About 28% of a coffee bean’s mass consists of dissolvable solids; the rest is insoluble plant matter. Not all of those solids should be extracted, however. A well extracted coffee requires precise control over the brewing process; the goal is to dissolve all of a coffee’s desirable flavors while excluding all the undesirables ones, achieving what we call an “ideal” extraction.
Part II covers how to diagnose, by taste, the distinct levels of extraction: under, over, and ideal.
Underextraction
When not enough of coffee’s solids are extracted, underextraction occurs. An indicator of underextraction is a lack of sweetness, and the flavor that persists in almost all forms of underextraction is an acute sourness. In aggressively underextracted coffee, the sensation can be mouth-puckering, akin to sucking on an unripe lemon: pure citric acid.
Acidity is not the problem with underextraction. On the contrary, acidity provides complexity and character. The difference between an unremarkable coffee and one that calls to mind a ripe Georgia peach lies in the acidity.
The problem with underextraction — like overextraction — is that causes the coffee to be unbalanced. The acids have been dissolved, but the sugars have not.
Note that we distinguish “sour” from other acid descriptors like “bright,” “juicy,” “ripe,” or “tart.” “Sour” has no depth of flavor; it is a single, loud note that drowns out other flavors.
Overextraction
Overextraction occurs as the result of extracting too much of the coffee.
Bitterness is the first obvious sign of overextraction. Coffee is naturally a little bitter due to the presence of caffeine, but this bitterness is imperceptible in a well-brewed coffee. Bitterness as a result of overextraction will taste like licking a piece of charcoal.
The other indication of overextracted coffee is a chalky mouthfeel evolving into dryness on the palate. The feeling is similar to the astringency felt after drinking dry wine. Polyphenols and tannins bind to proteins in saliva and deactivate its lubricating properties, causing the sensation of dryness to persist.
Overextracted coffees tend to taste very dull; overextracted flavors will overwhelm a coffee and mute any complexity within it.
Ideal extraction
Ideal extraction is balance, and its hallmark is clarity. The acidity is upfront, but it is tempered and shaped by sweetness. Well-extracted coffee expresses and distinguishes complex, nuanced flavors in an identifiable way.
Understanding the extraction profiles and their distinguishing factors are paramount to correct brewing, and will help you learn how to make your coffee taste better.