Ah, the coffee bean. Powerful. Majestic. Brown. But for something so essential to the lives of so many, it is truly a misunderstood entity. For instance, a coffee bean isn't even a bean; it's the seed of a fruit that's called a "cherry," even though it isn't technically a cherry, but a berry. And, as though to illustrate nature's keen sense of irony, that caffeine we're so eager to consume is thought to have originally been a defense mechanism.
Not only is coffee delicious, it's quite pretty.
Coffee beans can be just as confusing to actually work with; just think of how many bad cups of coffee you've had in your life. But don't worry. I have studied with them and I have learned their secrets. Here are a few principles essential to understanding how to brew a good cup of coffee.
Basically, there are four variables to keep in mind: Temperature, Ratio, Size, Time.
Temperature - This one is pretty simple. The higher the water temperature, the more flavor that's extracted more quickly (this is why cold brew needs to steep for 12-24 hours). But, if the water is too hot, it burns the beans. The ideal temperature is somewhere around 200 degrees F, so after you boil your water, let it sit for a minute or two to cool before you use it.
Ratio of coffee particles to water - I'm not going to go into exact ratios because, again, this is pretty intuitive. The higher the ratio of coffee to water, sometimes referred to as "the dose," the stronger the end product will be. Boom. But don't go too crazy; there's a threshold to how much coffee water can absorb, so after a certain point you're just wasting precious beans. I would start with something like 20 grams of coffee for every 10 ounces of water, and go up or down from there.
Size of particle - The finer the grind, the stronger the cup of coffee. But why? The coffee you drink is the result of water coming into contact with coffee grounds and absorbing oils and particulates from them. A finer grind creates smaller particles, and more of them, meaning there's more surface area exposed to the water. And the more surface area the water comes into contact with, the more flavor it can extract from the grounds.
Amount of time water is in contact with grinds - The longer the water spends in contact with the beans, the more it can absorb. This is pretty easy if you're brewing with a french press - you just plunge after about 4 minutes. Pour over methods, like a Chemex, are a little trickier because you have less control over how long the water remains in contact with the grounds. Stupid gravity!
This last one ties together all of these tenets. For example, the finer the grind, the less time the water needs to be in contact to extract the flavors, because you've made it so easy for the water to get at the grounds. But, making the particulates smaller will also make the water fall through them slower, so you may find that you need to lower the dose. It's a balancing act, but if you can identify the flavors in your cup, you can identify what went wrong in the brewing process.
A Note on Notes
Generally, the bright fruity flavors are among the first to be absorbed, and the sweet chocolatey flavors need a little more time. Bad cups of coffee are exaggerated results of this basic principle. So if your coffee is coming out tasting sour and watery, it's probably under-extracted, meaning the water has only been able to absorb those first notes. This is usually because the dosage is too low or the grind is too coarse. But stronger doesn't always mean better. Over-extraction, as I'm sure you can infer, occurs on the opposite end of the spectrum and results in a flat, bitter taste and a lingering astringency. This is because the water has had enough time with the grounds to absorb more of the bitter compounds from the grounds. This usually results from too fine a grind.
An expert artist's rendition of extraction. The coffee grounds on the right are finer, exposing more surface area for the water to interact with.
Beans, Beans, They're Good for Your Heart
Okay, but what beans should you buy in the first place? Blends tend to be more basic and are designed to be reliable. Single origins (beans from a single country and ideally from the same farm) can be a little pricier but are often more complex. For instance, some farms allow the fruit to dry out on the seed after harvesting, which imparts those fruity berry flavors into the coffee. (This is called "dry process" or "natural coffee" and is popular with African coffee growers.)
So different beans will certainly have inherently different profiles, but the actual roasting of the beans is what creates the most dramatic flavors. The same chemical reaction responsible for the flavor of a seared steak or a loaf of bread, called the Maillard reaction, is what produces the sweet, almost caramelized, coffee flavor. But the longer you roast it, the more bitter and burnt-tasting the beans will be. Recently there has been a trend toward shorter roast times and single origin beans, eschewing the practice of homogenized blends and overroasted "dark-roasts" and instead trying to highlight the varying delicate flavors. Overroasting also has the unfortunate side-effect of burning off some of the caffeine, so that pour over you spent $5 on at the hipster coffee shop does, in fact, pack more of a punch. If you're into that.
The roasting process creates over a thousand volatile compounds, and these are responsible for the flavors and aromas of a cup of coffee. But because these delicious compounds are so unstable, they begin to oxidize almost immediately, breaking down when exposed to oxygen. While grinding the beans allows the water to interact with more particles, it also allows oxygen to degrade the chemical compounds more quickly. If you are brewing with freshly ground beans, you may notice that the grounds will froth and rise when you pour water over them. This is called "the bloom," and it is caused by this degradation - those bubbles indicate that the beans are still full of these aromatic compounds. Older coffee has had more time to oxidize, so the bloom will be far less noticeable, and you may even need to raise the dosage to achieve the same flavors.
Long story short: Buy fresh beans (preferably beans that were roasted under a month ago), and don't grind them until just before you are going to brew. And drink it fast! Because these chemicals are so volatile, they actually continue to develop after you've brewed it. This is why an old pot of coffee doesn't taste as good as the fresh stuff - the sweet buttery lipids break down into acids and make it sour.
Which grinder is right for you? Probably the burr grinder.
Keep Your Nose to the Grindstone
Okay, so let's talk about equipment. Whatever method you use, I would highly suggest investing in a burr-grinder for your beans. The alternative, a blade-grinder, is basically just a blender you put coffee in. The grinds it produces may seem mostly the right size, but it will always be a little uneven, and this variance means you lose a lot of control. Burr grinders ensure a uniform particle size with settings that allow only particles of a certain size to slip through their teeth. They're a little pricier, but the rising interest in home coffee systems means you can get one for about 40 bucks. (You can also get hand operated ones for even less, which are great for trips.) You might also consider a food scale to have precise control over the dosage. It may seem pretty extra, but you can pick one up for 10 bucks. If you don't mind being a little less precise, you can always measure by scoop, about 2 tablespoons for every 10 ounces of water, and I promise I won't judge.
Just look at that beautiful bloom.
Look to Heaven, While it Goes the Other Way
Though these principles will always apply, the brewing method can have a dramatic effect on the end result. For instance, paper filters absorb some of the oils, so brewing methods that use metal filters, like espresso and french press, tend to be a little richer. I, myself, like to use an aeropress because of its versatility; I can make a pour-over or a french press style cup depending on my mood. It's also basically a modified turkey baster, so that's fun.
The final caveat is this: Brew what tastes good to you! If you like a light and fruity balance, try a slightly larger dose of coarsely ground natural beans. If you want that dark sludgy punch, grind that Columbian coffee a little finer. Keep in mind that single origin beans go in and out of season and that the blend you usually buy could be made up of entirely different beans from month to month. And while the roasters try to keep the same flavor profile, sometimes, for better or for worse, you may need to tweak your recipe. Just try to only change one variable at a time, so you can isolate its effects. Happy brewing!