Part II · Anatomy of a roast

The three phases

Roasters split every roast into three phases: drying, Maillard, and development. They aren't hard walls — one blends into the next — but they're the single most useful way to think about what should be happening, and for how long.

Phase 1 · Drying

From the moment the beans hit the heat (the charge) until they turn yellow — around 150 °C of bean temperature. The bean sheds most of its free water as steam. This phase is strongly endothermic: it eats energy, and almost no browning happens yet. The smell goes from grassy and vegetal to bready, like baking dough.

Your job here: drive heat steadily and evenly into the bean to set up everything that follows. Rush it and the outside dries far ahead of the inside, baking in unevenness you can't undo later. Drag it out with too little heat and you begin "baking" the bean — stripping out aroma before it ever forms. Drying is typically 30–50% of the total roast.

Phase 2 · Maillard (browning)

From yellowing to the first audible crack. This is the flavor factory. The Maillard reaction — sugars reacting with amino acids — kicks off alongside early caramelization, browning the bean and generating hundreds of new aromatic compounds. This is where most of a coffee's sweetness, body and brown-sugar depth are built.

The bean turns tan, then light brown, and smells toasty and nutty. Spend too little time here and the cup is thin and sharp; linger too long at low heat and you trade brightness for a dull, over-roasted flatness. Maillard is usually 25–40% of the roast.

Phase 3 · Development

From first crack to the drop (when you end the roast and dump the beans to cool). The bean is now brown, swelling, and — remember the energy story — partly cooking itself via exothermic reactions. This short window has enormous leverage: it sets the final roast level and the balance between acidity, body and roasty character.

Stop too early (a low development time ratio) and acids haven't mellowed — sour, grassy, sharp. Stretch it too far and you march toward dark and flat. The fraction of total time spent here, the DTR, is the headline number, and it usually lands around 18–25%. It gets its own chapter.

The percentages are a lens, not a recipe

It's tempting to chase numbers — "40 / 38 / 22 and I'm done." Resist it. The ratios are a way to describe and compare roasts, not a target to hit blindly. A 2-minute development means something different on a 10-minute roast than on a 14-minute one. Dense high-grown beans want more; soft naturals want less. And every machine reads temperature differently, so a friend's numbers may not map onto yours.

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Phases blend; your real signposts are the cracks and the color.

You don't switch phases by flipping a switch — you notice them: yellowing, the first crack, the smell changing. The percentages are most useful as a sanity check and, above all, for comparing one of your roasts to the next.