Acidity — often mistaken for acrid, bitter or pungent — is at the heart of the flavor chain.
In truth, acidity is the dry, bright, and sparkling sensation that sets a high-quality coffee from a mundane flat cup. Some call it snobbery, but it can be compared to nice wine or craft beer experiences.
Highly prized coffees are characterized by bright, nuanced qualities; quite often, they are unforgettable. A standard cup of coffee will have no movement or effervescence -- your takeaway, nothing special. Well-curated, freshly-roasted brew will show up with bubbles or brownish crema during brewing or after poured. This is a sign of the flavors active as a result of the acids contained in the coffee.
What starts with mother nature lives on with expertise — or dies with careless roasting or disregard (commodity coffee production).
Scientifically, acidity is measured on the pH scale, which uses 7.0 as an indicator of neutrality, ratings under 7 are more acidic and above 7 are lower acid (referred to as basic). Lemon juice registers at about 2.0; milk 6.5. Very basic coffee (black) is about 5 pH.
As you get lower in pH, you will discover those curated flavors from citrus to fruit and florals. Acidity and complexity go hand in hand in single-origin specialty coffee. Even savory flavors like nuts and malt are part of the wheel of flavors.