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coffee roasting machine

What is the first crack ? What is the second crack ?

Updated: Oct 17, 2023

During roasting, moisture begins to evaporate, and the bean expands. Besides the water vapor, gases released as the output of exothermic reactions build up extra pressure. Expanded gases and vapor, trapped inside the bean, escape by cracking the cellulose structure which is called first crack. First crack always occurs at 160 °C, where roasting temperature should not be increased.

first crack - second crack

After the first crack, gas release continues, and pressure restarts to be built up inside the bean cells. Immediately, bean structure becomes more fragile. Though primary reason of the first crack is the water vapor accumulation, the second crack's is the increase of carbon dioxide by volume during roasting.

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