In coffee production, “second crack” refers to the stage of the roasting process when the coffee beans begin to make a second set of popping sounds, similar to popcorn, as the internal pressure of the beans continues to increase.
The second crack is less intense than the first crack, and the popping sounds are more spaced out. It indicates that the coffee beans have reached a darker roast level than they were at first crack. The timing of the second crack and the duration of this stage can vary depending on the type of coffee bean, the size of the batch and the desired roast level.
As the roasting continues, the beans will continue to darken and more oils will be released, creating a more intense and full-bodied coffee, with smoky and chocolate notes. And the intensity and duration of second crack usually indicate the roast level, for example, a light roast will not reach the second crack stage, while a dark roast will pass through it quickly.
Roasters carefully monitor the second crack and adjust the roast profile to achieve their desired flavour profile, some roasters prefer to stop the roasting process just before the second crack and some prefer to end the roast just after the second crack.
Also see First Crack.
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