What roasting actually does
Over roughly eight to fourteen minutes, a green seed loses water and weight, nearly doubles in size, marches from sage-green through yellow and brown, and — crucially — flips from absorbing heat to producing its own. Understanding that journey is what lets you steer it.
The visible changes
| Property | Green → Roasted | Why |
|---|---|---|
| Weight | −12% to −18% | Water boils off; some dry matter is lost as gas and chaff. |
| Volume | +50% to +100% | Steam and CO₂ pressure puff the bean up like popcorn. |
| Color | green → yellow → brown → near-black | Browning reactions (next chapters) build pigments. |
| Moisture | ~11% → ~1–3% | The drying phase; never quite reaches zero. |
| Density | drops sharply | The bean is now bigger but much lighter — porous and brittle. |
| Surface oils | none → sheen (dark roasts) | Lipids migrate out as cell walls rupture in darker roasts. |
That weight loss is worth remembering practically: 100 g of green gives you only about 82–88 g of roasted coffee, and the darker you go, the less you keep.
The energy story — the bean fights back
Here's the single most important physical idea in roasting, and the reason the back half of a roast is harder than the front. For most of the roast, the bean absorbs energy — heating it up and boiling its water are both endothermic (energy-hungry) processes. But somewhere around first crack, the browning chemistry turns a corner and starts releasing energy: the reactions become exothermic, and the bean begins generating heat of its own.
Why care? Because if you keep feeding the same heat after the exothermic turn, the bean's temperature can accelerate right when you most want control — speeding you into a darker, flatter roast than you intended. Skilled roasting in the final minutes is largely about anticipating that turn and backing off. On a manual drum you do this by hand; on your Nano the profile does it for you — but you still have to design the profile that does.
First crack, in one line (for now)
That audible "first crack" you'll hear is steam pressure rupturing the bean's structure — the same physics as popcorn. It's such an important signpost that it gets its own chapter. For now, just file it as: the moment the bean pops, swells, and the exothermic back-half begins.
A roast is an energy balance that flips partway through. First you're pouring energy in to dry and heat the bean; later the bean starts giving energy back. Every technique in this guide — rate of rise, development, the drop — is ultimately about managing that balance.
With the foundations in place, we can now open up the roast itself: its three phases.