Before the Roast
Green coffee arrives to the roastery as an agricultural product with physical properties, not as a flavored coffee. Two of those physical properties help us know how we must roast it: moisture and density.
Moisture
A ripe cherry is about 45-55% water at picking. Drying at origin brings the seed down to somewhere around 10-12%, which is the range the industry settles on. Below about 10%, the coffee has dried too far and loses cup quality; above 12 percent it starts carrying a mould risk in storage1. Mould risk means a risk to human consumption. Water also carries heat, so moisture is not a compliance number, it is the first variable in how the roast will behave.
Density
Density is mass relative to volume and is mostly the fingerprint of elevation. A denser bean needs more energy to get heat through its structure. A softer, low-density bean is far more sensitive: too much heat or airflow and it scorches or bakes2. Kenyan and Ethiopian coffees tend to sit at the hard end, which is why they are roasted differently from a soft Brazilian.
Into the Drum
For the first stretch of roasting, very little happens that looks dramatic. The bean absorbs heat and drives off water as steam. The steam is what becomes the medium for the chemical reaction that follows. Sugars and amino acids react, browning the bean and building the caramel, chocolate, and nutty compounds that you think of when you taste coffee.
Then comes the first crack. Pressure from the steam builds inside of the bean until it can no longer contain it and the bean cracks. It fractures audibly, similar to how popcorn does3. Everything that happens after the first crack tells you about the roaster. Stop too early and the cup is sour. Roast too long and the origin characters disappear under the roast character, which is a dark commodity of coffee: the roast rather than the origin flavor.
By the end, the bean has lost roughly 1/5th of its weight, mostly water weight, and has expanded in size. It leaves the drum, not green and dense, but brown, brittle, and light.