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Physical changes of green coffee beans during roasting




green coffee beans

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The microscopic physical structure of green coffee beans appear remarkably homogenous, and oils on the surface cover the cellulose matrix. The production of water vapor and carbon dioxide release increase bean’s physical volume, and stimulate the formation of cavities during roasting. Bean expands, its color becomes silver, and cellulose gets trapped in the central fractures a few minutes before the first crack. As the cellulose can no longer withstand the increasing pressure, bean bursts the water vapor and the gas out of the cracks, and emits a sound like the corn popping. Roast masters obtain light or medium roasts by finishing the roasting right after the first crack and just before the second crack. Gas release continues, and bean’s interior cellular pressure starts to increase again after the first crack. In a short time, beans physical structure becomes fragile and the ambient becomes available for the second crack. While the main reason for the first crack is accumulated water vapor, it is carbon dioxide for the second crack. Oils inside the bean start to rise to the surface just before or after the second crack. This moment indicates the mark of dark roast for all the roast masters.


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