When adding the green beans to the preheated roasting drum, coffee starts to develop sugars while drying. Water is evaporating and the beans turn yellow-brown. Light roasts end at 180C-190C, and typically have a light body with fruity and earthy aromas. The coffee tends to be more acidic, as the oils didn't break through to the surface.
Further roasting causes vapors and pressure to crack thru the bean. This stage is known as the First Crack (205C), when the sugars begin to caramelize and the native acids have fully developed. The roaster is now balancing the bright cleanliness of acidity with the round, sugary sweetness. Aroma and body increases throughout this stage. Medium roasts tend to have a medium brown color, with a stronger flavor and a non-oily surface.
During the last stage of roasting a.k.a. the Second Crack (225C), carbon dioxide forms and the sugars have been caramelized to a crisp. The bean loses 15% of its weight due to pyrolysis. The coffee reaches its lowest acidity and full body. The bean is now exhibiting a rich, dark color, and oily surface. Aromas are nutty and caramelly, with a slight bittersweet aftertaste.
Beyond this stage (240C) the beans are burnt black due to too much caramelization. The acidity is almost fully gone in dryer beans, oils develop on the surface, and the bean looks very shiny and black. Very dark coffee roasts lose their origin flavors and caffeine levels. The coffee aromas are spicy, smoky, and bitter or burnt, with a lighter body.