All coffee guides · Roasting
Coffee roasting looks simple from the outside — hot drum, green beans in, brown beans out — but the chemistry happening inside each bean during the 8 to 15 minutes of a roast is extraordinarily complex. Understanding the basics helps you appreciate why roast date, roast degree and rest time matter as much as they do.
The roast begins with a drying phase where the bean's moisture content (around 10-12% in green coffee) is driven off. The bean heats up slowly at first as energy goes into evaporating water. Colour changes from green to yellow, and the grassy, hay-like smell of green coffee transitions into something more grain-like and then biscuity.
As temperature climbs past around 150°C, the Maillard reaction begins — the same reaction responsible for the browning of bread crusts and grilled meat. Amino acids react with reducing sugars to create hundreds of new flavour and aroma compounds. This is where most of coffee's complexity originates. The colour darkens through shades of brown, and the smell shifts from biscuity to rich and roasted.
At around 196-205°C, the bean undergoes first crack — a popping sound similar to popcorn as water vapour and CO2 pressure builds until the cell walls rupture. This is an exothermic event (the bean releases energy rather than just absorbing it), and it marks the transition from underdeveloped to drinkable coffee. Roasters who stop the roast shortly after first crack produce light roasts with high acidity and strong origin character.
After first crack, the roaster enters the development phase — the time between first crack and the end of the roast. This is arguably the most critical window. Development time ratio (DTR) — how long development takes as a percentage of total roast time — significantly affects sweetness, acidity and body. More development builds more sweetness and softens acidity. Under-developed coffee tastes baked and bland; over-developed coffee burns off delicate aromatics.
At around 224-230°C, second crack begins — a faster, more crackling sound as the cell structure of the bean breaks down more extensively and CO2 is released more rapidly. Surface oils begin to appear. Beyond second crack, the roast is in dark territory: bitterness increases, origin character disappears, and the smoky, carbon-like notes of very dark roasting emerge. Most specialty roasters stop before or just at second crack.
During and after roasting, CO2 is produced in large quantities inside the bean. It continues to off-gas for days after roasting. If you brew immediately after roasting, this CO2 escapes rapidly during extraction, disrupting the flow of water through the grounds and producing an uneven, gassy, underdeveloped cup. Most roasters recommend resting espresso for at least 5-7 days post-roast (often 10-14) and filter for 3-5 days. The visible sign of active degassing is the bloom in pour-over: a fresh coffee blooms vigorously; a stale one barely moves.
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