More than anything else, extraction determines how good your brew is going to be. Understand it, and you’ll be well on your way to an amazing cup.
Extraction describes the process of pulling flavor from each speck of ground coffee. It’s the reason the water goes in clear and comes out brown: As it passes through the grounds, it’s dissolving all kinds of compounds and taking them straight into your cup. Here’s the tricky part: Some of those compounds taste great, but others are kind of nasty. To get the good ones, and the right amount of them, you need to properly extract your coffee, meaning that the water dissolves the right stuff, and the right amount of it.
Too little, and your coffee is under-extracted; too much and it’s over-extracted.
Note that extraction isn’t the same as strength (which varies mostly according to how much ground coffee you’re using for a given quantity of water). The somewhat confusing truth is that you can have a strong cup of under-extracted coffee or a weak cup of over-extracted coffee. For now, we’ll focus on extraction; strength we’ll deal with soon.
– Whatever you usually use to brew coffee (e.g., a French press, Chemex, Mr. Coffee, etc.—a paper-filter method is best for this exercise, but it will really work with anything).
– Coffee ground to three different levels: Very coarse, somewhere in the middle and very fine. (If you don’t have a grinder, you can ask the baristas at a local café for help.) Make sure you have enough of each to brew a cup or pot, however you normally brew.
Now, go ahead and brew a batch of coffee using each grind size, doing your best to keep brew time and water temperature consistent.
The very coarse grind should yield an under-extracted brew. The grounds are so coarse that the water won’t be able to extract enough good-tasting coffee stuff from them, resulting in a brew that’s sour and grassy, with a sensation reminiscent of underripe fruit. (If you’re using a French press and want to get the full, exaggerated effect of under-extracted coffee, try brewing the very coarse-ground coffee for half the usual time. The coffee will be totally gross.)
The very fine grind should deliver an over-extracted cup. Because the grounds are so fine, the water will have no trouble extracting way too much caffeine and other pungent compounds. The result will be bitter and astringent. (If you’re using a French press and want to get the full, exaggerated effect of over-extracted coffee, try brewing the very fine-ground coffee for twice the usual time. The coffee will be totally gross.)
If your in-between grind is on target, the coffee will be properly extracted. That is, it won’t be sour and it won’t be bitter. If your brew went really well, it will have a natural sweetness.
Next time you make a cup, taste it for diagnosis: Is there a sort of sour and drying sensation when you take a sip, or does it leave a bitter aftertaste? Adjust your grind size slightly finer or coarser, respectively, to wind up with a more balanced and delicious cup.