When heated, water inside the bean evaporates and CO₂ and other gases form. Internal pressure builds until the cell structure gives way — that's the First Crack. At Second Crack, finer structures tear and oils emerge.

Why Do Coffee Beans Crack During Roasting?

When heated, water inside the bean evaporates and CO₂ and other gases form. Internal pressure builds until the cell structure gives way — that’s the First Crack. At Second Crack, finer structures tear and oils emerge.

Why that matters

A raw coffee bean contains 8–12 % moisture, enclosed in a dense cell structure made of cellulose. During roasting, this moisture and structure undergo a dramatic transformation.

In the first minutes of roasting (drying phase, up to around 150 °C), free water evaporates. The bean loses weight but remains largely unchanged on the outside — it shifts from green to yellow.

From around 150 °C, the Maillard reaction begins: amino acids and sugars react with each other, forming hundreds of new compounds — melanoidins (brown, bitter), aromatic compounds, and gases. Simultaneously, sugars in the bean caramelise. Both processes generate CO₂ and water vapour that build up inside the bean.

At 196–205 °C bean temperature, internal pressure is so high — estimated at 5–25 bar — that the cell walls give way. The bean cracks open: that’s First Crack. The energy is released suddenly, the bean nearly doubles in volume and loses up to 15 % of its weight. The sound comes from the cell structure breaking — an audible crack, comparable to popcorn kernels popping.

After First Crack, the bean is porous and noticeably larger. During the development phase that follows, chemical reactions continue. More sugar caramelises, more CO₂ forms, the cell structure breaks down further.

Second Crack (224–228 °C) is a different event: it’s no longer the large cell walls giving way, but the finer internal structures of the bean. Oils that were previously trapped inside the cells emerge on the surface — the bean becomes glossy. The sound is quieter and higher-pitched than First Crack, more of a fine crackling. From Second Crack onward, it’s considered a dark roast. Roasting well past Second Crack burns the bean — oils oxidise, carbon dominates, and the coffee tastes of ash.

At Green Wall Coffee

At our café on Sophienstraße 27 in Berlin-Lichtenberg, I like to explain the cracking with an analogy: think of the bean as a mini pressure cooker. The lid blows off at First Crack. If you take the coffee out of the roaster then, you have a light roast. If you wait until the inner walls tear too, you have a dark roast. In between lies a spectrum — and within that spectrum is where the coffee’s character is decided: fruity or chocolatey, acidity-forward or bitter.

Related Questions

• What is First Crack and Second Crack?
• At what temperature is coffee roasted?
• What is the difference between light, medium, and dark roast?

More depth on this topic in the article How to Make Perfect Espresso. Or stop by at Sophienstraße 27 — Mon–Fri 8am–5pm, Sat 10am–5pm.