The green bean
You can't roast your way out of a bad green coffee, and you can't fully hide a great one. Roasting is a transformation of whatever you put in — so it pays to know what that raw material actually is.
It's a seed, not a bean
What we call a coffee "bean" is the seed of a cherry-like fruit. Each cherry usually holds two seeds pressed flat against each other, which is why most beans have one rounded side and one flat side with a crease down the middle. That crease — the center cut — traps a papery silverskin that flakes off during roasting as chaff.
What's inside (and why it matters)
The green seed is a pantry of precursors. Heat turns these into color, aroma and taste:
| Component | Roughly | Becomes… |
|---|---|---|
| Water | 10–12% | Steam — drives the drying phase and carries heat through the bean. |
| Carbohydrates (incl. sugars like sucrose) | ~50% | Fuel for caramelization and the Maillard reaction → sweetness, brown color, body. |
| Chlorogenic acids | ~6–8% | Break down with heat → softer acidity plus some bitterness; lots is lost in dark roasts. |
| Proteins / amino acids | ~10% | The nitrogen partner in the Maillard reaction → savory, roasty aromas. |
| Lipids (oils) | ~15% | Carry fat-soluble aromatics and mouthfeel; migrate to the surface in dark roasts. |
Four things about a green coffee that change how it roasts
1. Species & variety
Almost all specialty coffee is Arabica, prized for its sweetness and complexity. Robusta is hardier, more bitter, far higher in caffeine, and roasts denser. Within Arabica, varieties (Gesha, Bourbon, Caturra, SL28…) differ in size and density, which nudges how they take heat.
2. Density & altitude
High-grown coffees are denser and harder — the slow maturation packs in more sugars and acids. Dense beans resist heat: they generally want more energy to get moving and reward a little more development. Soft, low-grown or aged beans heat faster and scorch more easily. You can feel this difference within a single bag.
3. Moisture content
Fresh-crop greens often sit around 10–12% moisture; older greens dry out below that. More water means more energy spent boiling it off, and a longer drying phase. Moisture also conducts heat into the bean, so very dry greens can heat unevenly.
4. Processing
How the fruit was removed from the seed leaves a chemical fingerprint that survives roasting:
- Washed — fruit stripped before drying. Clean, transparent, acidity-forward. Roasts predictably.
- Natural — dried whole in the fruit. Heavier body, fruity/boozy, often a touch more sugar at the surface — can brown faster and need a gentler hand.
- Honey / pulped natural — in between, with extra sweetness.
Write down the origin, variety, process and (if you have it) density and moisture for every green you buy, and keep it next to your roast notes. Patterns emerge fast: "dense washed Ethiopians like more drying time" is the kind of intuition you build one logged roast at a time.
Now that you know what's going in, let's look at what heat actually does to it.