What Actually Happens When Coffee Is Roasted

Green coffee doesn’t smell like coffee at all. It’s pale, dense, and smells vaguely of hay or wet grain, genuinely unremarkable, sitting in sacks in the corner of the roastery before I’ve touched it. Every time I open a new sack there’s a brief moment of “are you sure this turns into coffee.” Then it goes into the drum and something close to alchemy happens.

I upgraded from a tiny 1kg sample roaster to a proper 5kg drum last year, still parked in Blouberg, still slightly too loud for the room I’ve got it in, and I still find the actual chemistry of roasting kind of magic, even doing it most days now. Maybe especially doing it most days now.

The bit where chemistry does the work

A green bean is loaded with acids, sugars, proteins and chlorogenic acids sitting there quietly, more or less flavourless. Apply heat and they start reacting with each other, mostly two processes doing the heavy lifting: the Maillard reaction (the same thing that browns a steak or a crust of bread) and caramelisation, sugars breaking down under heat. Between them they generate literally thousands of aroma compounds. Skip this step and you’d basically be drinking grassy herbal tea, which, no thank you.

What I’m actually doing up there for twelve minutes

Roasting isn’t just “apply heat and wait.” I’m managing a temperature curve in real time, and the small decisions along the way change the cup completely.

• Drying phase: beans lose moisture, still yellow-green, smell like hay or fresh bread.
• Maillard phase: colour shifts to golden, then light brown, the good roasty smell starts showing up.
• First crack: an actual audible crack as the bean expands and releases CO2. Roughly where “light roast” starts. Drinkable here, bright, high acidity, a lot of the origin’s own character still intact.
• Development time: the stretch between first crack and, if I go that far, second crack, where I decide how far to push it. This single decision affects the cup more than almost anything else I do.
• Second crack, if I get there: darker, oilier, more bitter, proper espresso or French roast territory. I rarely go here.

Why I mostly stop early

A lighter roast keeps more of the bean’s own personality, the fruit, the florals, whatever the specific farm and altitude and processing gave it. Push it darker and you start replacing that with roasty, chocolatey, bitter notes from the roasting itself, which increasingly all taste the same no matter what bean you started with. Neither approach is wrong, but roasting a genuinely interesting natural Ethiopian dark feels a bit like ordering a well-marbled steak well done. Technically allowed. Questionable choice. I roast on the lighter side of medium across most of what I sell, light enough that the origin still gets to talk, developed enough that it works across a pour-over, a plunger or an espresso machine.

If you want the more chemistry-minded version of this, the National Coffee Association has a good breakdown of the roasting process. And if you want the practical side, what roast level and freshness actually do to your coffee once it’s home, I wrote about that separately in fresh coffee, the one thing most people get wrong.