First thing in the morning, it can be hard enough to find matching socks, much less troubleshoot what's awry in a cup of coffee gone south. While some problems can't be corrected with a simple re-brew, many can. Here's our quick guide to identifying some of the most common things you'll find wrong in your coffee, and how to fix them. Note that while these tips are broad enough to address problems found across a variety of filter brewing methods, they do assume you're starting with good quality water (filtered, not distilled) at the right temperature, and good quality, evenly ground coffee.
Symptom: Sour Flavor
The problem: Underextraction Coffee that's too sour to the taste hasn't had enough time to extract, resulting in not just "weak" flavor but a sour one as well. If the coffee you're drinking is one you expect to have natural sweetness, full mouthfeel, and a well-rounded flavor profile, but instead you're stuck with something hollow and sour-ish: an underdeveloped, underextracted cup.
The solution: Adjust grind or time To allow more soluble coffee particles to be extracted into your cup, you're going to have to get more out of them either with time (letting it steep more) or grind. Particle size determines exactly how your coffee will extract, so a grind size that's large will require more extraction time than a grind size that's finer. Bring your grind down a bit and try again. (Alternately, you can steep your coffee for longer—compare which resulting flavor gets you to the point you want, and stick with those parameters.)
But also... Coffee prepared as espresso can also ring a sour note if it's prepared too soon after roasting. This "fresh", sharp, sour flavor is associated with espresso coffee whose CO2 gasses haven't yet escaped and allowed the beans to, well, mellow. While this flavor can be found in filter coffee preparations of very fresh coffee, it (like any flaw) is exponentially more pronounced in espresso.
Symptom: Bitter Flavor
Coffee's bitter? You've likely extracted too much from your grounds. For a great lesson in overextraction, take your French press—and maybe you already do—and leave some coffee steeping in the press for an hour or so after you brew. Leaving the water/coffee in contact with the grounds will continue the extraction process: after about an hour you should have a brew of bitter, jittery-making sludge. See what we mean now? Sorry, we realize it's like making you smoke the entire pack of cigarettes at once.
The solution: Adjust grind or time Just like in underextracted coffee, the simplest solution is to attenuate your grind. In this case, coarsening your grind will slow the process of extraction and allow you to find the right balance of flavor. Or experiment with shorter and shorter extraction times to see if you hit the mark.
Symptom: Stale, Flat Taste
Stale-tasting coffee could come from a variety of factors, like underextraction and too low a brew temperature, but it's always good to double-check best practices here. Are you grinding your coffee right before you brew it? A few days before you brew it? Have you ever seen these beans in whole form? Coffee begins to lose subtlety of flavor—and important other elements, like body and acidity—both as it ages and as coffee particles are exposed to oxygen. If you're storing coffee for a long period of time, or grinding well ahead of brewing, stale and flat are unavoidable.
The solution: Pay attention to grinding and storage This is a good time for a public service reminder that even a low-end hand-grinder with conical burrs will run you about $40. It's worth it! Make your kid do the physical-labor part. And after opening a fresh bag of coffee, try to use it within a week to ten days, keeping it as well-sealed as you can during that time.
Symptom: Other Off Flavors
Of the countless things that can make coffee taste bad, here are a few: poor quality coffee, poor quality water, uneven particle size (as is customary from a blade-style coffee grinder), dirty coffee equipment, strong taste left over from paper or other coffee filters, and simple bad luck. Actual defects within the coffee—such as funky ferment flavor, or rogue beans plagued with Rwandan Potato Defect—are pretty rare, but they can happen. Check your parameters, make sure all the elements of your approach are locked-in to be as good as they can be, and if at first you don't succeed? Try, try again.