What Is Roasting?

Coffee roasting is the application of heat to green coffee beans in order to trigger a cascade of chemical reactions — collectively known as pyrolysis — that develop the flavors, aromas, and colors we associate with coffee. Before roasting, a coffee bean is dense, grassy, and nearly unrecognizable as the thing that brews into your morning cup. After roasting, it becomes porous, fragrant, and full of soluble compounds ready to be extracted.

The roaster's job is to apply heat in a controlled way that brings out the best of what's already present in the bean — not to impose a flavor, but to reveal one.

Starting with Green Beans

Everything downstream of roasting depends on the quality of the green bean. Green coffee is a living agricultural product — it carries the genetics of its variety, the minerality of its soil, the effects of altitude and rainfall, and the choices of the farmers and processors who handled it. At Mirlo, we spend as much time evaluating green lots as we do at the roaster.

Green beans are dense and have a moisture content of around 10–12%. They smell faintly of grass, hay, or peanuts. Their flavor potential is locked inside — roasting is the key.

The Roast Process

Phase 1 — Drying (0–5 min)

The first phase of a roast is purely about moisture. Beans enter the drum at ambient moisture and need to reach around 160°C before the real chemical work begins. During this phase the bean absorbs heat without visible color change. The roaster must be careful not to apply too much heat too fast — scorching the outside while the inside remains underdone. Good drying sets the foundation for an even roast.

Phase 2 — Maillard Reaction (5–8 min)

Around 150–160°C, the Maillard reaction begins: amino acids and reducing sugars combine to form hundreds of new aromatic compounds. This is where a roast's character starts to form — the nutty, bready, caramel-like notes that underpin most coffees emerge here. Beans shift from green to yellow to light brown during this phase.

Phase 3 — First Crack (8–10 min)

Around 196°C, the built-up CO₂ inside the bean causes it to fracture audibly — a sound like popcorn or distant gunfire. This is "first crack," and it marks the beginning of drinkable coffee. Light roasts are pulled just after or during first crack. The bean has expanded, its surface has smoothed, and flavors are bright and vibrant.

Phase 4 — Development Time (post-crack)

The time between first crack and the end of the roast is called development time (DT). It's typically 20–25% of the total roast time. Too short, and the coffee tastes underdeveloped — sour, grassy, sharp. Too long, and you bake out the character, producing flat, dull coffee. Mirlo targets a development time that lets sweetness fully form without letting delicacy burn off.

Roast Levels

Light

Pulled just after first crack ends. High acidity, complex fruit and floral notes, lighter body. The coffee's origin character is most expressive here. Less forgiving to brew — underextraction is easier to hit. Best for pour-over, filter, and cold brew.

Medium

The sweet spot for most specialty coffee. Brightness balanced with sweetness and body. Origin character is still present but roast notes (caramel, hazelnut, chocolate) begin to appear. Versatile — works well across brew methods. Mirlo's house roast level for most single-origins.

Dark

Pushed past second crack (around 224°C). Roast character dominates — bittersweet chocolate, smoke, low acidity, full body. Origin flavors are mostly masked. At Mirlo, we rarely go this dark — we prefer letting the bean speak rather than the drum.

How Mirlo Roasts

We use a small-drum roaster and roast in batches no larger than 3kg. Every lot is profiled manually — we don't set and forget. We listen for first crack, watch the color develop, and taste every batch before it ships. Our goal is always the same: get out of the bean's way and let it tell its own story.

If you want to taste the difference, try it at home: read our brewing guide.