Roasting: What Heat Does to a Green Bean
Drying, Maillard reactions, caramelisation, first crack and development — and why roast level dictates how you brew.
Green coffee is dense, grassy, and almost unbrewable. Roasting drives a cascade of chemical reactions that create nearly every aroma you associate with coffee, and physically transforms the bean from a hard seed into a brittle, porous, soluble one.
The stages
1. Drying (ambient → ~160 °C). Green coffee is 10–12 % water. The first minutes of a roast evaporate most of it. Little flavour develops here, but how evenly the bean dries determines how evenly everything afterwards happens.
2. Maillard reactions (~150–200 °C). Amino acids and reducing sugars react to form hundreds of new compounds: melanoidins (brown colour, body), pyrazines (nutty, roasty), furans (caramel), and many aromatic intermediates. This is where "coffee" flavour is born. The bean turns from yellow to tan to light brown.
3. Caramelisation (~170 °C+). Sugars break down and reform into caramel-flavoured compounds. Sweetness peaks and then declines as sugars are consumed — a dark roast has far less residual sugar than a light one.
4. First crack (~196–205 °C). Water vapour and CO₂ pressure inside the bean finally rupture the cell structure with an audible pop. The bean expands, becomes porous, and from here on extraction becomes possible. Almost all specialty coffee is dropped between the start of first crack and shortly before second crack.
5. Development. The time after first crack. Longer development converts acids to sweetness and body and increases solubility; too long and the roast loses origin character and moves toward generic "roasty" flavour.
6. Second crack (~225 °C+). Oils migrate to the surface, the cellular structure begins to fracture. This is dark roast territory: carbon, smoke, and very low acidity.
What roast level changes for the brewer
| Light | Medium | Dark | |
|---|---|---|---|
| Density | high | moderate | low (porous) |
| Solubility | low — hard to extract | moderate | high — easy to over-extract |
| Acidity | high | moderate | low |
| Sugars remaining | most | some | few |
| Bitterness | low | moderate | high |
| Grind for same extraction | finer | medium | coarser |
| Water temperature | 94–100 °C | 92–95 °C | 88–92 °C |
| CO₂ retained | high — rest 7–14 days | moderate | low — rest 3–7 days |
Light roasts hide an enormous amount of flavour behind their density; they reward hot water, fine grinds, high ratios and agitation. Dark roasts give everything up immediately and punish exactly the same treatment.
Acidity and chlorogenic acids
Green coffee is rich in chlorogenic acids (CGAs). Roasting breaks them into quinic and caffeic acid and, at higher temperatures, into chlorogenic acid lactones and phenylindanes — the compounds most responsible for the bitterness of dark roasts. This is why bitterness is a roast property as much as an extraction one: a dark roast brewed at a perfect 20 % EY is still bitter.
Degassing and freshness
Roasting generates CO₂ that stays trapped in the bean. In the first days after roasting the gas escapes rapidly; brew too early and it repels water, causes erratic blooming, and suppresses extraction — the cup tastes flat and sour. Most roasts brew best between 5 and 21 days off roast for filter, and a bit later (10–30 days) for espresso where CO₂ causes wild crema and channelling. After a month or two, oxidation takes over and aromatics fade.
Reading a bag
- Roast date, not "best before". Aim to use the coffee in the window above.
- Roast level is often unstated; "filter roast" is generally lighter than "espresso roast" from the same roaster, but there is no standard.
- Agtron / colour numbers where given: higher number = lighter. Specialty light roasts land around 70–85; dark roasts around 40–50.