Everyone knows basically how coffee works: you grind the beans, you run hot water through the grinds, and voilà, you have coffee. If you just need a kick of caffeine in the morning, this is probably all you need to know. If you are going to make a truly good cup of coffee worth savoring, however, you’ll probably need a little deeper knowledge of what’s going on in the process.
Brewing coffee is all about extraction. Imagine if you will, holding a granule of coffee grounds between your thumb and index finger. Inside that granule are dozens of soluble flavor compounds, some good, some not so good. Your job is to get out the good ones and leave behind the bad.
I tend to think about these flavor compounds in three general categories: acids (sour), sugars (sweet), and bitters. The trick is to extract as many of the desirable flavors and as few of the undesirable flavors from that granule as possible. If you extract too few, this is called under-extraction; too many, and it’s over-extraction.
When your granule comes in contact with hot water, the acids will dissolve the fastest, followed by the sugars, then the bitters. There is overlap, so it’s not as simple as that, but that’s the general rule. Under-extracted coffee will taste sour, as there aren’t enough sugars to counteract the acidity. Over-extracted coffee will taste bitter, because our taste buds are so sensitive to bitterness. I’d wager most Americans have never had a cup of under-extracted coffee, but they have had plenty of over-extracted ones.
An ideally extracted coffee will be neither sour nor bitter; it should be well balanced: both acidic and sweet, with minimal bitterness. This means you have extracted most of the acids and sugars, and then stopped extraction before too many of the bitters have dissolved.
“But”, you ask, “what if I like my coffee strong?” I’m so glad you brought that up.
So far I’ve been talking about extraction. Extraction refers to how much flavor is dissolved from the coffee grounds. Strength is something else entirely: strength is determined by how much water that flavor is dissolved into.
Most people never think about this distinction. This means you can have weak but over-extracted coffee; for an extreme example, put one teaspoon of grounds in a pot of french press and let it sit for ten minutes. You can also have strong but under-extracted coffee; this is probably harder to do to such an extreme, but run a couple ounces of water through a large filter full of beans and you’d probably be close.
If you want stronger coffee, then use more grounds. You want more flavor per ounce of water. So don’t try to squeeze out those last bits of nasty, bitter flavor compounds from your beans—you’re ruining your coffee by doing this. Instead, give yourself more beans from which you can extract more acids and sugars. You can have a very intense, strong cup of coffee that has very little bitterness to it.
A darker roast also has nothing to do with strength. Don’t buy “French Roast” thinking that’s how you’ll get stronger coffee (hint: that’s how you’ll get coffee beans with a lot more bitters, and fewer acids and sugars).
Remember, extraction is king. Don’t sacrifice it in the name of “strong” coffee.
Factors affecting brew
There are five key factors that affect brewing. I won’t go into them at length, but they are:
- Grind size — typically you want to match this to your brewing method; courser for slower methods like french press; finer for faster methods like pourover
- Water temperature — generally, 195°–202°F is ideal
- Time — aim for 4 minutes for French press; 2 1/2–3 minutes for pourover
- Coffee-to-water ratio — a good starting point is 1 gram of coffee grounds per 16 grams of water. Yes, it’s more accurate to weigh your beans instead of counting scoops, as beans vary by roast
- Agitation — Stirring your french press continually will speed up extraction and potentially even change the flavor profile of the coffee
Experiment with these and see how they affect your coffee. Of course, various brew methods give you varying measures of control over them: Automatic coffee makers don’t give you much leeway, as water temperature (they generally run too hot), time, and agitation are out of your hands. A French press gives a little more freedom. If you’re truly daring, get a pourover like a Chemex or a Hario V60, which give you complete control over all five (but also require some practice to get right).
You can also experiment with cold-brew Toddy. The flavor compounds of the bean extract very differently in cold water over long periods of time, resulting in very even extraction with little bitterness.
You’ll notice I gave a lot of attention to extraction of a single coffee granule. That’s because there’s another thing to consider when you start talking about brewing a whole cup: the evenness of the extraction.
It is quite possible to over-extract some grounds and under-extract others all at the same time, resulting in an imbalanced flavor. There’s a lot to it that I won’t get into, but if you decide to try a pourover brew method, this is something you should be aware of.
 How much of each type of flavor compound beans have depends on several factors: 1) the origin and varietal of the bean 2) How dark the roast is. Roasting brings out the sugars, but roast too long, and the acids and sugars get cooked out and replaced with bitters (burnt flavor) 3) How recently the coffee was roasted, and 4) How recently the coffee was ground. Some of the flavor compounds are volatile, and break down over time, especially after grinding. These latter two are why you should check the roast date on the beans you buy (ideally less than a few weeks) and only grind minutes before brewing.
 This is the weakness of the French Press: you cannot entirely stop extraction. After pushing the plunger down, some hot water in still in contact with the grounds beneath the filter. And then, even after pouring your cup, you still have tiny granules (“fines”) suspended in the coffee, due to the larger holes in the filter. These granules will continue to extract until everything they have to offer is in the water.