GeekPress points to The Chemistry of Great Coffee:
When it comes to great flavor, coffee chemistry boils down to roasting and brewing.When the inside of the bean reaches about 400 degrees, it begins to turn brown. Oil locked inside the beans begins to emerge. This is when the flavor takes shape. The more oil, the stronger the flavor.
Roasted coffee is a perishable food. The flavor peaks a few days after roasting and fades once the coffee is exposed to air, light or moisture. For this reason, aficionados keep their beans or fresh grounds in an airtight container at room temperature.
If you’re one of the many people who drink coffee for the pick-me-up, you might think an espresso or cappuccino would be most effective. Not necessarily so. Caffeine content goes up as the water spends more time in contact with the grounds. Espresso brewing takes 25 seconds. Other methods take several minutes. Darker roasts also yield more caffeine.
Flavor is the combination of aroma, acidity and body.
Body is the sensation of heft or viscosity, something like oil, on the palate. Longer roasting yields more body. But that also decreases acidity, the tingly taste on your tongue. So there’s a trade-off between body and acidity.
Acidity isn’t bitterness though. Bitterness comes from skimping on grounds when you brew, brewing for too long, and brewing in a pot or machine with residual grounds left from hours, days or weeks ago.