Just as their name would suggest, dark roasted coffee beans are dark brown in color or sometimes almost black. They have a sheen of oil on the surface, which is usually evident in the cup when the dark roast coffee is brewed. The coffee’s origin flavors are completely eclipsed by the flavors of the roasting process. Coffee brewed from these beans will generally have a bitter and smoky or even burnt taste. The amount of caffeine is substantially decreased compared with light roasts.
To reach the level of a dark roast, coffee beans are roasted to an internal temperature of 240°C (464°F) — about the end of the second crack — or beyond. They are seldom roasted to a temperature exceeding 250°C (482°F), at which point the body of the beans is thin and the taste is characterized by flavors of tar and charcoal.
Summary of Differences
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As coffee roasts get darker, they lose the origin flavors of the beans and take on more flavor from the roasting process.
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The body of the coffee gets heavier, until the second crack, where the body again thins.
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Lighter roasts have more acidity than darker roasts.
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Light roasted beans are dry, while darker roasts develop oil on the bean surface.
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The caffeine level decreases as the roast gets darker.