Drum roaster? Depending on the roasting procedure, green coffee beans can be transformed into brown beans with various flavors. A rotary is the most traditional – and still the most popular – roast coffee. While there are a variety of machines on the market, they all operate on the same basic principle: a huge metal drum keeps the coffee within, revolves around a shaft, and is heated from the outside (usually from under). The operator can alter several aspects of the procedure depending on the system. All of the airspeed, temperature, and drum speed can be changed.