The moisture content has a big impact on the rate of heat transfer into the bean. Coffee with a higher moisture content needs more heat during roasting because water takes a lot of energy to heat up. Coffee with a higher moisture content can handle higher temperatures in the roaster because water helps the bean carry heat away from the surface and into the centre of the bean.
Coffee with a high moisture content therefore both allows, and requires, more heat to be applied in the early part of the roast.
The higher the moisture content of the green coffee, the better it is at conducting heat (Farah 2020). This helps heat to transfer from the bean’s surface into its centre during the early part of the roast, before the moisture evaporates.
Coffee with a higher moisture content also has a higher specific heat (Pérez–Alegría et al 2001). This means that more energy is needed to raise the bean temperature.
The most important effect of moisture, though, is the amount of energy it takes to vaporise water. Water takes in energy as it vaporises, and it takes that energy with it when it leaves the bean as steam. The net effect is that the temperature of beans with a higher moisture content increases more slowly at the beginning of the roast (Schenker 2000).
The evaporation of moisture cools the centre of the bean until the bean temperature reaches around 150°C (302°F)— at which point most of the free water has evaporated (Fernandes 2019). After this, the rate of temperature rise inside the bean begins to accelerate (Bonnländer et al 2005).
The Evaporation Front
At the beginning of the roast, the bean temperature increases fastest near the bean surface, and heat travels more slowly to the inner layers. The moisture near the bean surface therefore evaporates first, creating a layer of water vapour in the outer part of the bean.