, maybe you should really call it a "Freedom press!" Oh, nevermind.
As you may recall from our discussion of the pourover method, I like to talk about coffee brewing as having three general phases: wetting, dissolution, and diffusion.
Wetting is the process of fully saturating the coffee grounds. Coffee grounds are made up of cells, and each of those cells holds some of the coffee solids that we want to extract. In fresh coffee, carbon dioxide gas is also trapped in those cells, and wetting releases that gas in a moment we call a "bloom."
The second step, dissolution, is all about dissolving the solids that will make up the coffee-part of our beverage with our solvent, hot water. The final stage is diffusion: the movement of that coffee-water concentrate out of the grounds into the surrounding liquid. Dissolution and diffusion are typically grouped together by the more common term "extraction," but I think it's helpful to look at those as separate processes.
In drip and pourover brewing, the liquid surrounding the coffee grounds is continually replenished with fresh hot water. This is an important factor because the purer the surrounding water, the stronger osmotic pressure drives coffee concentrate out of our grounds, and the more efficient our extraction. On the other hand, the constant flow of clean and hot water over the surface of our coffee grounds extracts those outer surfaces more aggressively, which means we have less time to brew before those outer surfaces are so extracted that they add less-tasty, 'overextracted' flavors to our brew. The best coffee brewing is about dialing in our many variables just right to get the best balance of maximizing good flavors and minimizing the less-delicious stuff.
The French press is a pretty different environment for coffee brewing. Drip or pourover brewing is a lot like a convection oven, where the convective heat (in the form of flowing water) speeds up the energy transfer in our little coffee chemistry set. In our low-and-slow French press, you're not adding more water in as you go, so the energy driving diffusion is decreased, resulting in slower, more gradual brewing. There's less of the surface-overextraction effect, and the brewing is ultimately a more gentle proposition. Put it all together, and French press brewing is less finicky than most other methods and can result in a more full-flavored brew with a deeper sweetness and syrupy body.
French presses have mesh filters that do a good job of holding back the grounds, but there will be a small quantity of powder-like coffee grounds, called fines, that will make it through the filter and remain suspended in your brew. Don't let those distract you too much. The fines can give the sensation of more viscosity and richness.
Try It at Home!
Here's my basic technique for making great French press coffee. As with all methods of coffee brewing, you'll need to experiment and tweak the variables a bit, tasting your results before you settle on your ideal settings. The good news is that French press is a good bit more forgiving than the faster brew methods.
Have a watch or stopwatch handy to time your brew. Your smartphone probably has one hidden in its 'Clock' app.
1. Start with a very coarse grind, maybe at the coarsest setting on your grinder. The particles should appear somewhere between coarse salt and steelcut oats. Take note of your grind size so you can make adjustments later: grind a little finer next time if your brew was weak, a bit coarser if you're tasting a lot of unpleasant, dish-raggy, overextracted flavors.
How much: While there's a maximum amount that your French press will make, there isn't really a minimum. A good coffee-to-water ratio is between 60-70 grams of coffee per liter of water (a mass ratio between 1:16 and 1:14). Decide how much brewed coffee you want to make and weigh out the right amount of coffee.
2. Get your clean (filtered if you need it) brew water ready. With French press, you're good to pour your water right off the boil unless you've got an insulated (or double-walled) press, in which case you should wait about 30 seconds off of boil. If you're brewing dark-roasted coffee or decaf, it's better with water about 10 to 15°F lower.
3. Start your clock and add your water. Some people like to add a little water, stir, and add the rest. It really doesn't matter. The important part is what you do after you add the water. If you were to just sit back and wait out your brew time now, you'd have an under-extracted brew, because the release of CO2 gas will cause your grounds to rise up and float on top of your water. Remember that first phase "Wetting?" Well, if you don't have good wetting, you don't have much of anything that follows, so you should give your coffee and water mix a gentle but thorough stir at about 30 to 45 seconds in. You'll know you're good to put the lid on and move on to the next step when most of the coffee has sunk and isn't floating anymore.
4. This may be very different from what you've heard before, but bear with me: shoot for a target brew time between 6 and 8 minutes. "What? I thought it was 3 to 4 minutes!" you might say. You can brew in 3 to 4 minutes if you want, but to get good flavor results, you'd be grinding a lot finer, and you're not getting the most out of the unique qualities of the French press. Give 6 to 8 minutes a try with the coarse grind, and see if you can dial that in.
5. When you're ready to stop your brewing, it's time to plunge. So as I've mentioned, French press is a nice, slow, gentle brew. One great way to ruin that niceness would be to violently agitate your coffee grounds, accelerating extraction right at the end when your coffee has already given up the good stuff and the bitter and astringent negative flavors are in danger of taking over. Plunge gently. If you feel the plunger start to get tight, back it up an inch or two and resume plunging. Once you get to the bottom, you're done!
6. If you've plunged your bed down nice and tight, there isn't a lot of brewing that will happen from this point on, but it's still ideal to pour off your entire beverage right after plunging to truly stop the brewing process.