What roasting does

Roasting dries the bean, then browns it: Maillard reactions first, then caramelizing sugars.
First crack comes around 190–205°C; second crack around 225°C, as oils reach the surface.
Roasted beans leave lighter, bigger and full of CO2, so fresh roasts want a few days’ rest.

First it dries, then it browns

Roasting first drives out the water: the beans fade from green to pale yellow. From roughly 150°C sugars and amino acids brown — the Maillard reaction, as toast does — a little hotter, sugars caramelize.

First crack, a modest bang

Steam and carbon dioxide build pressure inside each bean until its structure gives way with a pop, like popcorn: first crack. A light roast ends not long after, while the farm still does most of the talking.

How hot is that pop?

A roaster hears first crack. Where is the bean’s temperature?

Answer: Around 190–205°C, depending on the machine and where its probe sits. 150°C is where browning starts; 225°C is second crack.

Development

The minutes after first crack are called development. Longer development builds sweetness and body while acidity softens; many roasters aim to spend very roughly a fifth of the roast here.

Drop this roast so that about a fifth of its time falls after first crack.

Both cracks, by temperature

First crack arrives somewhere around 190–205°C, depending on the machine and its probe. Press on to around 225°C and a quieter, faster crackle begins: second crack. Oils creep to the surface, and the roast starts to drown out the farm.

Lighter, bigger

Beans leave the roaster roughly 12–20% lighter, more the darker they go, mostly from lost water, and close to double their size.

Full of gas

They are also full of carbon dioxide, most of the gas roasting makes. It seeps out over days and weeks: that’s degassing, and the reason a fresh roast wants about four days.

Roast level against the farm

Join each roast level to what it mostly tastes of.

Light — Fruit, florals, brightness
Medium — Caramel sweetness, more body
Dark — Smoke, bittersweet cocoa, weight

Light keeps what the farm worked for, medium meets it halfway, dark tastes of the roaster. None is wrong, but a dark-roasted Gesha is an expensive way to taste charcoal.

Choosing for a rare lot

You have a rare, expensive Gesha. Which roast serves it best?

Answer: A light roast. It keeps the fruit and florals the farm worked for, where a dark roast would mostly taste of the roaster.

“I would roast a rare coffee lightly; the farm has usually said something worth hearing.”