Extraction

Extraction is the process of water dissolving flavor compounds out of ground coffee, and it happens on a spectrum from under extracted to over extracted. Roughly eighteen to twenty two percent of the coffee's mass ends up dissolved in a well balanced cup, though the right number varies a little by method and taste.

Under extraction happens when water and coffee do not have enough contact time or the grind is too coarse, leaving mostly sour, sharp, and thin flavors because the sweeter and more complex compounds never dissolved. Over extraction happens when contact time is too long or the grind too fine, pulling out bitter, dry, and sometimes hollow tasting compounds along with everything else.

You control extraction with grind size, water temperature, brew time, and ratio, and changing more than one at once makes it hard to tell what fixed or broke a cup. A common mistake is assuming a sour shot needs more coffee when it usually needs a finer grind or longer contact time instead, and the reverse for a bitter shot.