Coffee beans typically lose 14–20% of their mass during roasting. Darker roasts lose relatively more mass — in fact, the amount of mass lost correlates linearly with the roast colour.
Fast, high-temperature roasts in fluid-bed roasters lose less mass than slower roasts for a given roast colour, partly because they lose less water in the short time they spend in the roaster. Over longer periods of time, however, roasting at high temperatures leads to a higher maximum mass loss, thus the roasting temperature has more of an effect on mass loss than the amount of time the beans spend in the roaster (Schenker 2000).
Mass loss depends on the final roast colour and the roasting temperature. The mass lost correlates linearly with roast colour (left), but fast, high-temperature roasts to the equivalent roast colour lose less mass. The maximum mass lost during roasting (right) is much higher in fast, high-temperature roasts. Adapted from Schenker (2000)
Most of the mass lost represents the bean’s initial moisture content, which escapes as steam. The remainder is made up of carbon dioxide, water that was created during reactions during roasting, and small amounts of volatile compounds. In practical measurements in the roastery rather than in a lab, the mass loss will also include any chaff, bean fragments, dust, or small stones that escape the roaster.
Because moisture makes up the biggest part of the lost mass, the initial moisture content has a big effect on the mass lost during roasting. Researchers often refer to ‘organic roast loss’ or ‘dry mass loss’, meaning the amount of mass lost after excluding water. The dry mass loss is typically around 4–6% (Fernandes 2019), but for very dark roasts it could be as high as 12% (Clarke 1987).
Some studies have found that the rate of mass loss accelerates at the end of the roast, perhaps because the chemical reactions that break down the beans’ structure begin at high temperatures.