The most typical way to describe coffee roast levels is by the colour of the roasted beans, which range from light to dark (or extra dark). As coffee beans absorb heat in the roasting process, their colour becomes darker. Oils appear on the surface of the beans at higher temperatures. Since coffee beans vary, colour isn’t an especially accurate way of judging a roast. But along with the typical roasting temperature that yields a certain shade of brown, colour is a convenient way to categorize roasting levels.
Roast level personal personal preferences are subjective. The roast level you like might depend on where you reside. In the US, folks on the West Coast have traditionally preferred darker roasts than those on the East Coast. Europeans have also favored dark roasts, lending their names to the so-called French, Italian, and Spanish roasts that dominate the darker end of the roasting spectrum.
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Light roasts are light brown in colour, with a light body and no oil on the surface of the beans. Light roasts have a toasted grain taste and pronounced acidity. The origin flavours of the bean are retained to a greater extent than in darker roasted coffees. Light roasts also retain majority of the caffeine from the coffee bean.
Light roasted beans usually reach an internal temperature of 180°C – 205°C (356°F – 401°F). At or around 205°C, the beans pop or crack and expand in size. This is known as the “first crack” (for the “second crack,” see below). So a light roast usually means a coffee which hasn’t been roasted beyond the first crack.
Medium roasted coffees are medium brown in colour with more body than light roasts. Like the lighter roasts, they’ve no oil on the bean surfaces. Nevertheless, medium roasts lack the grainy taste of the light roasts, exhibiting more balanced flavour, odor, and acidity. Caffeine is somewhat decreased, but there’s more caffeine than in darker roasts.
Medium roasts reach internal temperatures between 210°C (410°F) and 220°C (428°F) — between the end of the first crack and just before the beginning of the second crack.
Medium-dark roasts have a richer, darker colour with a few oil starting to show on the surface of the beans. A medium-dark roast has a heavy body in comparison with the lighter or medium roasts.
The beans are roasted to the beginning or middle of the second crack — about 225°C (437°F) or 230°C (446°F). The flavours and aromas of the roasting process become noticeable, and the taste of the coffee can be somewhat spicy.
Dark roasted coffees are dark brown in colour, like chocolate, or sometimes almost black. They’ve 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 flavours are eclipsed by the flavours of the roasting process. The coffee will usually have a bitter and smoky or burnt taste. The amount of caffeine is substantially decreased.
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.
Here are the summary of the differences:
But it’s all about the aroma and taste and it all depends on the personal preference of individuals.