The secret to unlocking a coffee's flavour, acids and aroma is a chemical process called coffee roasting.
The roasting process dramatically influences the taste of a particular coffee by changing the both the physical and chemical state of the bean. Technically, coffee beans do not begin roasting until the internal temperature of the bean reaches 200 °C, however, the moisture content of a coffee can differ greatly, affecting the rate at which some coffees develop.
The initial stage of the roasting process is considered the endothermic stage, where the beans rapidly absorb the roaster's heat.
As the temperature reaches 200 °C and above, the beans begin to double in size and caramelize. This stage, also called first crack, is where the bean's starches transform into sugars, losing moisture and darkening in colour the longer they are left to develop.
Another short endothermic period preceeds the final stage, also known as second crack, where the beans become exothermic and develop an oily sheen on their surface. Throughout the entire process, all of the coffees aromatic oils, acids, moisture and caffeine content continue to weaken, which further affects the coffee's eventual flavour and aroma.