We recently added a feature to the Angels' Cup coffee app that lets you record brew methods AND the results. 
If you're familiar with brew methods like the Chemex or Aeropress, recording the brew method is straight forward. As you can see in the screenshot to the left, you can record things like temp, grind, brew time, coffee, and water weight. When you have a coffee and water weight entered, the app shows you your brew ratio. There are a few apps out there that let you record brew data, but if you really want to learn how to make better coffee, you should also keep track of the results. To help you do this, we built a 2-axis chart to record your coffee's strength and extraction. Coffee strength is easy to discern. It either tastes too watery or not watery enough. If your coffee is ever too strong, you can solve the problem by adding a little water to it! Extraction is a little more challenging, and might take some practice to understand. Coffee people usually refer to extraction in terms of sour and bitter.
Sour
If coffee is under extracted it will taste
sour because the fruity acidity comes out first. Think about the tart flavor of
citrus fruit, or granny smith apples. Those flavors come from
acidic compounds found naturally in fruit, and often can be quite pleasant. Candy is often loaded with the same types of acidic compounds, think about Jolly Ranchers or Starbursts. However, for these sour notes to taste good in a cup of coffee, they need to be balanced with the sweetness and even some bitter notes that develop later in the brewing process.
Bitter
If coffee is over extracted, it will taste
bitter. Think about the flavor of really
dark chocolate, garlic, or medicine. The bitterness in coffee takes longer to develop than sour or sweet notes, but can quickly come to overpower a cup. Here's a quick table to help you fix bad coffee: Flavor, Brew Time, Water Temp, Grind Sour, Increase, Decrease, Finer Bitter, Decrease, Hotter, Coarser
It's easy to read about how something tastes, but how easy is it to pick up on in the cup? Understanding extraction, and training yourself to pick up on sour vs bitter, can be a challenge, but a fun one! The best way to really understand coffee is to deliberately brew multiple cups incorrectly, and try them side by side.
Experiment 1: Extraction through brew time
An easy way to mess up extraction is brew time. For this experiment, you will need a "submersion" brewer, one where the coffee grounds are suspended in water with no drainage (i.e. aeropress, french press, clever). A V60 can also work because the filters are thin enough to brew super fast if you want, but a Kalita or Chemex are bad options. You'll also need good coffee beans with at least medium acidity. You're going to make three cups. For one, just follow your normal recipe. For another, cut your recipe time in half, and for another double your recipe. For most people, that will probably mean a 2-minute brew, a 4-minute brew, and an 8-minute brew. When they're ready to try them side-by-side, what do you notice? I like to think of it as treble and bass. The under extracted coffee will have all treble and no bass. The over extracted coffee will have the bass dialed up. What if the experiment didn't work for you? Either a) your beans weren't very good or were stale, and didn't have treble or bass to begin with, or b) it just takes time to develop a palate, try again with 1, 4, and 12-minute brew times.
Experiment 2: Extraction through brew temp
This one is tricky, so buckle up. First, you need a variable temp electric kettle or a kettle with a thermometer. If you have that, make three cups of coffee, the first one at 210℉, the second at 195℉, and the third at 180℉. What you'll probably notice is that temperature affects the sour/treble notes, significantly more than the bitter/bass notes. Now here's where it gets tricky. Temperature doesn't simply increase or decrease extraction, it affects the rate at which specific compounds dissolve, namely acids and carbohydrates. The hotter the water, the more acidity and sweetness you will get, keeping all else equal. However, you will also taste a shift in the nuance of the acidity and sweetness, and this shift will be different for every damn coffee. Brewing a sweet coffee at lower temps seems to produce a beautiful candy-like sweetness. At higher temps we seem to get more of a starchy, bread-like sweetness. This might be acceptable if it's the only sweetness a coffee offers, or if the coffee needs sweetness to balance some bitterness or sourness. The basic rule of thumb I'll offer is this. For sour coffee, decrease temp. Yes that will reduce extraction, but think about cold brew as an extreme example. It's never too bright. If your coffee is bitter, increase temp. It won't pull out more bitterness, but it might pull out more acidity and/or sweetness to balance the bitter. And that's the basis of 'dialing in' a coffee recipe! You start with strength, and then adjust brew time and temp to get the perfect balance of treble and bass for a given coffee. Hope that helps, and addresses some of the comments below. :)
Experiment 3: Extraction through grind size
This is actually the hardest variable to experiment with. On something like a French Press, grind options are limited by what will pass through the filter. For filter brewing, changing the grind size will greatly affect your brew time. The finer you grind, the slower your brew time will be. That being said, one of the best tricks to making great coffee is to figure out exactly how fine a specific method needs to be. Go very fine, see if the coffee comes out bitter, then re-try with coarser coffee and a shorter brew time. At the end of the day, reading will only get you so far, you have to be willing to experiment. Don't think of it as a waste of beans. Gulping coffee without paying attention to flavor is a waste, experimentation is the best possible use of good beans. So set aside some time this weekend to have fun with your coffee!
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