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What happens when coffee is roasted ?

Updated: Oct 16, 2023

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Coffee beans undergo dramatic physical and chemical changes during the roasting process.

Most of the water in the bean, which is 14% to 21%, is lost. • Beans lose weight 12% to 24%. • Beans’ volume increase 50% to 100%. • Density of green coffee is around 0,8, i.e., one liter of green coffee weighs approximately 800 g. At the end of roasting, the density of coffee decreases down to 0,6 to 0,4, i.e., one liter of roasted coffee weighs 600 to 400 g.

Roasting process changes the color, reduces the moisture, increases the volume, and loosens the texture of the bean. Despite all professional roasting types depend on the bean color, there is no common view about naming the roast type

During the first stage of roasting, all beans lose moisture at the same ratio. This stage is commonly named as “drying stage”. Decomposition of chlorophyll during the first few minutes of roasting converts bean’s color from green to yellow, and Maillard reactions turn color from yellow to tan, and finally brown. As roasting goes further and first crack gets closer, bean’s color turns to deeper brown as a result of caramelization. Structural carbonisation in dark roast changes beans’ color to black.

Light roasts offer acidic, floral, and fruity flavors, more delicate aroma, and less body than dark roasts. Dark roasts develop smoky, pungent, bitter, and carbonized flavors. If one takes roasting to an extreme, completely bitter flavors develop. Over-roasted coffee is considerably hard to drink. Green beans of same characteristics, which are roasted in different profiles, temperature, time and drum pressure, may be in the same color but shall not have the same aroma. The foremost principle in roasting is to obtain the taste and aroma required by the roast master or the client.

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