The drying a necessary initial phase to roasting. When coffees are processed and dried in origin, they don’t completely dry. So, when roasters receive green coffee, there’s typically 10-12% moisture still left in the bean to preserve its quality. Because of the moisture content, there needs to be a final drying phase that begins at roasting.
After heating up the drum, in other words setting a charge temperature, the beans are dropped from above into the drum where the coffee immediately goes into drying. Keep in mind that timing and temperatures are usually based on the process, density, etc of the coffees in play. The timeline of this phase is usually about 4-8 minutes and typically ends at around 160C. At the point, the beans move into the browning stage, or the maillard stage.