what are First & Second Crack?

Coffee Roasting Basics: what are First & Second Crack?

When coffee is roasted, it undergoes a number of chemical changes that can be distinguished by various sensory cues. For specialty coffee roasters, it’s important to pay close attention to these cues in order to achieve a desired flavour and aroma for your coffee.

Two of the most distinctive indicators of how beans are changing during the roast are the temperature thresholds known as first and second crack. The two “cracks” are named for their audible popping sound, and typically occur at around 180-190°C and 200-220°C.

What Is First Crack?

During roasting, coffee beans undergo a series of chemical reactions as a result of heat transfer, known as endothermic and exothermic reactions.

Endothermic reactions are when the beans absorb heat energy from the drum. In the initial stage of a roast, the heat moves from the roasting environment into the green bean. This initiates a reaction between the amino acids and reducing sugars (called the Maillard reaction), which creates a range of flavour and aromatic compounds within the beans.

The Maillard reaction accelerates between 120°C and 150°C, before slowing down when the bean temperature reaches approximately 170°C, giving way to caramelisation.

Caramelisation is a form of pyrolysis (chemical decomposition as a result of heat) that decreases sweetness and increases bitterness in the beans. Up until this stage, the beans have been absorbing heat energy and building up internal pressure.

At around 196°C, the beans will emit a cracking sound from within the drum, not unlike the sound of corn kernels popping. This is called “first crack”.

What Is Second Crack?

After the first crack, a short endothermic phase follows. The colour of the coffee continues to darken, while gas and pressure start building up within the bean once more.

During this phase, the cellulose structure of the bean continues to break down and becomes more brittle, paving the way for a second crack. Ishan tells me that the beans will also start developing more sweetness.

When second crack occurs (typically around 224°C) the oils within the coffee beans start to migrate to the surface. The roasting smoke also becomes darker and more pungent. This is characteristic of dark roast profiles.

It’s important to carefully monitor this stage of the roast as the character of the beans begins changing at a rapid rate; if left for too long, they may even ignite.